THE NEW DUAL APERTURE TELESCOPE FOR THE MT. EVANS OBSERVATORY
(1996 SPIE Proc. Vol. 2828:463-471)
Eric T. Meyer
The Meyer Foundation, 4 Leeds Court
Lake Forest, Illinois 60045
Robert E. Stencel
Dept. of Physics & Astronomy, University of Denver
2112 E. Wesley, Denver, Colorado 80208
Donald G. Bruns
Stellar Products, P.O. Box 720879
San Diego, California 92172
An unusual dual-aperture 28.5-inch, f/21 Ritchey-Chretien telescope has been completed and will be installed in the
recently upgraded University of Denver extreme high altitude observatory facility, atop 14,268 ft. Mount Evans in Colorado.
Designed to optimize high spatial resolution imaging, the Meyer Binocular Telescope incorporates active thermal
management of the telescope structure. The secondary mirror support elements are fabricated from INVAR and permit active
tip-tilt and focusing capability. The optics were fabricated from Zerodur by Contraves USA, and each system has a measured
total wavefront error <0.050 at 633nm. All optical surfaces are coated with a multi-layer dielectric enhanced silver,
providing high reflectance from below 350nm to beyond 26æm.
The telescope control system has been designed to allow initial operation from an insulated control room. Long-
term plans call for totally remote operation from the University of Denver campus via direct microwave radio link.
Instrumentation planned for the telescope at first light includes: 1) a low order 400nm to 1,000nm band adaptive
optics system (AO5: Adaptive Optics, 5 mode) equipped with a large format CCD camera; 2) a mid-infrared array camera
(TNTCAM: Ten and Twenty micron Camera); and 3) a mid-IR moderate dispersion spectrometer (TGIRS: Two Grating IR
Spectrometer). Some of the science problems the dual aperture telescope is uniquely situated to tackle include the study of
planetary atmosphere, detection of planetary systems around nearby stars and the analysis of evolutionary changes in stars.
The Mount Evans site (at 4,303 meters elevation, the highest operating astronomical facility in the world) is located
70 km west of Denver and can be reached via a paved state highway which extends all the way to the summit. The
observatory is currently under construction with installation of the telescope planned for late summer 1996.
Keywords: telescope, thermal management, adaptive optics, infrared instrumentation, Mount Evans
Several years ago, a bequest from alumnus William Herschel Womble enabled the University of Denver to revitalize
its Astronomy program, and this has enabled the enhancement of the extreme high altitude observatory facility atop 14,268 ft.
Mount Evans, 70 km. west of Denver. The Meyer Foundation of Lake Forest, Illinois has agreed to provide its dual aperture
0.7m Ritchey-Chretien telescope for this observatory.
The intention of this paper is to highlight some of the more interesting aspects of the telescope, the dedicated
adaptive optics system, the observatory, and site. This information should be particularly timely in that dedication of the new
facility and instrumentation will nearly coincide with this conference, and the University of Denver will be leading tours for
SPIE meeting participants to the Mount Evans site.
3. THE TELESCOPE
The Meyer Binocular Telescope is of an equatorially-mounted English yoke design (Fig. 1). The overall mass of the
telescope is 9,000 kg. with a moving mass of 4,100 kg. The declination drive incorporates a 0.9m worm gear while the right
ascension worm gear is 1.25m in diameter. Both worms have been ground to a total indicated run out of less than 0.00005
cm/cm and are spring loaded to minimize backlash.
Major structural components of the support columns and yoke have been fabricated from 12mm and 18mm steel
plate. Based on FEA, the lowest frequency vibration of the telescope is a 20Hz twisting of the yoke about the ascension axis.
Nevertheless, approximately 75% of the surface of the yoke has been covered with a sandwich of 6mm of elastomer
composite and 3mm of steel plate (Fig. 2). This system of constrained layer damping is anticipated to provide an
improvement in damping of as much as two orders of magnitude at the eigen frequency of the yoke.
The low expansion Zerodur substrate used for the primary and secondary mirrors and the continually changing
temperature within the observatory necessitated the use of a low expansion support system to avoid dangerous stresses within
the optics or unacceptable levels of slippage due to excessive clearances. It was decided to avoid the complexity of thermally
compensated structures and fabricate the primary mirror central support cylinder as well as the secondary mirror cell from
Super Invar. The primary mirror receives no less than 90% of its lateral support from a peripheral flotation system of very
high compliance allowing the central pillar to maintain, with modest forces, the orientation of this mirror due to the very
small clearance between the cassegrain perforation and the outer diameter of the hollow central pillar. Similarly, the
secondary is supported in a Super Invar cell 0.01mm larger than the outer diameter of the secondary disk. Silastic adhesive
bridges this gap at 12 contact points located equidistant around the circumference and midway between the front and back
surfaces of the mirror.
It is likely that heating and cooling of the telescope will cause mainly axial translation of the optics which is easily
compensated by adjustment of the motorized secondary focuser. Furthermore, we intend to control this motor with periodic
input from the adaptive optics system, thus constraining focal plane shift within the functional limits of the high speed
adaptive optics system.
We anticipate some gravitational flexure of the telescope structure will lead to decollimation of the optics. To
compensate, the secondary mirror cell has been secured to a central pivot point defined by a self-aligning linear bearing. The
cell is then oriented by three piezoactuators 120o apart contacting the outer edge of the back of the cell. The maximum
secondary mirror tilt offered by the piezoactuators is 12 arc minutes; this corresponds to a shift at the cassegrain focal plane of
4.5mm. The potential to actively optimize the collimation of this relatively small telescope through input from the adaptive
optics system is intriguing.
The dual Ritchey-Chretien optical systems were fabricated by Contraves USA. They incorporate a 0.7m F3 primary
mirror and a 12cm, 7 power secondary mirror for a combined focal length of 14.92m. The measured total system wavefront
error for both telescopes is less than 0.05 at 633nm RMS. The Strehl ratio for both systems is approximately 94%. It
is noteworthy that these two telescopes are nearly identical in focal length and aperture thus simplifying the exchange of
instrumentation between, and comparison of data obtained at, the two telescopes.
All four mirrors are coated with a multilayer enhanced (protected) silver FSS 99 from Denton Vacuum that promises
very high reflectivity from 0.35 to beyond 26 æm. The low emissivity of this coating in the thermal infrared should
complement the low precipitable H2O levels measured at the Mount Evans site.
3.3 Thermal control systems
The relatively large thermal mass of this telescope presents a major source of thermal disequilibrium within the
observatory enclosure. In order to reduce this effect, approximately 150m of 18mm copper tubing has been mounted on the
telescope with 500 copper heat transfer nodes. A solution of 50/50 glycol and water is circulated at 3,000 L/hr. between the
telescope and a liquid to air heat exchanger mounted in the air flow duct which guides air from the observatory enclosure to
the 5 m3/second exhaust fan located down wind of the observatory. Control of this system is passive, necessitating these
large flow rates to minimize thermal gradients at each stage.
The decision to utilize liquid cooling of this telescope was based on a desire to compare this method with the more
popular direct air flow systems being utilized in most of the advanced telescopes currently being constructed. We plan to
utilize thermographic techniques to visualize temperature gradients and make estimates of heat flow between the telescope and
the observatory environment beginning shortly after installation. If successful, liquid cooling offers the opportunity to
retrofit more conventional telescopes with minimal structural alteration and the flexibility of dumping thermal loads to
conveniently located heat exchangers outside the observatory.
Thermal management of the Zerodur primary mirrors of our telescope presents another challenge. Each mirror has a
thermal mass of only 20 kcal/o K but due to the extremely low thermal conductivity of Zerodur 1.64W/(múo K) and the
sensitivity of image quality to small air temperature differences directly in the optical path,1 active cooling of the back surface
of each primary mirror is required.
The primary mirror cells incorporate a 10 cm high space behind the mirror to accommodate the flotation system and
are essentially air tight due to the thin wall inflatable seals on the outer and inner edges of the mirror. We elected to insulate
this space and incorporate three compact thermoelectric heat pumps behind each mirror. Driving voltage is provided by a
programmable power supply directed to achieve an arbitrary air temperature within the mirror cell. This temperature is in turn
determined by a dedicated personal computer programmed to maintain the front surface at or slightly below ambient air
temperature measured within the telescope tubes. The changing air temperature at the observatory site necessitates the
continual flow of heat between the back surface of the mirror and the heat pumps. Based on the work of RJS Greenhalgh et al.
for the Gemini telescope,2 we anticipate for each degree of offset between back surface and ambient, we will achieve a mirror
temperature change of 0.12 degrees/hour. The low conductivity of the 9 cm thick Zerodur substrate requires that the thermal
control program maintain a substantial offset between ambient air and the rear surface of each mirror. We are uncertain as to
the maximum night time cooling rates on Mount Evans and therefore we have conservatively designed the heat pumps to
maintain as large as a 20oC offset at a -10oC ambient air temperature. An added benefit of these high capacity pumps is that
by simply reversing voltage polarity to the thermoelectric coolers for a short period, significant warming of the mirrors can
occur, inhibiting dew formation and preventing coating damage.
Thermal gradients that persist within the telescope tubes will be minimized by air drawn from the observatory
enclosure through low resistance filters and directed around the mirror cell to pass up the telescope tube and out the open end.
the slight positive pressure within the telescope may also help to reduce dust build-up on the optical surfaces and in turn
reduce scattered light, potential coating damage, and thermal emissivity.
4. ADAPTIVE OPTICS SYSTEM
A low order adaptive optics system for one half of the Meyer Binocular Telescope, based on simple optics and
commercially available key components, has been designed and built. For this 0.7 meter aperture telescope, (D/ro) is small
enough even in the visible spectral region, that only five correction orders are required to produce significantly improved
images. The system, called the AO-5, uses translating lenses to produce smooth aberration corrections.
4.1 Optical design
The optical layout is shown schematically in Fig. 3. At the prime focus is a movable pinhole and set of LEDs
which are used to calibrate the wavefront sensor and wavefront aberrators. After collimating the light, the beam goes through
a set of five individually actuated wavefront aberrators to remove defocus, then two cylinders, then tip and tilt. A beamsplitter
passes some of the light to a lens which reimages the light to a silicon CCD science camera, while the rest of the light is
diverted to four prisms which split the light into subapertures. The four resulting beams are focused onto another small
camera (based on the Texas Instruments TC211 CCD), where only the active pixels in the four quad cells are read out.
The wavefront aberrators are made with voice coil actuated lens pairs. The defocus aberrator consists of a nearly zero-
power pair of simple lenses spaced about 1 mm apart. Changing the separation with the actuators changes the optical power
very slightly, just enough to correct the defocus measured over the telescope aperture. The astigmatism correction is
accomplished in the same way, but using matched cylindrical lenses. The astigmatic power at the actuator bias point of the
cylinders is made zero by using different glass types in lenses with the same (but opposite) curvature. Tip and tilt are
corrected by translating very weak lenses transverse to the optical beam.
4.2 Mechanical and electronic design
The optical bench measures 18 cm x 31 cm x 60 cm, and bolts directly to the telescope. Without the science
camera, it weighs about 18 kg. Cables 15 meters long connect the optics head to a 486DX computer, which contains the
camera controller and the actuator digital-to-analog converter card. A small amount of electronics, such as the stepping motor
drivers for the filter wheel and the calibration source, and the voice coil drivers, are located in the optical head.
4.3 Software control
The computer software performs the functions of reading the wavefront sensor camera, calculating the wavefront
errors, and sending the appropriate command signals the aberrator actuators. The software also does semi-automatic
calibration as frequently as desired. The operator manually corrects the image of the calibration source into the perfect Airy
pattern on the science camera by adjusting the aberrator actuators (Fig. 4). When completed, the computer then stores those
offset voltages and the corresponding wavefront tilts on the wavefront sensor. This corrects any alignment drifts. The gain
coefficients of the actuators is also automatically measured on demand, which can be used to generate a new reconstructor.
Since only five actuators and eight wavefront tilts are measured, the software is fast and simple.
The system has been assembled, and is undergoing preliminary calibration and testing. The system design is limited
by the wavefront sensor integration period, with a goal to operate on magnitude 12 stars with 10 millisecond integration
time. Actuator response time to a step input is less than 5 millisec, while the readout time of the wavefront sensor camera,
including tilt calculations, matrix multiplication, and actuator output, is less than 3 milliseconds.
5. INFRARED INSTRUMENTATION
The focal plane instrumentation complement planned for the telescope at first light also includes a mid-infrared array camera
(TNTCAM: Ten and Twenty micron Camera and a mid-IR moderate dispersion spectrometer (TGIRS: Two Grating IR
Spectrometer). Details may also be found on our web page: www.du.edu/physastron/obs.html.
TNTCAM is Denver University's Ten aNd Twenty micron mid-IR array CAMera.3 TNTCAM utilizes a Rockwell HF-16
Si:As focal plane array, 128 x 128 in size, with 75micron pixels and 16 channel outputs, and efficiently detects wavelengths
from shortward of 4 microns to longward of 26 microns, with a quantum efficiency ranging between 5 and 50%. The camera
incorporates a compact optical design, wherein the entrance aperture is placed at the rotation axis of the filter wheel, which
minimizes off-axis angles from the spherical collimating and re-imaging mirrors. This optical configuration produces
diffraction limited images at the Mt. Jelm (WIRO) 2.3 meter and the Mt.Lemmon (MLOF) 1.5 meter telescopes. The filters
are located at the Lyot stop, thereby minimizing the effects of inhomogeneities within the filters on photometric results.
The collimating and re-imaging mirrors are housed in separate compartments, thereby shielding the array from unwanted stray
radiation. TNTCAM has demonstrated background noise limited performance (not due to electronics) at both WIRO and
MLOF. The array, optics and filter wheel are housed in a LN2/LHe dewar provided by the University of Minnesota IR
Astronomy group, and has an operational hold time of 18 hours.
The Two Grating Infrared Spectrometer (TGIRS)4, a new mid-IR array, dual grating spectrometer for the 7.0-13.8 æm region
built at the University of Denver (DU). This instrument has been designed to monitor silicate features in evolved stars, but is
flexible enough to accommodate a variety of astrophysical investigations. The instrument uses diamond-turned aluminum
optics to allow warm optical alignment and eliminate differential contraction of the optics while operating at 6.5oK. Two
gratings are used in the optical design to provide a resolution of about 800. The first grating cross disperses the flux into
several orders, 8-14. The second grating is the high resolution grating which disperses the flux into each of the above orders
over the wavelength range of the instrument. This second grating has two position settings controlled by a swing arm device
to allow for both maximum spectral coverage and efficient use of the array detector with the least amount of moving
hardware. The entire assembly is cooled with a Gifford-McMahon refrigerator so that it may later be adapted for use during
remote observing. The array is a Rockwell 128X128 Si:As BIB Hybrid Focal Plane Array, with 75 æm pixels, sensitive to
26 æm. The dewar is being custom built by J. K. Henricksen and Assoc. in Vista, CA. Short wavelength IR and optical
radiation is blocked with a long pass filter. The slit of the instrument is 2"x 8" allowing for both spectral and spatial
coverage of the objects being studied. The electronics package and software for readout were developed by Wallace
Instruments and are already in use on our TNTCAM at DU. With "first light" scheduled for summer 1996.
5.3 Initial Science Goals
Some of the science problems the dual aperture telescope is uniquely situated to tackle include the study of planetary
atmosphere, detection of planetary systems around nearby stars and the analysis of evolutionary changes in stars. Among the
research projects planned after the Meyer Binocular Telescope is installed include:
-- infrared imaging and spectroscopy of the planets -- such studies will better characterize the atmospheric conditions
of these bodies and the relation of atmospheric conditions elsewhere to earth's weather systems;
--studies of members of our outer solar system, including comets and Kuiper Belt objects -- these denizens of deep
space record the origin and evolution of our solar system in ways not seen among the inner planets; such studies will help
characterize their numbers and orbital motions;
--studies of the birth and death of stars and their planetary systems -- infrared observations have discovered
increasingly strong evidence for planetary systems around other stars, which require careful followup studies to verify and to
determine the relationship between the evolutionary state of the central star and the "viability" of the planetary system;
--surveys for novae, supernovae, clusters, active galaxies and other astrophysical phenomena -- as the recent Jupiter-
comet collision indicated, explosive events occur from time to time and only telescopes in excellent locations are prepared to
fully explore these events.
The University of Denver wishes to acknowledge the estate of DU Alumnus, William Herschel Womble, for the generous
bequest he provided for educational research in astronomy and astrophysics, which has made development of the new
observatory facilities at Mt.Evans possible. RES also thanks W.J.Williams, D.I. Klebe, H.Neumann, J.P.Meyer,
J.Greathead and many others for valuable assistance in developing the new observatory for Mt.Evans.
1. Barr, L.D., Fox, J., Poczulp, G.A. and Roddier, C.A., "Seeing Studies on a 1.8 m mirror" in Advanced Technology
Optical Telescopes IV, SPIE Vol. 1236, pp. 492-506, 1990.
2. Greenhalgh, R.J.S., Stepp, L. and Hansen, E., "The Gemini Primary Mirror Thermal Management System", SPIE Vol.
2199, pp. 911-921, Aug.1994.
3. Klebe, D., Dahm, M. and Stencel, R., "TNTCAM: Ten and Twenty micron infrared array camera", in Polarimetry of
the Interstellar Medium, eds. W.Roberge and D.Whittet, Astron. Soc. Pacific Conf. Series Vol. 97, pp. 79-84, 1996.
4. Creech-Eakman, M., Klebe, D., Stencel, R. and Williams, W.J. "TGIRS: A New Two Grating mid-infrared Spectrometer",
Bulletin Amer. Astron. Soc. Vol. 28, p.962, 1996.