A spherometer is often used to precisely measure the radius of curvature of a spherical surface. It can also measure the vertex radius of a more complex surface such as an off-axis parabola (OAP). This paper provides a reliable algorithm to find the vertex radius of an OAP by solving a few equations based on the test geometry. This algorithm can also be easily expanded to any conic surface with high-order aspheric coefficients. The algorithm was verified by measuring an 8-inch diameter OAP and comparing the results with its known prescription. Results show good agreement. An example of measuring the vertex radius of a 4-m diameter OAP is also presented. In addition to this, a calculation was done to show that the coma and astigmatism are independent of the clocking of the spherometer on the optic.
Daniel K. Inouye Solar Telescope (formerly known as Advanced Technology Solar Telescope) will be the largest optical solar telescope ever built to provide greatly improved image, spatial and spectral resolution and to collect sufficient light flux of Sun. To meet the requirements of the telescope the design adopted a 4m aperture off-axis parabolic primary mirror with challenging specifications of the surface quality including the surface figure, irregularity and BRDF. The mirror has been completed at the College of Optical Sciences in the University of Arizona and it meets every aspect of requirement with margin. In fact this mirror may be the smoothest large mirror ever made.
This paper presents the detail fabrication process and metrology applied to the mirror from the grinding to finish, that include extremely stable hydraulic support, IR and Visible deflectometry, Interferometry and Computer Controlled fabrication process developed at the University of Arizona.
Low order aberration was founded when focused Gaussian beam imaging at Kodak KAI -16000 image detector, which is integrated with lenslet array. Effect of focused Gaussian beam and numerical simulation calculation of the aberration were presented in this paper. First, we set up a model of optical imaging system based on previous experiment. Focused Gaussian beam passed through a pinhole and was received by Kodak KAI -16000 image detector whose microlenses of lenslet array were exactly focused on sensor surface. Then, we illustrated the characteristics of focused Gaussian beam and the effect of relative space position relations between waist of Gaussian beam and front spherical surface of microlenses to the aberration. Finally, we analyzed the main element of low order aberration and calculated the spherical aberration caused by lenslet array according to the results of above two steps. Our theoretical calculations shown that , the numerical simulation had a good agreement with the experimental result. Our research results proved that spherical aberration was the main element and made up about 93.44% of the 48 nm error, which was demonstrated in previous experiment. The spherical aberration is inversely proportional to the value of divergence distance between microlens and waist, and directly proportional to the value of the Gaussian beam waist radius.
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 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.
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.
A 4-mirror prime focus corrector is under development to provide seeing-limited images for the 10-m aperture Hobby- Eberly Telescope (HET) over a 22 arcminute wide field of view. The images created by the spherical primary mirror are aberrated with 13 arcmin diameter point spread function. The University of Arizona is developing the 4-mirror wide field corrector to compensate the aberrations from the primary mirror and present seeing limited imaged to the pickoffs for the fiber-fed spectrographs. The requirements for this system pose several challenges, including optical fabrication of the aspheric mirrors, system alignment, and operational mechanical stability. This paper presents current status of the program which covers fabrication of mirrors and structures and pretest result from the alignment of the system.
The Giant Magellan Telescope (GMT) is one of the extremely large telescopes of the next generation. The GMT adaptive optics (AO) system uses an adaptive secondary mirror and natural and laser guide stars to achieve diffraction-limited images. The AO calibration source provides sources at the telescope prime focus which replicate the properties of the natural and laser guide stars, to calibrate and verify the performance of the AO system. We present an optical design for this calibration source, and discuss the expected accuracy based on the tolerance analysis.
ITF is usually over looked during the deflectometry measurements, especially when low spatial frequency errors are the main test focus. However, real data shows that the effect of ITF cannot be ignored to reach high accuracy measurements of high spatial frequency features. We illustrated with simulation that ITF of SCOTS is proportional to the camera imaging MTF. We then applied this result to the edge measurement data of a large mirror, where a better agreement is achieved between SCOTS test and a test-plate interferometric test after the compensation. Experimental verification of the ITF theory for deflectometry is preliminary performed. The results will be summarized in our following paper.
We present a new device, the diffractive optics calibrator, for measuring duty cycle and etching depth for computer-generated holograms (CGHs). The system scans the CGH with a collimated laser beam and collects the far-field diffraction pattern with a CCD array. The relative intensities of the various orders of diffraction are used to fit the phase shift from etching and the duty cycle of the binary pattern. The system is capable of measuring variations that cause 1-nm peak-to-valley (PV) phase errors in the wavefront created by the CGH. The measurements will be used primarily for quality control CGHs, but the data can also be used to provide a lookup table for corrections that allow calibration of the lithography errors. Such calibrations may be necessary for us to achieve our goal of measuring freeform aspheric surfaces with 1-nm RMS accuracy.
We present a new device, the Diffractive Optics Calibrator (DOC), for measuring duty-cycle and etching depth for
computer generated holograms (CGH). The system scans the CGH with a collimated laser beam, and collects the far
field diffraction pattern with a CCD array. The relative intensities of the various orders of diffraction are used to fit
the phase shift from etching and the duty cycle of the binary pattern. The system is capable of measuring variations
that cause 1 nm PV phase errors in the wavefront created by the CGH. The measurements will be used primarily for
quality control CGHs, but the data can also be used to provide a lookup table for corrections that allow calibration of
the lithography errors. Such calibration may be necessary for us to achieve our goal of measuring freeform aspheric
surfaces with 1 nm RMS accuracy.
Computer-generated holograms are often used to test aspheric surfaces. This paper provides a parametric model for
the CGH phase function using the exact geometric model. The phase function is then used to derive the sensitivity
functions to alignment errors in testing. When using the CGH to test aspheric surface, it is important to separate the
diffraction orders and only allow the desired order to pass the system. This paper also provides a recipe for
determining the amount of carriers needed to eliminate the ghost images.
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.
Aspheric surfaces are often measured using interferometers with null correctors. The null corrector creates a
wavefront that matches the surface under test, but also introduces imaging aberrations, such as mapping distortion
and field curvature. These imaging aberrations cause high frequency features in the surface under test to be filtered
out and create artifacts at edges. We provide a concise methodology for analyzing these effects using field
curvatures, and showing how they couple with diffraction as represented by the Talbot effect and Fresnel edge
diffraction.
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.
Aspheric surfaces are measured using standard interferometers coupled with computer generated holograms (CGHs) that
compensate the aspheric wavefronts. Such systems can measure complex aspheric shapes with accuracy of a few
nanometers. However, the imaging properties of the interferometer-CGH combination can provide limitations for data
mapping, resolution, and accuracy. These effects are explored, with an emphasis on the diffraction effects that are
unique to interferometry.
Aspheric surfaces, such as telescope mirrors, are commonly measured using interferometry with computer generated
hologram (CGH) null correctors. The interferometers can be made with high precision and low noise, and CGHs can
control wavefront errors to accuracy approaching 1 nm for difficult aspheric surfaces. However, such optical systems
are typically poorly suited for high performance imaging. The aspheric surface must be viewed through a CGH that was
intentionally designed to introduce many hundreds of waves of aberration. The imaging aberrations create difficulties
for the measurements by coupling both geometric and diffraction effects into the measurement. These issues are
explored here, and we show how the use of larger holograms can mitigate these effects.
The random ball test (RBT), also known as the CaliBall test, is often used to calibrate interferometer transmission
spheres. This paper provides a way to estimate the total errors remaining after interferometer calibration using the
RBT. Errors that cannot be removed by calibration include random errors due to measurement noise in the calibration,
geometric errors, and errors due to diffraction. The random errors can be reduced by averaging multiple random ball
tests. The geometric errors and diffraction errors are systematic, and arise when the radius of the CaliBall is different
from that of the test optic.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.