optica designs of astronomical instruments ,
and Earth viewing cameras in space
Profile Summary
EHR Optical Systems 1871 Elmhurst Place Victoria, B. C. V8N 1R1 Phone 250 472 0105 Fax 250 472 0145 Biographical: Born Portland, Oregon, U.S.A., 14 August 1927 Graduate Oak Bay High School, Victoria, Canada, 1945 Victoria College, Craigdarroch Castle, 1946 Victoria College, Landsdowne Campus, 1947 B.A. Physics, University of British Columbia, 1949 M.A. Nuclear Physics, University of British Columbia, 1951 Ph.D. Molecular Spectroscopy, University of Toronto, 1960
• President, Commission 9 of International Astronomical Union, Instruments and Techniques, 1979-1982 • Optical Consultant on NASA Lunar Laser Ranging Experiment, McDonald Observatory, 1969 1991-4
2005-8
Optical design consultant to EMS Technologies, Ottawa, on HERO resource satellite. Optical design consultant to MDA, Vancouver, on RapidEye and another project, Optical designs for Routes, Inc., of UV Auroral Imager called Ravens. Optical designs of corrector mirrors and lenses for a proposed 8-m Liquid Mirror Telescope Optical designs for U. Saskatchewan of a laser imager to power an experimental Space Elevator. Optical design of near infrared imager and spectrograph for Mt Abu telescope, India Optical design of prismatically cross-dispersed Echelle grating spectrograph for Mt Abu telescope, India. Modifications to fit manufacturer’s tools and materials (ongoing) Optical designs of a High Definition High Contrast, HDHC, spectrograph for the U. of Illinois (ongoing) Optical design options for a wide field adaptive optics system for the Canada-France-Hawaii Telescope, CFHT, (ongoing)
Publications (15)
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We present the first light commissioning results from the Physical Research Laboratory (PRL) optical fiber-fed high
resolution cross-dispersed Echelle Spectrograph. It is capable of a single- shot spectral coverage of 3700A to 8600A at R
~ 63,000 and is under very stable conditions of temperature (0.04°C at 23°C). In the very near future pressure control
will also be achieved by enclosing the entire spectrograph in a low-pressure vacuum chamber (~0.01mbar). It is attached
to a 1.2m telescope using two 50micron core optical fibers (one for the star and another for simultaneous Th-Ar spectral
calibration). The 1.2m telescope is located at Mt. Abu, India, and we are guaranteed about 80 to 100 nights a year for
observations with the spectrograph. The instrument will be ultimately used for radial-velocity searches of exoplanets
around 1000 dwarf stars, brighter than 10th magnitude, for the next 5 years with a precision of 3 to 5m/s using the
simultaneous Th-Ar spectral lamp reference technique. The spectrograph has already achieved a stability of 3.7m/s in
short-term time scale and in the near future we expect the stability to be at 1m/s once we install the spectrograph inside
the vacuum chamber.
We describe the design and optimization of a wide-field near-infrared camera and spectrograph (NICAS) for Mt Abu 1.2
m, f/13 Cassegrain telescope of Physical Research Laboratory. The principal science goals include photometric mapping
of star forming regions and medium resolution spectroscopy of Young Stellar Objects, evolved stars and transient
sources. The design goals are to achieve seeing-limited angular resolution in an un-vignetted field of view of ~ 8'x8' with
0.5" per pixel (of 18.5 μm) on a HgCdTe 1024×1024 infrared array, requiring a two-fold Cassegrain focal reduction. In
addition to the imaging, the instrument is required to have spectroscopic capability with a resolving power of 103 in the
0.85 - 2.5 μm region, needing a dispersion of 1 nm per pixel. Finally, since our telescope has a moderate aperture, the
throughput losses need to be minimized. The specifications are achieved by an optical design using 9 singlet lenses.
Only those lens materials are chosen for which measured values are available for refractive indices at 77 K (detector
operating temperature), changes of indices with temperature, and thermal coefficients of expansion. The design is
optimized to give sharpest images at 77 K. The optical path is folded by 90° after collimation by a fold-mirror and re-imaged
on the detector. The fold-mirror is replaced by a diffraction grating for spectrograph mode. In order to minimize
the reflection losses, all the lenses will be anti-reflection coated for the full operating wavelength range. Details of the
design are presented.
We present here the optical design of an efficient Fiber-fed, Prism Cross-dispersed, Echelle Spectrograph (Resolution
~70,000 @seeing limited ~2arcsecs conditions) which will operate in the wavelength region of 3700A to 8100A. It will
be used for extra-solar planets searches down to the precision of 3m/s and as well as for follow-up observations for new
transit discoveries. The spectrograph design is such that with a beam size of 100mm (4inch) it should suit the existing 1
to 2m class of telescopes available in India. The fiber-fed spectrograph will be installed with a 1.2m telescope, which is
situated at Mt. Abu (5800feet), Rajasthan, India. We estimate the spectrograph to be >30% efficient from the slit to the
CCD detector, and up to 15% efficient including sky, telescope, fiber-fed optics etc. We expect to reach the S/N ratio of
70 on a 10mag star for an integration time of 40mins. We aim to achieve 5m/s to 3m/s Radial Velocity accuracies on
such a star using the simultaneous ThAr referencing method. Since thermal stability is absolutely necessary to achieve
<5m/s RV accuracies, the whole spectrograph is planned to be kept inside a vibration free isolated tank under low
vacuum (0.001 mbar) in a thermally isolated room at 28C +/- 0.01C. It should see the first light by the summer of 2009.
We are guaranteed at least 120 nights per year for the planet search program, more nights are possible.
Differential atmospheric refraction does not cause a net rotation of a field of stars thus a correctly tracking equatorial telescope does not require rotation of the detector during observations. Ira S. Bowen was mistaken in 1966 in his calculation of star trails. In the publication of his lecture titled "Future Tools of the Astronomer" after being awarded the Gold Medal from the Royal Astronomical Society in 1966, on page 18, he said "At this point I should digress to make a few remarks about the ultimate limitation on the size of field we may hope to reach. For many purposes this will be set by differential refraction in the atmosphere". It shows trails for a ring of stars at a declination of 60 degrees for a site at 30 degrees latitude, for an 8-hour exposure over a 2 degree field diameter. The tangential component of the East and West trails is 3 seconds of arc (arcsec). The correct separation is 0.4 arcsec in the opposite direction which balances the 0.4 arcsec clockwise trails of the North and South stars. Also his 0.6 arcsec radial trail should be 1.1 arcsec. The source of the errors is in the application of spherical trigonometry.
In this paper, we describe the progress of the construction of the Multi-Conjugate Adaptive Optics laboratory test-bed at the University of Victoria, Canada. The test-bench will be used to support research in the performance of multi-conjugate adaptive optics, turbulence simulators, laser guide stars and miniaturizing adaptive optics. The main components of the test-bed include two micro-machined deformable mirrors, a tip-tilt mirror, four wavefront sensors, a source simulator, a dual-layer turbulence simulator, as well as computational and control hardware. The paper describes changes in the opto-mechanical design, characteristics of the hot-air turbulence generator, performance achievements with the tip-tilt and MEMS deformable mirrors as well as the design and performance of the wavefront sensors and control software.
The concept and design for a novel four-quadrant position and focus error sensor are presented. Expected performance and theory of operation of the astigmatic focus sensor are presented. Features include wide field of view, broad wavelength coverage, high efficiency, integral field and pupil stops, and alignment and assembly benefits. A new method for sectioning the field is utilized, an internal mirrorlette array (IMA). The advantages of the IMA are given. This error sensor is implemented in the WIYN Tip-Tilt Module, an add-on imaging instrument for the 3.5 m WIYN telescope at Kitt Peak National Observatory.
The 8-meter Gemini telescope's adaptive optics (AO) module, Altair, is 'transparent' in that it does not change the focal ratio, being f/16 in and f/16 out; it has the same focal position as the bare telescope, with insignificant change in the exit pupil. However, Altair has a flat focal surface, unlike other AO designs which have focal surfaces curved more than the focal surface of the bare telescope and in the opposite direction. An unusual requirement for Altair is that the atmospheric layer 6.5 km above the telescope should be imaged onto the deformable mirror. Other requirements are minimization of distortion in the wavefront sensor module for both the imaging of the deformable mirror onto the lenslet array and for the reimaging of the approximately 230 lenslets' images onto a CCD, for a natural guide star, and also for a Sodium laser guide star ranging in object distance form 85 km to 156 km. The separation of natural and laser star beams is done with minimum light loss by passing the in-focus natural star image through a pinhole which is smaller than the shadow of the secondary mirror of the telescope in the out-of-focus laser beam which is reflected by the tilted pinhole mirror.
The Gemini Adaptive Optics System, under construction at the Dominium Astrophysical Observatory of the National Research Council of Canada's Herzberg Institute of Astrophysics is unique among AO systems. Altair is designed with its deformable mirror (DM) conjugate to high altitude. This concept is only practical at an observatory where extraordinary measures have been taken to reduce local seeing degradation. We summarize these measures. We then describe Altair. Both the wavefront sensor foreoptics and control system are unconventional, because the guide star footprint on an altitude-conjugated DM moves as the guide star position varies. During a typical nodding sequence, where the telescope moves 10 arcseconds between exposures, this footprint moves by half an actuator and/or WFS lenslet. The advantages of altitude conjugation include increased isoplanatic patch size, which improves sky coverage, and improved uniformity of the corrected field. Although the initial installation of Altair will use natural guide stars, it will include features to use a laser guide star with minimal rework. Altitude conjugation also reduces focal anisoplanatism with laser beacons. The infrastructure of Gemini observatory provides a variety of wavefront sensor and nested control loops that together permit some unique design concepts of Altair.
Progress in Active Optics Methods has led to the invention and production of blazed aspherical gratings. These developments use jointly 'vase form' submasters and a two-stage replication technique. It has been shown that the use of aspherized gratings greatly minimizes the number of optical surfaces. This improves the optical throughput of astronomical spectrographs and has a capability of correcting camera mirror aberrations up to f/1.2. With respect to refractive designs, the full achromaticity in correcting mirror aberrations by constant line spacing reflective gratings allows much broader spectral coverages -- hereafter [(lambda) (lambda) ] approximately equals 2 octaves. In addition, and also due to a full reflective design, such instruments provide quasi- constant spectral dispersions and are distortion free. These latter features increase the accuracy in the data reduction process (sky substraction, etc. ...), and are particularly convenient in the multi-aperture mode. Recent developments in this field are presented with imager-spectrograph ISARD, dedicated to the Cassegrain focus of the 2m Bernard Lyot Telescope at Pic-du-Midi Observatory for faint object studies in the optical domain [320 - 1200 nm], and with spectrograph OSIRIS, to be launched in a ODIN orbital mission in 1998 and built by the Canadian Space Agency for studies in the spectral range [295 - 800 nm].
For spectroscopic observations with a segmented mirror 28- meter telescope, an alternative to image slicing of a single composite image is to separate the 141 images and stack the unsliced images from the segments along the slit of the spectrograph where an array of micro optics redirects the beams from the segments to superimpose on the grating of a spectrograph. The focal length of the collimator can then be increased in proportion to the square root of the number of beams, resulting in a proportional increase in the slit width without loss of spectral resolution. An additional advantage is that the longer focus collimator introduces less aberration and can be used off-axis thus avoiding central obstruction near the grating. Following the collimator, an aspherized grating, developed by Gerard Lemaitre and manufactured by Jobin-Yvon, has the advantage of minimizing the number of optical surfaces in a spectrograph.
The paper examines an off-axis three-mirror telescope with a flat local surface. Designed to image aurora in UV light from a satellite, this 20-cm telescope has an f/2 primary and an f/8 tertiary focus. The focal surface has a diameter of 4 cm which subtends 1.3 deg. The telescope is engineered to have very low distortion so that resolution would not be degraded during integration of images moving on the CCD detector at the same rate as that of the CCD readout.
An alt-alt (altitude-altitude) mounting for an 8-metre telescope has several operational advantages over an alt-az (altitude-azimuth) design. In this alt-alt arrangement the yoke (or major) axis, which is horizontal, lies East-West and the second (or minor) altitude axis is located across the yoke and lies in the North-South plane. By comparison, the alt-az has a vertical, azimuth axis pointed at the zenith and its minor axis is always horizontal. Consequently, the altaz telescope cannot track through the zenith because of the infinitely high rotation rate required about the vertical (azimuth) axis, thus it cannot be used to observe at the zenith where the images are sharpest and the atmospheric transmission highest, and its axle and field rotation speeds are disadvantageously high throughout a considerable area of sky in the vicinity of the zenith. The rapid rotation of spectrographs at folded Cassegrain and Nasmyth foci can cause the spectrum to drift on the detector during an exposure because of fiexure within the spectrographs. By comparison, the alt-alt telescope tracks readily through the zenith at slow rotation speeds, and the field rotation rate is zero when tracking along the Celestial Equator where the telescope becomes pseudo-equatorial because one axis (the minor axis) is then parallel to the axis of the Earth. It is at the Eastern and Western horizons where the rate of rotation about the horizontal axis becomes excessive, which is not critical because telescopes are not scheduled to observe there, where the seeing and transparency are very poor, and where in practice the light is usually obstructed by the building enclosing the telescope. When pointed at the zenith, the primary mirror in an alt-alt mounting is suspended over the cooled observing floor, thus minimizing seeing degradation caused by heat generated in the bearings and drives. (It is when the telescope is pointed closer to the horizon that the light path within the telescope tube is over a bearing.) Because of the rapid rotation and acceleration during tracking the alt-az mountings are specified to have exceptionally high resonant frequencies to enable quick response to the drive motors. The alt-alt telescope does not need such high resonance frequencies because its rotation speeds are low, usually even lower than for an equatorial mounting. In summary, the characteristics of the alt-alt mounting are in phase with the astronomical requirements, whereas the alt-az mounting is 90 degrees out of phase with these requirements in the sense that its worst performance is at the zenith instead of at the horizon.
The optical design of an f/4 coude echelle spectrograph for the 3.6-m Canada-France-Hawaii Telescope is described and illustrated with drawings and diagrams. The basic configuration comprises a tilted spherical f/20 collimator mirror, a 30-cm 316-g/mm grating near the slit, a paraboloid camera mirror, and a small triplet corrector lens. Two sets of optics are provided, coated for optimal performance in the UV-blue and green-IR regions, respectively, and small remotely selectable variable-wedge grism or grens modules are located in the diverging beam from the slit to the collimator to prevent other orders from overlapping the part of the spectrum being recorded. Consideration is given to the Hartmann mask system, the mosaic grating controls, the collimator and camera mirror turrets, and the detector support.
A 1.6-meter diameter f/0.95 all-reflecting telescope was designed to observe orbital debris particles as small as 1 mm from the shuttle payload bay. The telescope was specified to have a flat focal surface without the imposition of refractive elements. Two design configurations involving three mirrors were evaluated - a reflective Schmidt-Cassegrain and a modified Paul corrector. The Paul system was found to be more compact and appropriate for this application.
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