In the development of space-based large telescope systems, having the capability to perform active optics correction allows correcting wavefront aberrations caused by thermal perturbations so as to achieve diffraction-limited performance with relaxed stability requirements. We present a method of active optics correction used for current ground-based telescopes and simulate its effectiveness for a large honeycomb primary mirror in space. We use a finite-element model of the telescope to predict misalignments of the optics and primary mirror surface errors due to thermal gradients. These predicted surface error data are plugged into a Zemax ray trace analysis to produce wavefront error maps at the image plane. For our analysis, we assume that tilt, focus and coma in the wavefront error are corrected by adjusting the pointing of the telescope and moving the secondary mirror. Remaining mid- to high-order errors are corrected through physically bending the primary mirror with actuators. The influences of individual actuators are combined to form bending modes that increase in stiffness from low-order to high-order correction. The number of modes used is a variable that determines the accuracy of correction and magnitude of forces. We explore the degree of correction that can be made within limits on actuator force capacity and stress in the mirror. While remaining within these physical limits, we are able to demonstrate sub-25 nm RMS surface error over 30 hours of simulated data. The results from this simulation will be part of an end-to-end simulation of telescope optical performance that includes dynamic perturbations, wavefront sensing, and active control of alignment and mirror shape with realistic actuator performance.
KEYWORDS: Space telescopes, Design and modelling, Telescopes, Observatories, Mirrors, James Webb Space Telescope, Space mirrors, Equipment, Astronomy, Coronagraphy
New development approaches, including launch vehicles and advances in sensors, computing, and software, have lowered the cost of entry into space, and have enabled a revolution in low-cost, high-risk Small Satellite (SmallSat) missions. To bring about a similar transformation in larger space telescopes, it is necessary to reconsider the full paradigm of space observatories. Here we will review the history of space telescope development and cost drivers, and describe an example conceptual design for a low cost 6.5 m optical telescope to enable new science when operated in space at room temperature. It uses a monolithic primary mirror of borosilicate glass, drawing on lessons and tools from decades of experience with ground-based observatories and instruments, as well as flagship space missions. It takes advantage, as do large launch vehicles, of increased computing power and space-worthy commercial electronics in low-cost active predictive control systems to maintain stability. We will describe an approach that incorporates science and trade study results that address driving requirements such as integration and testing costs, reliability, spacecraft jitter, and wavefront stability in this new risk-tolerant “LargeSat” context.
The Richard F. Caris Mirror Lab at the University of Arizona continues production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT’s 25 m primary mirror consists of a center segment surrounded by six off-axis segments, with an additional off-axis segment to allow continuous operation as segments are removed for coating. We cast the sixth segment (5 off-axis segments + center segment) in March 2021. As of June 2022 we have tentatively completed polishing of the third off-axis segment, and we are in the process of grinding the optical surface of off-axis Segment 5. For Segment 3, we improved the measurement accuracy for small-scale structure near the edge of the mirror, which has been a challenge for the off-axis segments. In addition to full-aperture interferometry and deflectometry, we used a 20 cm test plate to obtain high-resolution interferometric measurements of the edge.
The Vera C. Rubin Observatory Primary Tertiary Mirror (M1M3), together with the fully-assembled mirror support system, underwent two optical testing campaigns at the University of Arizona Richard F. Caris Mirror Lab. The objectives of the testing campaigns were: (1) optimizing the M1M3 surfaces with support forces, and (2) characterizing how the surfaces respond to actuator forces, including measuring the bending modes and single actuator influence functions. Both objectives were successfully achieved. The differences between the measured bending modes and the Finite Element Analysis (FEA) predicted modes were shown to be less than a few percent. The surface optimizations routinely resulted in Root-Mean-Square (RMS) surface errors below 30 nm for M1 and M3, simultaneously. The entire system was shown to be robust and repeatable. In this paper, we present the results of the optical testing and the analyses performed using the data acquired.
The Richard F. Caris Mirror Lab at the University of Arizona continues production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT will have a center segment surrounded by six off-axis segments, plus an additional off-axis segment to allow continuous operation as segments are removed for coating. The second off-axis segment was completed and accepted in 2019. We are currently polishing the optical surface of off-axis Segment 3. We have completed work on the rear surfaces of Segment 4, the center segment, and offaxis Segment 5. We are assembling the mold for off-axis Segment 6 with the spin-casting scheduled for March 2021.
Advances in our understanding of the Universe depend on improvements in sensitivity and angular resolution that can come only with larger telescopes. Telescope diameters increased by almost an order of magnitude in the last century, but that growth has been sporadic, limited mainly by the ability to make bigger mirrors that hold their shape against the dynamic effects of gravity, wind and temperature. Three major advances in mirror technology occurred in the 1980s, including the lightweight honeycomb mirrors made at the Richard F. Caris Mirror Lab at the University of Arizona. In this informal paper, I will describe these technologies and show how they enabled the current generation of 8- to 12-m telescopes and how they are now being used to build telescopes of 25 to 39 m.
Large optic fabrication is a delicate and time consuming process. Obtaining a large prime optic is often in the critical path of a project and poses a serious risk to both the schedule and budget. The Optical Engineering and Fabrication Facility (OEFF) at the College of Optical Sciences, the University of Arizona, has developed a new way of optimizing its large optic fabrication process for maximum efficiency in convergence. The new process optimization takes the amount of stock material removal, tool characteristics, metrology uncertainty, optic prescription, optic material properties, and resource availability as input parameters and provides an optimized process along with an achievable convergence. This paper presents technical details of the new process optimization and demonstrates performance on 6.5m mirror fabrication at the University of Arizona. Two case studies for an 8.4m GMT off-axis primary mirror segment and a 3.1m TMT convex secondary mirror fabrication are also presented.
The Richard F. Caris Mirror Lab at the University of Arizona continues the production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT will have a center segment surrounded by six identical off-axis segments, plus an additional off-axis segment to allow continuous operation as segments are removed for coating. Production highlights of the last two years include the spin-casting of Segment 5, preliminary polishing of Segment 2, and completion of the rear surface processing for Segments 3 and 4. We completed a preliminary design of the significant modifications of the test systems required for Segment 4, the center segment. We finished an upgrade of the 8.4 m polishing machine; both the upgrade and experience gained with Segment 1 have contributed to much faster polishing convergence for Segment 2. Prior to polishing Segment 2, we verified the stability and accuracy of the measurement systems by re-measuring Segment 1, achieving good agreement among multiple independent tests as well as good agreement with the original acceptance tests of Segment 1.
A turnkey observatory with 6.5-m telescope has been developed for a broad range of science applications. The observatory includes the telescope, mount and enclosure, installed on site and ready for operation. The telescope’s primary mirror is an f/1.25 honeycomb sandwich of borosilicate glass, similar to that of the MMT and Magellan telescopes. The baseline optical design is for a Gregorian Nasmyth focus at f/11. A Gregorian adaptive optics secondary that provides a wide-field focus corrected for ground layer turbulence (0.25 arcsecond images over a 4 arcminute field) as well as a narrow-field diffraction-limited focus is optional. Another option is a corrected f/5 focus with a 1° field. The observatory, built by partners from academia and industry with extensive experience, can be delivered within five years at a fixed price.
At the core of the Large Synoptic Survey Telescope (LSST) three-mirror optical design is the primary/tertiary (M1M3) mirror that combines these two large mirrors onto one monolithic substrate. The M1M3 mirror was spin cast and polished at the Steward Observatory Mirror Lab at The University of Arizona (formerly SOML, now the Richard F. Caris Mirror Lab at the University of Arizona (RFCML)). Final acceptance of the mirror occurred during the year 2015 and the mirror is now in storage while the mirror cell assembly is being fabricated. The M1M3 mirror will be tested at RFCML after integration with its mirror cell before being shipped to Chile.
The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in nearsimultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope’s active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small highresolution maps, and used these to show the impact of the small features on images is acceptably small.
The Giant Magellan Telescope (GMT) primary mirror consists of seven 8.4 m light-weight honeycomb mirrors that are being manufactured at the Richard F. Caris Mirror Lab (RFCML), University of Arizona. In order to manufacture the largest and most aspheric astronomical mirrors various high precision fabrication technologies have been developed, researched and implemented at the RFCML. The unique 8.4 m (in mirror diameter) capacity fabrication facilities are fully equipped with large optical generator (LOG), large polishing machine (LPM), stressed lap, rigid conformal lap (RC lap) and their process simulation/optimization intelligence called MATRIX. While the core capability and key manufacturing technologies have been well demonstrated by completing the first GMT off-axis segment, there have been significant hardware and software level improvements in order to improve and enhance the GMT primary mirror manufacturing efficiency. The new and improved manufacturing technology plays a key role to realize GMT, the next generation extremely large telescope enabling new science and discoveries, with high fabrication efficiency and confidence.
The Richard F. Caris Mirror Lab at the University of Arizona is responsible for production of the eight 8.4 m segments for the primary mirror of the Giant Magellan Telescope, including one spare off-axis segment. We report on the successful casting of Segment 4, the center segment. Prior to generating the optical surface of Segment 2, we carried out a major upgrade of our 8.4 m Large Optical Generator. The upgrade includes new hardware and software to improve accuracy, safety, reliability and ease of use. We are currently carrying out an upgrade of our 8.4 m polishing machine that includes improved orbital polishing capabilities. We added and modified several components of the optical tests during the manufacture of Segment 1, and we have continued to improve the systems in preparation for Segments 2-8. We completed two projects that were prior commitments before GMT Segment 2: casting and polishing the combined primary and tertiary mirrors for the LSST, and casting and generating a 6.5 m mirror for the Tokyo Atacama Observatory.
The Richard F. Caris Mirror Lab is in the process of fabricating 8.4 meter mirror segments for the Giant Magellan Telescope. Seven of the segments are off-axis with 14 mm of aspheric departure. In order to successfully fabricate these mirrors we are constantly taking steps towards faster, more deterministic methods, from diamond generating to stressed lap polishing. The Large Optical Generator (LOG) is celebrating its 30-year anniversary at the University of Arizona with a complement of technological updates and enhancements. This paper shows how some of these upgrades will aid in the manufacture of the GMT segments.
The Steward Observatory Mirror Lab is nearing completion of the combined primary and tertiary mirrors of the Large
Synoptic Survey Telescope. Fabrication of the combined mirror requires simulation of an active-optics correction that
affects both mirror surfaces in a coordinated way. As is common for large mirrors, the specification allows correction of
large-scale figure errors by a simulated bending of the substrate with the 156 mirror support actuators. Any bending
affects both mirrors, so this active-optics correction is constrained by the requirement of bending the substrate so both
mirrors meet their figure specifications simultaneously. The starting point of the simulated correction must be
measurements of both mirrors with the substrate in the same shape, i. e. the same state of mechanical and thermal stress.
Polishing was carried out using a 1.2 m stressed lap for smoothing and large-scale figuring, and a set of smaller passive
rigid-conformal laps on an orbital polisher for deterministic small-scale figuring. The primary mirror is accurate to about
25 nm rms surface after the active-optics correction, while work continues toward completion of the tertiary.
Segment production for the Giant Magellan Telescope is well underway, with the off-axis Segment 1 completed, off-axis
Segments 2 and 3 already cast, and mold construction in progress for the casting of Segment 4, the center segment. All
equipment and techniques required for segment fabrication and testing have been demonstrated in the manufacture of
Segment 1. The equipment includes a 28 m test tower that incorporates four independent measurements of the segment's
figure and geometry. The interferometric test uses a large asymmetric null corrector with three elements including a 3.75
m spherical mirror and a computer-generated hologram. For independent verification of the large-scale segment shape,
we use a scanning pentaprism test that exploits the natural geometry of the telescope to focus collimated light to a point.
The Software Configurable Optical Test System, loosely based on the Hartmann test, measures slope errors to submicroradian
accuracy at high resolution over the full aperture. An enhanced laser tracker system guides the figuring
through grinding and initial polishing. All measurements agree within the expected uncertainties, including three
independent measurements of radius of curvature that agree within 0.3 mm. Segment 1 was polished using a 1.2 m
stressed lap for smoothing and large-scale figuring, and a set of smaller passive rigid-conformal laps on an orbital
polisher for deterministic small-scale figuring. For the remaining segments, the Mirror Lab is building a smaller, orbital
stressed lap to combine the smoothing capability with deterministic figuring.
The Giant Magellan Telescope (GMT) primary mirror is a 25 meter f/0.7 surface composed of seven 8.4 meter circular
segments, six of which are identical off-axis segments. The fabrication and testing challenges with these severely
aspheric segments (about 14 mm of aspheric departure, mostly astigmatism) are well documented. Converting the raw
phase data to useful surface maps involves many steps and compensations. They include large corrections for: image
distortion from the off-axis null test; misalignment of the null test; departure from the ideal support forces; and
temperature gradients in the mirror. The final correction simulates the active-optics correction that will be made at the
telescope. Data are collected and phase maps are computed in 4D Technology's 4SightTM software. The data are saved
to a .h5 (HDF5) file and imported into MATLAB® for further analysis. A semi-automated data pipeline has been
developed to reduce the analysis time as well as reducing the potential for error. As each operation is performed, results
and analysis parameters are appended to a data file, so in the end, the history of data processing is embedded in the file.
A report and a spreadsheet are automatically generated to display the final statistics as well as how each compensation
term varied during the data acquisition. This gives us valuable statistics and provides a quick starting point for
investigating atypical results.
The Large Synoptic Survey Telescope (LSST) is a three-mirror wide-field survey telescope with the primary and tertiary
mirrors on one monolithic substrate1. This substrate is made of Ohara E6 borosilicate glass in a honeycomb sandwich,
spin cast at the Steward Observatory Mirror Lab at The University of Arizona2. Each surface is aspheric, with the
specification in terms of conic constant error, maximum active bending forces and finally a structure function
specification on the residual errors3. There are high-order deformation terms, but with no tolerance, any error is
considered as a surface error and is included in the structure function. The radii of curvature are very different, requiring
two independent test stations, each with instantaneous phase-shifting interferometers with null correctors. The primary
null corrector is a standard two-element Offner null lens. The tertiary null corrector is a phase-etched computer-generated
hologram (CGH). This paper details the two optical systems and their tolerances, showing that the uncertainty
in measuring the figure is a small fraction of the structure function specification. Additional metrology includes the radii
of curvature, optical axis locations, and relative surface tilts. The methods for measuring these will also be described
along with their tolerances.
Large telescope mirrors have stringent requirements for surface irregularity on all spatial scales. Large scale errors, typically represented with Zernike polynomials, are relatively easy to control. Errors with smaller spatial scale can be more difficult because the specifications are tighter. Small scale errors are controlled with a combination of natural smoothing from large tools and directed figuring with precisely controlled small tools. The optimization of the complete process builds on the quantitative understanding of natural smoothing, convergence of small tool polishing, and confidence in the surface measurements. This paper provides parametric models for smoothing and directed figuring that can be used to optimize the manufacturing process.
High performance optical systems aiming for very low background noise from scattering or a sharp point spread function with high encircled energy often specify their beam wavefront quality in terms of a structure function or power spectral density function, which requires a control of mid-to-high spatial frequency surface errors during the optics manufacturing process. Especially for fabrication of large aspheric optics, achieving the required surface figure irregularities over the mid-to-high spatial frequency range becomes a challenging task as the polishing lap needs to be compliant enough to conform to the varying local surface shapes under the lap. This compliance degrades the lap’s smoothing capability, which relies on its rigidity. The smoothing effect corrects the mid-to-high spatial frequency errors as a polishing lap removes low spatial frequency (i.e. larger than the lap size) errors on the optical surface. Using a parametric smoothing model developed to quantitatively describe the smoothing effects during Computer Controlled Optical Surfacing (CCOS) processes, actual CCOS data from large aspheric optics fabrication projects have been analyzed and studied. The measured surface error maps were processed with the model to compare different polishing runs using various polishing parameters. The results showing the smoothing effects of mid-to-high spatial frequency surface irregularity will be presented to provide some insights for a CCOS process optimization in terms of smoothing efficiency.
The Computer-controlled Large-tool such as the stressed-lap which firstly developed in the Steward Observatory Mirror
Lab (SOML) [1]and the Computer controlled active lap which developed in the IOE (Institute of Optics and Electronics,
Chinese Academy of Science), those large tools are controlled by computer to manufacturing large optics, especially
for grinding with loose abrasive and polishing with slurry. Comparing the fixed orbital lap, computer-controlled largetool
can bend its lap surface timely to match the local sub-aperture, so it always strike the high area preferentially, due to
its large diameter , computer-controlled large-tool possess highly remove efficiency and generate less middle-frequency
and high-frequency errors comparing some small tools such as computer controlled optical surface (CCOS), but on the
other hand how to calculate the dwell time for those computer-controlled large-tool becomes a challenge comparing
those small tools. Based on the mathematical removal equation for computer controlled active lap we have none negative
least square algorithm to calculate the dwell time, after the simulation, a optimized algorithm based on none negative
least square is provided, the dwell time calculated by this optimized algorithm meet the wanted removal volume with
little residual errors.
As previously reported (at the SPIE Astronomical Instrumentation conference of 2010 in San Diego1), the Large
Synoptic Survey Telescope (LSST) utilizes a three-mirror design in which the primary (M1) and tertiary (M3) mirrors
are two concentric aspheric surfaces on one monolithic substrate. The substrate material is Ohara E6 borosilicate glass,
in a honeycomb sandwich configuration, currently in production at The University of Arizona’s Steward Observatory
Mirror Lab. We will provide an update to the status of the mirrors and metrology systems, which have advanced from
concepts to hardware in the past two years. In addition to the normal requirements for smooth surfaces of the appropriate
prescriptions, the alignment of the two surfaces must be accurately measured and controlled in the production lab,
reducing the degrees of freedom needed to be controlled in the telescope. The surface specification is described as a
structure function, related to seeing in excellent conditions. Both the pointing and centration of the two optical axes are
important parameters, in addition to the axial spacing of the two vertices. This paper details the manufacturing process
and metrology systems for each surface, including the alignment of the two surfaces. M1 is a hyperboloid and can utilize
a standard Offner null corrector, whereas M3 is an oblate ellipsoid, so it has positive spherical aberration. The null
corrector is a phase-etched computer-generated hologram (CGH) between the mirror surface and the center-of-curvature.
Laser trackers are relied upon to measure the alignment and spacing as well as rough-surface metrology during looseabrasive
grinding.
Production of segments for the Giant Magellan Telescope is well underway at the Steward Observatory Mirror Lab. We
report on the completion of the first 8.4 m off-axis segment, the casting of the second segment, and preparations for
manufacture of the remaining segments. The complete set of infrastructure for serial production is in place, including the
casting furnace, two 8.4 m capacity grinding and polishing machines, and a 28 m test tower that incorporates four
independent measurement systems. The first segment, with 14 mm p-v aspheric departure, is by some measures the most
challenging astronomical mirror ever made. Its manufacture took longer than expected, but the result is an excellent
figure and demonstration of valuable new systems that will support both fabrication and measurement of the remaining
segments. Polishing was done with a 1.2 m stressed lap for smoothing and large-scale figuring, and a series of smaller
passive rigid-conformal laps for deterministic figuring on smaller scales. The interferometric measurement produces a
null wavefront with a 3-element asymmetric null corrector including a 3.8 m spherical mirror and a computer-generated
hologram. In addition to this test, we relied heavily on the new SCOTS slope test with its high accuracy and dynamic
range. Evaluation of the measured figure includes simulated active correction using both the 160-actuator mirror support
and the alignment degrees of freedom for the off-axis segment.
A software configurable optical test system (SCOTS) based on fringe reflection was implemented for measuring the
primary mirror segments of the Giant Magellan Telescope (GMT). The system uses modulated fringe patterns on an
LCD monitor as the source, and captures data with a CCD camera and calibrated imaging optics. The large dynamic
range of SCOTS provides good measurement of regions with large slopes that cannot be captured reliably with
interferometry. So the principal value of the SCOTS test for GMT is to provide accurate measurements that extend
clear to the edge of the glass, even while the figure is in a rough state of figure, where the slopes are still high.
Accurate calibration of the geometry and the mapping also enable the SCOTS test to achieve accuracy that is
comparable measurement accuracy to the interferometric null test for the small- and middle- spatial scale errors in
the GMT mirror.
Precision optical surfaces can be efficiently manufactured using a computer-controlled optical surfacing (CCOS)
process. Most CCOS processes are based on control of the dwell time of a tool on the workpiece, according to the
desired removal and the tool influence function (TIF), which is the material wear function of the tool. Several major
topics were investigated to improve current CCOS processes and provide new solutions for the next generation of CCOS
processes. A rigid conformal (RC) lap using a visco-elastic non-Newtonian medium was invented. It conforms to the
aspheric surface shape, yet maintains stiffness on short time scales to provide natural smoothing. The smoothing
removes mid- to high-frequency errors while controlled dwell time removes low-frequency errors. A parametric
smoothing model was also introduced to predict the smoothing effects. A parametric edge TIF model to represent
measured edge TIFs was developed and demonstrated. This model covers the removal behavior as the tool overhangs the
edge of the workpiece. These new tools and models were applied in a new process optimization technique called nonsequential
optimization. The non-sequential approach performs a comprehensive optimization of dwell time using
multiple TIFs (multiple tools) simultaneously. An overview of these newly implemented CCOS features** is presented
along with some actual CCOS results.
The Large Synoptic Survey Telescope (LSST) utilizes a three-mirror design in which the primary (M1) and tertiary (M3)
mirrors are two concentric aspheric surfaces on one monolithic substrate. The substrate material is Ohara E6 borosilicate
glass, in a honeycomb sandwich configuration, currently in production at The University of Arizona's Steward
Observatory Mirror Lab. In addition to the normal requirements for smooth surfaces of the appropriate prescriptions, the
alignment of the two surfaces must be accurately measured and controlled in the production lab. Both the pointing and
centration of the two optical axes are important parameters, in addition to the axial spacing of the two vertices. This
paper describes the basic metrology systems for each surface, with particular attention to the alignment of the two
surfaces. These surfaces are aspheric enough to require null correctors for each wavefront. Both M1 and M3 are concave
surfaces with both non-zero conic constants and higher-order terms (6th order for M1 and both 6th and 8th orders for M3).
M1 is hyperboloidal and can utilize a standard Offner null corrector. M3 is an oblate ellipsoid, so has positive spherical
aberration. We have chosen to place a phase-etched computer-generated hologram (CGH) between the mirror surface
and the center-of-curvature (CoC), whereas the M1 null lens is beyond the CoC. One relatively new metrology tool is the
laser tracker, which is relied upon to measure the alignment and spacings. A separate laser tracker system will be used to
measure both surfaces during loose abrasive grinding and initial polishing.
The scanning pentaprism system for testing the 8.4 m off-axis segments for the Giant Magellan Telescope has recently
been completed. The system uses a fiber source and a carriage mounted pentaprism to scan a 40 mm collimated beam
across the surface of the segment under test. Since the scanning beam is parallel to the optical axis of the parent mirror, it
comes to focus on a detector at the telescope's prime focus, where displacement of the spot is proportional to the slope
error. A second collimated beam from a stationary reference pentaprism is used to compensate for any changes in the
relative positions of the optical components during testing. The optical components are suspended over the mirror on a
rail system that can be rotated so that scans can be made across any diameter of the segment. The test is capable of
measuring wavefront slope errors to 1 μrad rms, adequate to verify that power, astigmatism, coma, and other low-order
aberrations are small enough to be corrected easily at the telescope with the segment's active support system.
The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter
diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis
segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated
spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger
mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out
a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at
an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement
hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the
alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror
and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100
microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a
separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT
measurement, allowing active correction and compensation for residual errors. We describe the details of the design,
alignment, and use of this unique off-axis optical test.
We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, has
limited sensitivity to the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system
couples a commercial laser tracker with an advanced calibration technique and a system of stability references to
mitigate numerous error sources. This system was built to guide loose abrasive grinding and initial polishing of the off-axis
primary mirror segments for the Giant Magellan Telescope (GMT), and is also being used to guide the fabrication of
the Large Synoptic Survey Telescope primary and tertiary mirrors. In addition to guiding fabrication, the system also
works as a verification test for the GMT principal optical interferometric test of the polished mirror segment to
corroborate the measurement in several low-order aberrations. A quantitative assessment of the system accuracy is
presented, along with measurement results for GMT, including a comparison to the optical interferometric test of the
polished surface.8
The primary mirror of the Giant Magellan Telescope consists of seven 8.4 m segments which are borosilicate
honeycomb sandwich mirrors. Fabrication and testing of the off-axis segments is challenging and has led to a number of
innovations in manufacturing technology. The polishing system includes an actively stressed lap that follows the shape
of the aspheric surface, used for large-scale figuring and smoothing, and a passive "rigid conformal lap" for small-scale
figuring and smoothing. Four independent measurement systems support all stages of fabrication and provide redundant
measurements of all critical parameters including mirror figure, radius of curvature, off-axis distance and clocking. The
first measurement uses a laser tracker to scan the surface, with external references to compensate for rigid body
displacements and refractive index variations. The main optical test is a full-aperture interferometric measurement, but it
requires an asymmetric null corrector with three elements, including a 3.75 m mirror and a computer-generated
hologram, to compensate for the surface's 14 mm departure from the best-fit sphere. Two additional optical tests
measure large-scale and small-scale structure, with some overlap. Together these measurements provide high confidence
that the segments meet all requirements.
The scanning pentaprism test has provided an important absolute test method for flat mirrors, parabolic mirrors and also
collimation systems. We have developed a scanning pentaprism system to measure off-axis paraboloidal mirrors such as
those for the Giant Magellan Telescope (GMT) primary mirror. Special characteristics of the pentaprism testing of an
off-axis mirror are discussed in the paper. We provide performance results for the final measurement of a 1.7 m off-axis
parabolic mirror and present a technique used to determine the radius of the parent, off-axis distance and the clocking of
the mirror from the data from the scanning pentaprism system.
We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, is nearly
independent of the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system couples a
commercial laser tracker with an advanced calibration technique and a system of external references. This system was
built to guide loose abrasive grinding and initial polishing of the off-axis primary mirror segments for the Giant
Magellan Telescope, and will be used to guide the fabrication of the Large Synoptic Survey Telescope primary and
tertiary mirrors as well. The results obtained using this system during the fabrication of the first segment of the Giant
Magellan Telescope are presented along with an assessment of the expected system accuracy.
The pentaprism test is based on the property of a paraboloidal surface where all rays parallel to the optical axis will go
through its focal point. We have developed a scanning pentaprism system that exploits this geometry to measure off-axis
paraboloidal mirrors such as those for the Giant Magellan Telescope primary mirror. Extension of the pentaprism test to
off-axis mirrors requires special attention to field effects that can be ignored in the measurement of an axisymmetric
mirror. The test was demonstrated on a 1. -m diameter off-axis mirror and proved to have about 50nm rms surface
accuracy. This paper gives detailed performance results for the measurement of the 1.7 m mirror, and designs and
analysis for the test of the GMT segments.
The Giant Magellan Telescope achieves 25 meter aperture and modest length using an f/0.7 primary mirror made from
8.4 meter diameter segments. The systems that will be used for measuring the aspheric optical surfaces of these mirrors
are in the final phase of development. This paper discusses the overall metrology plan and shows details for the
development of the principal test system - a system that uses mirrors and holograms to provide a null interferometric test
of the surface. This system provides a full aperture interferometric measurement of the off-axis segments by
compensating the 14.5 mm aspheric departure with a tilted 3.8-m diameter powered mirror, a 77 cm tilted mirror, and a
computer generated hologram. The interferometric measurements are corroborated with a scanning slope measurement
from a scanning pentaprism system and a direct measurement system based on a laser tracker.
The Large Synoptic Survey Telescope uses a unique optomechanical design that places the primary and tertiary mirrors
on a single glass substrate. The honeycomb sandwich mirror blank was formed in March 2008 by spin-casting. The
surface is currently a paraboloid with a 9.9 m focal length matching the primary. The deeper curve of the tertiary mirror
will be produced when the surfaces are generated. Both mirrors will be lapped and polished using stressed laps and other
tools on an 8.4 m polishing machine. The highly aspheric primary mirror will be measured through a refractive null lens,
and a computer-generated hologram will be used to validate the null lens. The tertiary mirror will be measured through a
diffractive null corrector, also validated with a separate hologram. The holograms for the two tests provide alignment
references that will be used to make the axes of the two surfaces coincide.
Surface measurements represent a significant part of the cost for manufacturing large aspheric optics. Both polished and
rough ground surfaces must be measured with high precision and spatial resolution. We have developed a system that
couples a commercial laser tracker with an advanced calibration technique and a system of external references. This
system was built to measure the off-axis primary mirror segments for the Giant Magellan Telescope where it will guide
loose abrasive grinding and initial polishing. The system is further expected to corroborate the optical interferometric
tests of the completed mirrors, in several low-order aberrations. The design, analysis, calibration, and measured
performance of this system will be presented.
The first of the 8.4 m off-axis segments for the primary mirror of the Giant Magellan Telescope is being manufactured at
the Steward Observatory Mirror Lab. In addition to the manufacture of the segment, this project includes the
development of a complete facility to make and measure all seven segments. We have installed a new 28 m test tower
and designed a set of measurements to guide the fabrication and qualify the finished segments. The first test, a laser-tracker
measurement of the ground surface, is operational. The principal optical test is a full-aperture interferometric test
with a null corrector that includes a 3.75 m spherical mirror, a smaller sphere, and a computer-generated hologram. We
have also designed a scanning pentaprism test to validate the measurement of low-order aberrations. The first segment
has been cast and generated, and is in the process of loose-abrasive grinding.
The Large Optical Test and Integration Site (LOTIS) at the Lockheed Martin Space Systems Company in Sunnyvale,
CA is designed for the verification and testing of optical systems. The facility consists of a large, temperature
stabilized vacuum chamber that also functions as a class 10k cleanroom. Within this chamber and atop an advanced
vibration-isolation bench are the 6.5 meter diameter LOTIS Collimator and Scene Generator, LOTIS alignment and
support equipment. The optical payloads are also placed on the vibration bench in the chamber for testing. The Scene
Generator is attached to the Collimator forming the Scene Projection System (SPS) and this system is designed to
operate in both air and vacuum, providing test imagery in an adaptable suite of visible/near infrared (VNIR) and
midwave infrared (MWIR) point sources, and combined bandwidth visible-through-MWIR point sources, for testing
of large aperture optical payloads. The heart of the SPS is the LOTIS Collimator, a 6.5m f/15 telescope, which projects
scenes with wavefront errors <85 nm rms out to a ±0.75 mrad field of view (FOV). Using field lenses, performance
can be extended to a maximum field of view of ±3.2 mrad. The LOTIS Collimator incorporates an extensive integrated
wavefront sensing and control system to verify the performance of the system, and to optimize its actively controlled
primary mirror surface and overall alignment. Using these optical test assets allows both integrated component and
system level optical testing of electro-optical (EO) devices by providing realistic scene content. LOTIS is scheduled to
achieve initial operational capability in 2008.
The first of the two Gregorian Adaptive Secondary Mirror (ASM) units for the Large Binocular Telescope (LBT) has
been fully integrated and tested for laboratory acceptance. The LBT unit represents the most advanced ASM device
existing in hardware. The unit has 672 electro-magnetic force actuators to change the shape of the 1.6mm-thick and
911mm-diameter Zerodur shell. The actuators control the mirror figure using the position feedback from the internal
metrology provided by co-located capacitive sensors. The on-board real-time control electronics has a parallel
computational power of 163Gflop/s providing not only the internal control of the unit with a 72kHz loop but also the
wavefront reconstruction for the 1kHz Adaptive Optics loop. The paper describes the final configuration of the system
and reports the results of the characterization and optimization process together with the results of the laboratory
acceptance tests.
We report on the current status of production of the thin shells for the LBT adaptive secondary mirrors. These shells are
made of Zerodur and have a front (optical) surface highly aspherical whereas the other (rear) surface is spherical. They
have a 910mm diameter and an average thickness of 1.6mm. The manufacturing of these shells starts with a thick blank
of Zerodur and follows the steps of: 1) optical surface figuring, 2) blank thinning, 3) rear surface grinding and polishing,
4) edges machining and 5) rear surface aluminizing. Of the three (two plus a spare) shells planned for LBT the first shell
was completed and shipped to Italy for integration with magnets and the second is in advanced state of production. The
third shell (spare) is planned to start production soon. In the paper we report details of production of these shells as well
as the 'as built' characteristics. Details concerning the operations that follow the production, i.e. surface aluminum
coating as well as handling and shipping fixtures are also reported.
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image planetary systems of
nearby stars simultaneously in a few wide spectral bands covering the visible light (0.4-0.9 μm). It achieves its
power by combining a high accuracy wavefront control system with a highly efficient Phase-Induced Amplitude
Apodization (PIAA) coronagraph which provides strong suppression very close to the star (within 2 λ/D). The
PIAA coronagraphic technique opens the possibility of imaging Earthlike planets in visible light with a smaller
telescope than previously supposed. If sized at 1.2-m, TOPS would image and characterize many Jupiter-sized
planets, and discover 2 RE rocky planets within habitable zones of the ≈10 most favorable stars. With a larger
2-m aperture, TOPS would have the sensitivity to reveal Earth-like planets in the habitable zone around ≈20
stars, and to characterize any found with low resolution spectroscopy. Unless the occurrence of Earth-like planets
is very low (η⊕ <~ 0.2), a useful fraction of the TPF-C scientific program would be possible with aperture much
smaller than the baselined 8 by 3.5m for TPF, with its more conventional coronagraph. An ongoing laboratory
experiment has successfully demonstrated high contrast coronagraphic imaging within 2 λ/d with the PIAA
coronagraph / focal plane wavefront sensing scheme envisioned for TOPS.
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image in the visible (0.4-
0.9 μm) planetary systems of nearby stars simultaneously in 16 spectral bands (resolution R≈20). For the
≈10 most favorable stars, it will have the sensitivity to discover 2RΕ rocky planets within habitable zones and
characterize their surfaces or atmospheres through spectrophotometry. Many more massive planets and debris
discs will be imaged and characterized for the first time. With a 1.2m visible telescope, the proposed mission
achieves its power by exploiting the most efficient and robust coronagraphic and wavefront control techniques.
The Phase-Induced Amplitude Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2λ/d
with nearly 100% throughput and preserves the telescope angular resolution. An efficient focal plane wavefront
sensing scheme accurately measures wavefront aberrations which are fed back to the telescope active primary
mirror. Fine wavefront control is also performed independently in each of 4 spectral channels, resulting in a
system that is robust to wavefront chromaticity.
We have nearly completed the manufacture of a 1.7 m off-axis mirror as part of the technology development for the Giant Magellan Telescope. The mirror is an off-axis section of a 5.3 m f/0.73 parent paraboloid, making it roughly a 1:5 model of the outer 8.4 m GMT segment. The 1.7 m mirror will be the primary mirror of the New Solar Telescope at Big Bear Solar Observatory. It has a 2.7 mm peak-to-valley departure from the best-fit sphere, presenting a serious challenge in terms of both polishing and measurement. The mirror was polished with a stressed lap, which bends actively to match the local curvature at each point on the mirror surface, and works for asymmetric mirrors as well as symmetric aspheres. It was measured using a hybrid reflective-diffractive null corrector to compensate for the mirror's asphericity. Both techniques will be applied in scaled-up versions to the GMT segments.
The Giant Magellan Telescope (GMT) uses seven 8.4-m diameter segments to create a giant primary mirror,
25 meters across with focal ratio f /0.7. The off-axis segments will be difficult to measure accurately, as they
have 14.5 mm departure from the nearest fitting sphere! The test configuration adopted uses a large 3.75-m
powered mirror to fold the light path and provide most of the aspheric correction, with a smaller mirror and
computer generated hologram (CGH) providing the additional correction. These optics will be aligned to a
vibration-insensitive interferometer using a combination of optical references created by the CGH and
metrology with a laser tracker. Some key challenges for this system are presented here including, the system
alignment, the large fold mirror, and the mechanical structure. Analysis of the optical test shows that it will
meet GMT specifications, including the difficult requirement that the separate segments have matching radius
of curvature. Additional corroborative testing will be performed to assure that the mirror segments are correctly
figured.
The second 8.4 m primary mirror and its active support system were delivered to the Large Binocular Telescope in September 2005. The mirror was figured to an accuracy of 15 nm rms surface after subtraction of low-order aberrations that will be controlled by the active support. The mirror was installed into its operational support cell in the lab, and support forces were optimized to produce a figure accurate to 20 nm rms surface with no synthetic correction. The mirror was polished on a new 8.4 m polishing machine that gives the Mirror Lab the capacity to process up to four 8.4 m mirrors simultaneously, with each mirror going through a sequence of stations: casting furnace, generating machine, polishing machine, and integration with its support cell. The new polishing machine has two carriages for polishing tools, allowing use of two 1.2 m stressed laps during loose-abrasive grinding and early polishing, followed by final figuring with a stressed lap and a small tool for local figuring.
The primary mirror for the 25-m Giant Magellan Telescope is made of seven circular segments, each of 8.4-m
diameter. The lack of axisymmetry and the steep aspheric departure present significant technical challenges for
the metrology. These segments will be measured interferometrically using a complex system of mirrors and
holograms to give a null test with high spatial resolution. While analysis predicts this system will meet
requirements, an additional set of measurements will be used to corroborate the principal interferometric
measurement. The set of tests, including these alternate surface measurements, assures that all aspects of the
mirror surface are measured completely and redundantly. The corroboration tests discussed in this paper are:
Direct surface profile using metrology system based on a laser tracker, measuring low order shape errors
Shear testing with full aperture interferometer, separating small scale errors in the null test from those in
the mirror
Slope testing with scanning pentaprism, measuring low order shape errors and sampling small scale errors
The design, manufacture and support of the primary mirror segments for the GMT build on the successful primary mirror systems of the MMT, Magellan and Large Binocular telescopes. The mirror segment and its support system are based on a proven design, and the experience gained in the existing telescopes has led to significant refinements that will provide even better performance in the GMT. The first 8.4 m segment has been cast at the Steward Observatory Mirror Lab, and optical processing is underway. Measurement of the off-axis surface is the greatest challenge in the manufacture of the segments. A set of tests that meets the requirements has been defined and the concepts have been developed in some detail. The most critical parts of the tests have been demonstrated in the measurement of a 1.7 m off-axis prototype. The principal optical test is a full-aperture, high-resolution null test in which a hybrid reflective-diffractive null corrector compensates for the 14 mm aspheric departure of the off-axis segment. The mirror support uses the same synthetic floatation principle as the MMT, Magellan, and LBT mirrors. Refinements for GMT include 3-axis actuators to accommodate the varying orientations of segments in the telescope.
We describe the manufacture of thin shells for the deformable secondary mirrors of the LBT adaptive optics system. The secondary mirrors are thin shells, 910 mm in diameter and 1.6 mm thick. Each mirror will have its shape controlled by 672 voice-coil actuators. The main requirement for manufacture of the shell is smoothness on scales too small to be adjusted by the actuators. An additional requirement is that the rear surface match the reference body within 30 μm peak-to-valley. A technique was developed for producing smooth surfaces on the very aspheric surfaces of the shells. We figure the optical surfaces on a thick disk of Zerodur, then turn the disk over and thin it to 1.6 mm from the rear surface. Figuring is done primarily with a 30 cm diameter stressed lap, which bends actively to match the local curvature of the aspheric surface. For the thinning operation, the mirror is blocked with pitch, optical surface down, onto a granite disk with a matching convex surface. Because the shell may bend during the blocking operation and as its thickness is reduced to 1.6 mm, figuring of the rear surface is guided by precise thickness measurements over the surface of the shell. This method guarantees that both surfaces of the finished shell will satisfy their requirements when corrected with small actuator forces. Following the thinning operation, we edge the shell to its final dimensions, remove it from the blocking body, and coat the rear surface with aluminum to provide a set of conductive plates for capacitive sensors.
Steward Observatory Mirror Lab is currently polishing an off-axis parabola which will be the primary mirror of the New Solar Telescope. To test this mirror, we built a test equipment to combine a spherical mirror and a Computer Generated Hologram (CGH) as null lens. The spherical mirror is tilted to compensate much of the astigmatism and some coma. And the CGH compensates rest of aberrations. The combination of a spherical mirror and a CGH makes the test system compact. The technology developed here will be used to test the Giant Magellan Telescope's primary mirror segment--a five times larger off-axis parabola.
We describe the active support system for the 8.4 m LBT primary mirrors and the use of this system to optimize the mirror figure in the lab before installation in the telescope. We figured the mirror, mounted on passive supports, to an accuracy of 18 nm rms surface after subtraction of spherical aberration and several flexible bending modes that would be corrected with the active supports. After installing the mirror on its active supports, we optimized the 160 support forces based on interferometric wavefront measurements and a finite-element model of mirror bending. We verified the accuracy of the model and determined a scale factor-the model is stiffer than the real mirror-by measuring a number of bending modes using forces calculated from the model. We then optimized the forces to obtain an accuracy of 28 nm rms surface. The optimization included correcting 200 nm of spherical aberration (Zernike coefficient of surface error) with a maximum correction force of 26 N.
We describe the requirements for manufacturing and maintaining alignment of the 8.4 m off-axis segments of the Giant Magellan Telescope’s primary mirror, and a demonstration of the manufacturing techniques on the 1.7 m off-axis primary mirror of the New Solar Telescope. This mirror is approximately a 1/5 scale model of a GMT segment. We show that the stressed lap polishing system developed for highly aspheric primary and secondary mirrors is capable of figuring the GMT segments and the NST mirror. We describe an optical test with a null corrector consisting of a tilted spherical mirror and a computer-generated hologram, and derive accuracy requirements for the test. The criterion for accuracy of low-order aberrations is that the active support system can correct any figure errors due to the laboratory measurement, with acceptably small forces and residual errors.
The adaptive secondary for the MMT is the first mirror of its kind. It was designed to allow the application of wavefront corrections (including tip-tilt) directly at the secondary mirror location. Among the advantages of such a choice for adaptive optics operation are higher throughput, lower emissivity, and simpler optical setup. Furthermore, this specific implementation provides capabilities that are not found in most correctors including internal position
feedback, large stroke (to allow chopping) and provision for absolute position calibration. The mirror has now been used at the MMT during several runs where it has performed reliably. In this paper we discuss the mirror operation and AO performance achieved during these runs in which the adaptive secondary has been operating in conjunction with a Shack-Hartmann wavefront sensor as part
of the MMT adaptive optics system. In particular we mention a residual mirror position error due to wind buffeting and other errors of ≈ 15 nm rms surface and a stable closed loop operation with a 0dB point of the error transfer function in the range 20-30 Hz limited mainly by the wavefront sensor maximum frame rate. Because of the location of the adaptive secondary with respect to the wavefront sensor camera, reimaging optics are required in order to perform the optical interaction matrix measurements needed to run the AO loop. This optical setup has been used in the lab but not replicated at the telescope so far. We will discuss the effects of the lack of such an internal calibration on the AO loop performances and a possible alternative to the lab calibration technique that uses directly light from sky objects.
In this communication, we present the progress of the 6.5m MMT adaptive optics system. During the last part of 2001 and the 1st part of 2002, the system has been validated in the laboratory statically and dynamically with sample frequencies of up to 550 Hz. In June 2002, an attempt has been made to make this system work on the telescope but has been hampered by mechanical failures. However, ease of installation of the system and open-loop operation of the mirror was demonstrated at this occasion and offers reasons to be optimistic on the future of the system.
The MMT-AO system is the first AO system to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. Its realization presents many technical challenges, which have now been overcome. Today, the deformable mirror is characterized and accepted. It features a 1.8 mm thick 640mm diameter convex aspheric mirror (manufactured at the Steward Observatory Mirror Lab), mounted on a 50 mm thick ULE reference body with 336 actuators, as well as a cluster of 168 DSP’s and associated analog circuitry for position sensing and actuator driving. The system has been characterized in the laboratory at sampling speeds up to 550 Hz and had been integrated on the telescope.
The adaptive secondary for the MMT (called MMT336) is the first mirror of its kind. It was designed to allow the application of wavefront corrections (including tip-tilt) directly at the secondary mirror location. Among the advantages of such a choice for adaptive optics operation are higher throughput, lower emissivity, and simpler optical setup. The mirror also has capabilities that are not found in most correctors including internal position feedback, large stroke (to allow chopping) and provision for absolute position calibration. The 336 actuator adaptive secondary for MMT has been used daily for over one year in our adaptive optics testing facility which has built confidence in the mirror operation and allowed us to interface it to the MMT adaptive optics system. Here we present the most recent data acquired in the lab on the mirror performance. By using interferometer measurements we were able to achieve a residual surface error of approximately 40nm rms. Coupling the mirror with a Shack-Hartmann wavefront sensor we obtained a stable closed loop operation with a -3dB closed loop bandwidth of approximately 30Hz limited by the wavefront sensor frame rate. We also present some preliminary results that show a 5Hz, 90% duty cycle, ±5 arcsec chopping of the mirror. Finally the experience gained and the problems encountered during the first light adaptive optics run at the telescope will be briefly summarized. A more extensive report can be found in another paper also presented at this conference.
In an adaptive optical system, it is essential that the wavefront sensor be accurately calibrated and aligned to the wavefront corrector. For the case of the Shack Hartmann sensor, there are at least three quantities which must be measured: a. the response of the subapertures to local wavefront tilt, b. the location of the zero point for each subaperture from a plane wave input, and c. the spatial relationship between the wavefront sensor subapertures and the wavefront corrector actuators. This paper present a method which verifies the calibration of a Shack Hartmann wavefront sensor by simultaneously measuring static wavefront with a phase shifting interferometer. These measurements were made using an apparatus we constructed in the laboratory to build and test the adaptive optical system for the 6.5 m MMT.
KEYWORDS: Actuators, Digital signal processing, Electronics, Mirrors, Sensors, Adaptive optics, Telescopes, Prototyping, Optical testing, Control systems
The two adaptive secondary (AS) mirrors for LBT (LBT672) represent the new generation of the AS technology. Their design is based on the experience earned during the extensive tests of the previous generation unit (the MMT AS mirror). Both the mechanics and the electronics have been revised, improving the stability, reliability, maintenance and computational power of the system. The deformable mirror of each unit consists of a 1.6mm-thick Zerodur shell having a diameter of 911mm. The front surface is concave to match the Gregorian design of the telescope. Its figure is controlled by 672 electro-magnetic force actuators that are supported and cooled by an aluminum plate. The actuator forces are controlled using a combination of feed-forward and de-centralized closed loop compensation, thanks to the feedback signals from the 672 co-located capacitive position sensors. The surface reference for the capacitive sensors is a 50mm-thick Zerodur shell faced to the back surface of the thin mirror and rigidly connected to the support plate of the actuators. Digital real-time control and unit monitoring is obtained using new custom-made on-board electronics based on new generation 32bit floating-point DSPs. The total computational power (121 Gflop/s) of the LBT672 units allows using the control electronics as wave-front computer without any reduction of the actuator control capability. We report the details of the new features introduced in the LBT672 design and the preliminary laboratory results obtained on a prototype used to test them. Finally the facility in Arcetri to test the final LBT672 units is presented.
We describe the fabrication and testing of the 6.5 m f/1.25 primary mirrors for the Magellan telescopes and the 8.4 m f/1.14 primary mirrors for the Large Binocular Telescope (LBT). These mirrors, along with the 6.5 m MMT primary, are the fastest and most aspheric large mirrors made. Steward Observatory developed special methods to polish and measure these and other fast mirrors. We use a stressed-lap polishing tool to fit the aspheric surface while providing strong passive smoothing, and computer-generated holograms to verify the measurement of up to 1.4 mm peak-to-valley asphericity to an accuracy of 0.01%. The Magellan mirrors are diffraction-limited at visible wavelengths, with surface accuracies of about 20 nm rms on active supports. We are currently polishing the first LBT primary mirror and preparing to make the thin shells for the LBT adaptive secondary mirrors.
The Magellan active optics system has been operating continuously on the Baade 6.5-m since the start of science operations in February 2001. The active optical elements include the primary mirror, with 104 actuators, and the secondary mirror, with 5 positional degrees of freedom. Shack-Hartmann (SH) wavefront sensors are an integral part of the dual probe guiders. The probes function interchangeably, with either probe capable of guiding or wavefront sensing. In the course of most routine observing stars brighter than 17th magnitude are used to apply corrections once or twice per minute. The rms radius determined from roughly 250 SH spots typically ranges between 0.05" and 0.10". The spot pattern is analyzed in terms of a mixture of 3 Zernike polynomials (used to correct the secondary focus and decollimation) and 12 bending modes of the primary mirror (used to compensate for residual thermal and gravitational distortions). Zernike focus and the lowest order circularly symmetric bending mode, known affectionately as the "conemode," are sufficiently non-degenerate that they can be solved for and corrected separately.
Existing design rules break down as we plan for a new generation of giant optical telescopes of 20, 30, 50, even 100 meters in diameter. One might expect these telescopes to converge on the design universally ac-cepted for similarly sized radio telescopes, with their highly aspheric, ~ f/0.4, primary dish. But most of the optical design concepts now under consideration have favored spherical or relatively slow paraboloidal surfaces, leading to a much larger telescope, more subject to wind buffeting, and requiring gargantuan en-closures for protection. This paper explores issues and limitations for building and operating telescopes as the primary focal ratio is reduced to a value as small as f/0.4. Such compactness will be particularly impor-tant for mechanical stability, cost control and for large telescopes that must move continuously on a track, as in the 20/20 concept. We find that fabrication and alignment methods for telescopes using numerous small (1-m class) segments are driven to long focal ratios, while those using few large, actively controlled segments can be made as fast as f/0.5.
Any future giant ground-based telescope must, at a minimum, provide foci for seeing-limited imaging over a wide field and for diffraction-limited imaging over ~1 arcminute fields corrected by adaptive optics (AO). While this is possible with a number of design concepts, our choices are constrained if we anticipate wanting to later add a second telescope for imaging with still higher resolution, and very high contrast imaging for exoplanet studies. This paper explores designs that allow for such future development. Higher resolution imaging by interferometric combination of the AO-corrected fields of two telescopes is possible without loss of point-source sensitivity or field of view, as long as the baseline can be held perpendicular to the source and can be varied in length. This requirement is made practical even for very large telescopes, provided both can move continuously on a circular track. The 20/20 telescope illustrates this concept. Telescopes so mounted can additionally be operated as Bracewell nulling interferometers with low thermal background, making possible the thermal detection of planets that would have been unresolvable by a single 20 m aperture. In practice, limits set by funding and engineering experience will likely require a single 20 or 30 m telescope be built first. This would be on a conventional alt-az mount, but it should be at a site with enough room for later addition of a companion and track. In anticipation of future motion it should be compact and stiff, with a fast primary focal ratio. We envisage the use of large, highly aspheric, off-axis segments, manufactured using the figuring methods for strong aspherics already proven for 8 m class primaries. A compact giant telescope built under these guidelines should be able to perform well on its own for a broad range of astronomical observations, with good resistance to wind buffeting and simple alignment and control of its few, large segments. We compare here configurations with adjacent hexagonal segments and close-packed circular segments. For given segment parent size and number, the largest effective aperture is achieved if the segments are left as circles, when also the sensitivity and resolution for diffraction-limited operation with AO is higher. Large round segments can also be individually apodized for high-contrast imaging of exoplanets with the entire telescope-for example 8.4 m segments will yield 10-6 suppression 0.05 arcsec from a star at 1 μm wavelength, and at 0.25 arcsec at 5 μm.
We present a plan for making the optics of a 21 m telescope that builds on advances in mirror design and fabrication developed for the Large Binocular Telescope and other large telescopes. The 21 m telescope, with a fast f/0.7 primary mirror made of only seven large honeycomb-sandwich segments and an adaptive secondary mirror with matching segments, is much stiffer than other designs and offers simpler and more accurate wavefront control. It can be a powerful stand-alone telescope, or one of a pair that move on a circular track to achieve coherent imaging with baselines up to 120 m (the 20/20 telescope). Each segment of the 21 m primary mirror is similar to an 8.4 m LBT primary, and each segment of the 2.1 m adaptive secondary mirror is similar to an LBT secondary. The off-axis segments of both mirrors can be made with the same methods and equipment currently used at the Steward Observatory Mirror Lab, and can be polished with the same stressed-lap polishing system used for the LBT mirrors. A change in algorithm to accommodate the asymmetric surface is required, but no new hardware development is needed because the lap bending is similar to that for the LBT mirrors. Each segment can be measured interferometrically, with a combination reflective and diffractive null corrector producing an accurate template wavefront and alignment references for the segments.
The adaptive optics (AO) system for the 6.5 m MMT conversion telescope is the first to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. Its realization presents many technical challenges, which have now been overcome. The deformable mirror is now characterized and accepted. It features a 1.9mm thick 640mm diameter convex aspheric mirror (manufactured at the Steward Observatory Mirror Lab), mounted on a 50 mm thick ULE reference body with 336 actuators, as well as a cluster of 168 DSP's and associated analog circuitry. A wavefront sensor with integrated CCD and lenslet array has also been completed. The complete system is now starting to produce laboratory results, which we present below. Closed loop operation is tested under an auto-collimation illumination system that reflects aberrated artificial starlight from the convex secondary.
We describe the optical fabrication and the active support system of the 6.5 m f/1.25 primary mirror for the first Magellan telescope. Figuring was performed with a 1.2 m stressed lap, which bends under active control to match the local curvature of the optical surface, and small passive tools. The figure was measured with IR and visible interferometers, using refractive null lenses to compensate 810 microns of aspheric departure. After subtraction of Seidel astigmatism and spherical aberration, the finished mirror is accurate to 14 nm rms surface and has an encircled energy of 80% in 0.06' diameter at 500 nm. The mirror was integrated with its active support system in the laboratory, and support forces were adjusted to optimize the figure. The optimization was performed by singular value decomposition of the influence functions into normal bending modes. Using the first 20 modes and a maximum correction force of 46 N, the surface accuracy is 24 nm rms with 80% of the light in 0.11' diameter.
The 336-actuator adaptive secondary unit (MMT336) for the new MMT is being assembled in Italy and will be delivered in June 2000 for the acceptance test at Steward Observatory (Tucson, AZ). The latest results obtained on a reduced-size (36 actuators) prototype called P36 are reported, confirming a settling time less than 1 ms measured in previous tests. The flattening procedure has been successfully tested on the P36 unit, reducing the initial surface error of 1.1 micrometer down to 43 nm rms. Moreover the dynamical tests on the P36 unit show that the system is able to attenuate the atmospheric-induced error from 466 nm to 31 nm rms. This in the case of median seeing condition at MMT (0.75 arcsec) a high wind speed (48 m/s) and a 1 kHz command rate per actuator. Finally, in the same conditions the atmospheric error is effectively attenuated up to a frequency of 100 Hz (OdB attenuation level).
We describe the optical fabrication of the adaptive secondary mirror for the MMT. The 640 mm f/15 secondary consists of a flexible glass shell, 1.8 mm thick, whose shape is controlled by 336 electromagnetic actuators. It is designed to give diffraction-limited images at a wavelength of 1 micron. For generating and polishing, the shell was supported by attaching it to a rigid glass blocking body with a thin layer of pitch. It could then be figured and measured using techniques developed for rigid secondaries. The highly aspheric surface was polished with a 30 cm stressed lap and small passive tools, and measured using a swing-arm profilometer and a holographic test plate. The goal for fabrication was to produce diffraction-limited images in the visible, after simulated adaptive correction using only a small fraction of the typical actuator forces. This translates into a surface accuracy of less than 19 nm rms with correction forces of less than 0.05 N rms. We achieved a surface accuracy of 8 nm rms after simulated correction with forces of 0.02 N rms.
The adaptive optics system for the 6.5 m MMT conversion telescope will be the first to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. We report here the present state of construction, and the results of static and dynamic performance tests of the Cassegrain optical package.
We present a design and new approach to certify a null corrector with tight manufacturing tolerances. This involves a diamond turned asphere and a hologram that provide certification redundancy.
We describe the optical fabrication and testing of the 6.5 m f/1.25 primary mirror for the first Magellan telescope. Figuring was performed with a 1.2m stressed lap, which bends under active control to match the local curvature of the optical surface, and a variety of small passive tools. The figure was measured with RI and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. After subtraction of Seidel astigmatism and spherical aberration, the finished mirror is accurate to 14 nm rms surface error, and has an encircled energy of 80 percent in 0.06 inch diameter at 500 nm.
The next generation of space telescopes will require primary mirrors that push beyond the current state of technology of mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/m2 and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes--diffraction limited optical performance of a 0.5-m diameter mirror and launch survival of a 1-m diameter mirror. We have also built and tested a prototype actuator that achieves 25 nm resolution at cryogenic temperatures. We are now building a 2-m mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators and control electronics. It will be actively controlled and interferometrically measured at 35 K.
Hubert Martin, R. Allen, James Roger Angel, James Burge, Warren Davison, Scott DeRigne, Lee Dettmann, Dean Ketelsen, W. Kittrell, Stephen Miller, Peter Strittmatter, Steve West
The primary mirror for the Multiple Mirror Telescope Conversion is the first 6.5 m honeycomb sandwich mirror cast and polished by the Steward Observatory Mirror Lab. We describe the optical fabrication and testing of the f/1.25 paraboloid, and present the final measurements of figure accuracy and inferred image quality. Figuring was performed with a 1.2 m stressed lap--which bends under active control to match the local curvature of the optical surface--and a variety of small passive tools. The mirror was pressurized to compensate for polishing loads and thereby eliminate print-through of the honeycomb structure. The net result is a smoother surface on scales of 5 - 20 cm than has been achieved on previous honeycomb sandwich mirrors. The figure was measured with IR and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. The final measurements were used to calculate synthetic stellar images in a variety of seeing conditions.
We describe the active support system and optimization of support forces for the 6.5 m primary mirror for the Multiple Mirror Telescope Conversion. The mirror was figured to an accuracy of 26 nm rms surface error, excluding certain flexible bending modes that will be controlled by support forces in the telescope. On installation of the mirror into its telescope support cell, an initial optimization of support forces is needed because of minor differences between the support used during fabrication and that in the telescope cell. The optimization is based on figure measurements made interferometrically in the vibration- isolated test tower of the Steward Observatory Mirror Lab. Actuator influence functions were determined by finite- element analysis and verified by measurement. The optimization is performed by singular value decomposition of the influence functions into normal modes. Preliminary results give a wavefront accuracy better than that of the atmosphere in 0.11 arcsecond seeing.
A new class of telescope is being built with primary mirrors as large as 8.4 meters in diameter and as fast as f/1. Fabricating the secondary mirrors for these telescopes has presented tough challenges because of their large sizes, up to 1.7 meter diameter; their aspheric departure of more than 300 microns; the required figure accuracy of a few tens of nanometers; and the fact that they are typically convex and difficult to measure. We have developed tools and techniques to meet these demands to produce secondary mirrors efficiently and accurately. A dedicated facility was constructed in the mirror lab that integrates a 1.8-m stressed-lap polishing machine with interferometric and mechanical measuring systems. This paper presents data from a 1.15-m secondary that was finished in our shop, and from two other large mirrors that are currently being fabricated.
The upgraded 6.5 m MMT in Arizona will use an adaptive secondary to optimize performance in the near infrared spectral region. The secondary mirror is a 2 mm thick, 640 mm diameter Zerodur shell suspended only by a flexible center hub. Three hundred voice coil actuators installed in an aluminum reference surface deform the shell according to commands from a wavefront sensor. Capacitor position sensors surrounding each actuator provide feedback in an inner servo loop, much faster than the exterior wavefront sensor control bandwidth. A 60 actuator prototype, nearly identical to the final adaptive secondary size, has been built and is currently being tested.
We describe the optical fabrication of thin glass shells which will be combined with rigid active supports for adaptive secondary mirrors and for space optics. These applications require glass shells about 2 mm thick, with diameters up to about 1 m for adaptive optics and possibly 6 m for space optics. The extreme flexibility presents unique fabrication challenges which are overcome by a simple adaptation of traditional glassworking techniques. Here we describe the fabrication of concave spherical shells 20 cm and 55 cm diameter. A method of handling and supporting the thin substrates for loose abrasive grinding and polishing is demonstrated and some variations on this approach are compared. Extension of the technique to aspheric adaptive secondary mirrors and to ultra-light mirrors up to 6 meters in diameter is discussed. The subsequent integration and optical testing of a 55 cm shell with a 36 point active support is reported.
The new 6.5 m single mirror multiple mirror telescope (MMT) will be equipped with adaptive optics capabilities to enhance high resolution infrared astronomy. Before we build the 64 cm diameter adaptive secondary, we fabricated a smaller prototype mirror. The adaptive secondary uses voice coil force actuators with an average spacing of 30 mm. Surrounding each actuator is an analog capacitor position sensor operating in a digital closed loop at 10 kHz. This allows the force actuators to be controlled as if they were position actuators. The adaptive secondary configuration and performance test results are presented, followed by the changes to be incorporated into the next curved shell prototype.
The Steward Observatory Mirror Lab is in the process of fabricating the 6.5 m mirror for the conversion of the multiple mirror telescope (MMT) to a single primary mirror. For this purpose the lab has developed a versatile polishing system built around the stressed lap polishing tool. The system must produce an f/1.25 parabolic surface with an accuracy corresponding to 0.09 arcsecond FWHM seeing and 1.5% scattering loss at 500 nm wavelength.
James Roger Angel, Hubert Martin, David Sandler, Nick Wolf, Pierre Bely, Piero Benvenuti, Robert Fosbury, Robin Laurance, James Crocker, Riccardo Giacconi
We present a concept for a Next Generation Space Telescope with a monolithic 8 X 4 meter primary, optimized for the near infrared region (2 - 5 microns). The observatory is radiatively cooled to about 35 K and would be launched on an Ariane 5 to the Lagrange Point L2.
A prototype adaptive-optic secondary mirror consisting of a centrally located single fixed voice coil actuator and surrounded by 20 fixed point supports has been characterized optically. A phase-shifting interferometer was used to determine the static influence function and calibrate the capacitive sensor of the prototype secondary. A distance measuring interferometer sampling a single point on the secondary surface at 32 kHz was used to obtain time series data of the dynamic response of the optic. The system impulse response and transfer function were measured directly and the mechanical modes of the structure identified. Measurement philosophy, hardware, and test results are discussed.
We describe the development of techniques for the optical fabrication of the adaptive secondary mirror for the 6.5-m MMT Conversion Project. The f/15 secondary is 640 mm in diameter and consists of a 2-mm-thick convex mirror supported on 320 actuators. This mirror will be polished using the stressed lap method and measured using the holographic test plate system developed at the Mirror Lab, but it presents unique challenges because of its flexibility. During fabrication, the support of the thin mirror must be uniform and stiff enough to keep it from bending significantly under polishing forces which are 25 - 50 times the weight of the mirror. We plan to support the thin mirror by attaching it to a rigid glass substrate, and are pursuing two experimental approaches to the attachment: optical contact and blocking with pitch. The experiments are being performed by fabricating 200-mm concave prototypes of the secondary mirror.
In 1996, the Multiple Mirror Telescope will be replaced with a 6.5 m single primary mirror. Development is currently underway on a sodium laser guide star adaptive optical system for the new telescope. One unique feature will be an adaptive secondary mirror, consisting of 320 individually controlled voice coil actuators on the back side of a thin, 64 cm diameter mirror. This paper describes initial tests on a 15 cm diameter, thin, flat prototype mirror with a single actuator. The thin mirror is held near a thick substrate which also serves as an immovable reference surface. A novel voice coil actuator connects the two glass parts. A custom capacitance sensor surrounding the voice coil actuator measures the absolute distance between the mirror back side and the reference surface. This prototype provides detailed performance measurements, including temporal and spatial actuator response functions. Assembly, alignment, and calibration techniques for the 64 cm mirror will be debugged. The data will help optimize the design and performance of the adaptive secondary.
KEYWORDS: Mirrors, Actuators, Polishing, Finite element methods, Surface finishing, Telescopes, Error analysis, Active optics, Monochromatic aberrations, Control systems
We describe the active support system and optimization of support forces for a 3.5-m honeycomb sandwich mirror. The optimization was based on interferometric figure measurements made in a vibration-isolated test tower. We obtained actuator influence functions by measurement and by finite-element analysis. The two sets of influence functions are similar in shape, but the computed figure changes are 25% smaller in magnitude than the measured figure changes. We achieved a figure accuracy of 25 nm rms surface error with the computed influence functions and only slightly worse with the measured influence functions, but were unable to reproduce the 21-nm rms surface error obtained on the passive polishing support. This implies that subtle differences between the polishing support and operational support caused small, uncorrectable figure changes. The optimization was performed by singular-value decomposition of the influence functions into normal modes. The best results were obtained using 20 - 30 out of a possible 37 modes.
The pursuit of economical fabrication of large (8 m) fast (< f/2), astronomical optics has led to the development of efficient fabrication and testing methods at the Mirror Lab. These methods rely on a mix of advanced technology blended with some traditional practices. Two fabrication strategies have been developed, one for primary mirrors and one for secondary mirrors. Both of these plans rely heavily on the use of the stressed lap both as a grinder as well as for polishing. For secondary fabrication novel methods of testing the convex, severely aspheric mirrors are used.
The continuing development of rapid fabrication methods for large optics at the Steward Observatory Mirror Lab has resulted in the completion of the WIYN 3.5 m primary mirror in only five months. The use of these methods, though rapid, also resulted in one of the best surface figures we have produced (16 nm rms), excellent microroughness (8 angstroms rms), and very smooth small-scale figure error as determined by the structure function of the surface errors. In this paper, we review the important techniques in the grinding, polishing, and testing of the mirror used to achieve these results.
David Anderson, James Roger Angel, James Burge, Warren Davison, Scott DeRigne, B. Hille, Dean Ketelsen, W. Kittrell, Hubert Martin, Robert Nagel, Thomas Trebisky, Steve West, R. Young
The stressed-lap polishing technique has been developed to meet the challenge of polishing 8- m-class mirrors with highly aspheric figures to an accuracy consistent with the best ground- based telescope sites. The method is currently being demonstrated in the polishing of two primary mirrors, a 1.8-m f/1.0 ellipsoid and a 3.5-m f/1.5 paraboloid. The figure accuracies achieved at the time of writing are 43 nm rms surface error for the 1.8-m mirror, and 190 nm rms surface error for the 3.5-m mirror. Polishing is proceedings on both mirrors. In this paper we describe the process used for the 3.5-m mirror and the progress through the early stages of fabrication. We also summarize progress on the 1.8-m mirror.
In contrast to cytophotomnetric studies of nuclear detail quantitative inorphomnetry of histologic specimens has only been developed to a limited degree as a diagnostic tool /13/. Counting of mnitoses is one of the oldest methods for quantification and has become a valuable predictor of good or poor prognosis of uterine soft tissue tumors /5/. In case of malignant melanoma the determination of tumor thickness has proven an important prognostic factor /4/. Measurements of endometrial gland volume enable the pathologist to discriminate endomnetrial carcinoma from hyperplasia /1/2/. Other applications of mnorphometry to special diagnostic problems could be added /13/. L
Caryoxnetry and histoinetry are the two characterization levels for histological
patterns. A wide spectrum of caryoinetrical methods exists (characterization of size
and form, densitometry, nuclear texture analysis) and there is often a good correlation
between measured parameters and diagnosis as well as proosis, especially in
tumor diagnostic /14/15/20/. The development of histoinetry has just begun.
A technique has been developed for casting glass in the form of a honeycomb structure possessing good stiffness despite its low weight, and facilitating thermal control via forced ventilation of the honeycomb cells. To date, 3.5 m diameter mirrors of this type have been successfully cast; fully 8 m diameter mirrors are expected to be cast by 1992. No mirror of diameter as large as 8 m has ever been polished, however, and the difficulties which will be encountered shall be compounded by the shorter focal lengths required for advanced telescopic optics. A novel method, designated 'stressed lap polishing', has been developed to address these problems.
We are in the process of polishing a 1.8-rn f/i ellipsoid with an actively stressed lap. As a preliminary
exercise, we have polished the mirror as a sphere using a rigid subdiameter lap. The overall surface error
was 25 nm rms, and the surface met a specification corresponding to i/8-arcsec image quality. A stressed
lap 600 mm in diameter was designed and built to polish the mirror as an f/i ellipsoid. It consists of an
aluminum disk which changes shape continuously under the influence of 12 moment-generating actuators.
These actuators are programmed to produce the shape changes necessary to make the lap fit the mirror
surface as it moves across that surface and rotates. In this paper we describe the principles and design
of the lap, test results, and progress to date in polishing the 1.8-rn mirror.
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