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 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.
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.
In optical lens assembly, metal retaining rings are often used to hold the lens in place. If we mount a lens to a sharp metal
edge using normal retention force, high compressive stress is loaded to the interface and the calculated tensile stress near
the contact area from Hertzian contact appears higher than allowable. Therefore, conservative designs are used to ensure that glass will not fracture during assembly and operation. We demonstrate glass survival with very high levels of stress. This paper analyzes the high contact stress between glass lenses and metal mounts using finite element model and to predict its effect on the glass strength with experimental data. We show that even though contact damage may occur under high surface tensile stress, the stress region is shallow compared to the existing flaw depth. So that glass strength will not be degraded and the component can survive subsequent applied stresses.
While the concept of wavefront decomposition is a foundation of active optics systems, the choice of basis functions for
mirror figure control is divided. The common functions are Zernike polynomials, ubiquitously used for wavefront
descriptions, and bending (also called minimum energy or vibration) modes which offer optimal performance. We
present a look at the comparative performance between the two approaches, and discuss an implementation approach
which seeks to combine much of the analytic and interface simplicity of Zernike polynomials with the improved
performance of bending modes.
Lockheed Martin Corporation (LMC) tasked The University of Arizona Steward Observatory (UASO) to conduct an
engineering study to examine the feasibility of creating a 4m space telescope based on mature borosilicate technology
developed at the UASO for ground-based telescopes. UASO has completed this study and concluded that existing launch
vehicles can deliver a 4m monolithic telescope system to a 500 km circular orbit and provide reliable imagery at NIIRS
7-8. An analysis of such an imager based on a lightweight, high-performance, structured 4m primary mirror cast from
borosilicate glass is described. The relatively high CTE of this glass is used to advantage by maintaining mirror shape
quality with a thermal figuring method. Placed in a 290 K thermal shroud (similar to the Hubble Space Telescope), the
orbit averaged figure surface error is 6nm rms when earth-looking. Space-looking optical performance shows that a
similar thermal conditioning scheme combined with a 270 K shroud achieves primary mirror distortion of 10 nm rms
surface. Analysis shows that a 3-point bipod mount will provide launch survivability with ample margin. The primary
mirror naturally maintains its shape at 1g allowing excellent end-to-end pre-launch testing with e.g. the LOTIS 6.5m
Collimator. The telescope includes simple systems to measure and correct mirror shape and alignment errors
incorporating technologies already proven on the LOTIS Collimator. We have sketched a notional earth-looking 4m
telescope concept combined with a wide field TMA concept into a DELTA IV or ATLAS 552 EELV fairing. We have
combined an initial analysis of launch and space performance of a special light-weighted honeycomb borosilicate mirror
(areal density 95 kg/m2) with public domain information on the existing launch vehicles.
The Discovery Channel Telescope (DCT) is a 4.3-meter telescope designed for dual optical configurations, featuring an
f/6.1 Ritchey-Chretien prescription with a 0.5° field-of-view, and a corrected f/2.3 prime focus with a 2° field-of-view.
The DCT Active Optics System (AOS) maintains collimation and mirror figure to provide seeing limited images across
the focal planes and rapid settling times to minimize observing overhead, using a combination of feed-forward and lowbandwidth
feedback control via wavefront sensing. Collimation is maintained by tip-tilt-piston control of the M2
assembly and articulating M1 within its cell, taking advantage of the 120 degree-of-freedom support used for figure
control. We present an overview of the AOS design and principles of operation, and a summary of progress and results
to date.
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.
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.
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 Large Binocular Telescope (LBT) is built around two lightweight borosilicate honeycomb mirrors which, at
8.4 meters in diameter, are the largest operational examples of this technology. Since the mirrors are relatively
stiff, the LBT mirror support system relies on passive position control and active force control. Passive position
control is performed by six extendable hardpoints organized as a truncated hexapod, which may be positioned
as required by the active optics control loop. The hardpoints rely on their axial stiffness to maintain the mirror
position against residual external disturbances. The active force control system minimizes the force exerted by
the hardpoints on the glass. Additionally, the axial component of the nominally uniform active support forces
can be perturbed to distort the mirror as required by the active optics control loop. Because of the relatively
large CTE of borosilicate glass, the differential temperature of the mirror is critical. Thus, the force control
system must support a 16 metric ton mirror using less than 100 Watts of electrical power. The authors present
a description of the primary mirror support system as implemented at the LBT. Initial stability problems made
the mirrors nearly unusable in freezing temperatures. The authors explain the reason for this instability and
describe the solutions implemented. Data demonstrating the current performance of the primary mirror support
system are also 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 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.
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.
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.
TOPS (Telescope to Observe Planetary Systems) is the first coronagraphic telescope concept designed specifically to
take advantage of Guyon's method of Phase Induced Amplitude Apodization PIAA).1 The TOPS primary mirror may
incorporates active figure control to help achieve the desired wavefront control to approximately 1 angstrom RMS accurate
across the spectral bandwidth. Direct correction of the primary figure avoids the need for a separate small deformable
mirror. Because of Fresnel propagation, correction at a separate surface can introduce serious chromatic errors unless it
is precisely conjugated to the primary. Active primary control also reduces complexity and mass and increases system
throughput, and will likely enable a full system test to the 10-10 level in the 1 g environment before launch. We plan to
use thermal actuators with no mechanical disturbance, using radiative heating or cooling fingers distributed inside the
cells of a honeycomb mirror. The glass would have very small but finite coefficient of expansion of ~ 5x10-8/C. Low
order modes would be controlled by front-to-back gradients and high order modes by local rib expansion and
contraction. Finite element models indicate that for a mirror with n cells up to n Zernike modes can be corrected to
better than 90% fidelity, with still higher accuracy for the lower modes. An initial demonstration has been made with a
borosilicate honeycomb mirror. Interferometric measurements show a single cell influence function with 300 nm stroke
and ~5 minute time constant.
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.
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.
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.
The 6.5m Multiple Mirror Telescope Observatory (MMTO) installed a new f/5 secondary system in April 2003. We describe the design and performance of the mirror cell and supports for the 1.7 m diameter Zerodur mirror. Pneumatic actuators divided into one lateral and three axial zones support this 318 kg mirror. The control feedback for the high bandwidth pressure transducers for these four zones is obtained from six load cells attached to rigid positioning rods. The mirror cell includes thermal control, force limiters, passive supports, installation and handling, and alignment metrology. Optical test results are described and compared to the original design specifications.
A concept design has been developed for the Giant Magellan Telescope (GMT). The project is a collaboration by a group of U.S. universities and research institutions to build a 21.5-meter equivalent aperture optical-infrared telescope in Chile. The segmented primary mirror consists of seven 8.4-meter diameter borosilicate honeycomb mirrors that will be cast by the Steward Observatory Mirror Laboratory. The fast primary optics allow the use of unusually compact telescope and enclosure structures. A wide range of secondary trusses has been considered for the alt-az mount. The chosen truss employs carbon fiber and steel and, due to its unique geometry, achieves high stiffness with minimal wind area and primary obscuration. The mount incorporates hydrostatic supports and a C-ring elevation structure similar in concept to those implemented on the Magellan 6.5-m and LBT dual 8.4-m telescopes. Extensive finite element analysis has been used to optimize the telescope structure, achieving a lowest telescope resonant frequency of ~5 Hz. The design allows for removal and replacement of any of the 7 subcells for off-telescope mirror coating with no risk to the other
mirrors. A wide range of instruments can be used which mount to the top or underside of a large instrument platform below the primary mirror cells. Large instruments are interchanged during the day while small and medium-sized instruments can be enabled quickly during the night. The large Gregorian instruments will incorporate astatic supports to minimize flexure and hysteresis.
The proposed science missions of the LSST require a telescope with an optical etendue of greater than 250 meters square degrees square. The current LSST Baseline Configuration has a field of view of 3.5 degrees and an optical etendue of 302 m2d2. The etendue calculation includes the effect of gradual vignetting by the camera as the field angle increases. A current optical point design includes spun cast light-weighted borosilicate mirrors (primary and tertiary) of 8.4 and 5 m diameter respectively. Thermal control systems are needed to optimize telescope seeing and to minimize the thermal distortion of the mirrors. The goals of this study are to determine the airflow requirements for the specified ambient temperature rate of change, to identify thermal time constants and to predict the magnitude and form of thermal distortions that can be developed by environmental conditions. Operational data taken at the 6.5 m MMT (Multi-Mirror Telescope Observatory) and at the Magellan Observatory are presented for comparison with this study. Finally, the results from the thermal analysis were used to simulate the LSST focus control over one night of observation and to estimate the effect on the image quality for different correction frequencies.
The Large Synoptic Survey Telescope (LSST) is an 8-meter class telescope with a proposed field of view between 3.0 and 3.5 degrees. The scientific goals of the survey establish a cadence that sets the telescope performance. The proposed cadence of the LSST telescope will typically require movements and settling of the telescope of approximately 3 degrees in 5 seconds. This dictates a high bandwidth to the telescope servo and thus a high locked rotor resonant frequency. In this study, the structure must accommodate three optical surfaces, the 8.4-meter primary, the 3-meter class secondary, and a 5-meter class tertiary in a long-tube configuration. The instrument must be accommodated in a "Trapped Focus" in the middle of the telescope. This imposes very stringent requirements on the structure and drives. This structure will require performance beyond the existing class of 8-meter telescopes. This can be achieved with the C-ring and azimuth platform concept demonstrated with the Large Binocular Telescope. The structure requires a low rotational inertia and a very high locked rotor resonant frequency at all angles of the sky. This is a challenging problem that can be overcome with this innovative solution.
The current LSST Baseline Configuration has a field of view of 3.5 degrees and an optical etendue of 302 meters square degrees square. The etendue calculation includes the effect of gradual vignetting by the camera as the field angle increases. A current optical point design includes an 8.4 m spun cast light-weighted borosilicate primary mirror, a 3.2 m secondary mirror and a 5.0 m tertiary mirror. The goal of this study is to determine if these mirrors can be actively supported and retain figure control over elevation angles without closed-loop control based on wave-front measurement. Support systems for the tertiary and primary mirrors are adapted from proven systems utilized on 6.5 and 8.4 m class primaries developed by the University of Arizona's Mirror Laboratory. The number and locations of axial and lateral supports is determined for each mirror and the gravitational and support induced surface distortions are calculated and are shown to be within budgeted limits. The support components and their performance are described and it is demonstrated that predicted mirror distortion attributable to the support system is consistent with the known performance of the support components.
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 University of Arizona has built a 2-m lightweight active
mirror prototype for the next generation of space telescopes. This
paper briefly reviews the mirror's opto-mechanical design, and it
describes the three different metrology systems that were used to
measure it during the actuation process. We also present a list
of lessons learned while working on this project. We conclude by
discussing one of the successful projects that has come out of
this technology.
Future space telescopes will require apertures that are larger than the current state of the art, yet fit within the exiting launch restrictions on size and mass. The mass can be reduced by using a thin flexible substrate for the optical surface and a rigid, lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. We have designed, built and tested a 0.5-m demonstration mirror for use in geosynchronous Earth-imaging systems. The mirror has an areal density of 5 kg/m2 and is the lightest mirror we have made using the thin substrate design. This paper discusses the design, fabrication and performance of the 0.5-m mirror.
Future space telescopes require primary mirrors that are much lighter than those currently being manufactured. They also must maintain optical tolerances while operating at cryogenic temperatures. We present a Mirror System Demonstrator for the Next Generation Space Telescope (NGST) that uses a thin glass facesheet with active control to achieve low mass and high surface quality. A 2-mm thick glass facesheet is controlled by miniature actuators and held together by a rigid carbon fiber frame. The 2-m diameter mirror system weighs only 13 kg/m2, including the glass, supports, actuators, support structure, and cabling. We present the status of the development and testing of this revolutionary mirror.
Lightweight mirrors for space can be made using a thin flexible substrate for the optical surface and a rigid lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. The University of Arizona is now in the final stages of fabricating two such mirrors. A 2-m NGST Mirror System Demonstrator, with an areal density of 13 kg/m2, is being built for NASA and will be tested at cryogenic temperatures. A 50 cm development mirror, with an areal density of only 5 kg/m2, is also being fabricated. This paper discusses the fabrication processes involved with both of these mirrors.
Lawrence Craig, Brian Cuerden, David Jacobson, Andrew Kissil, Gregory Mehle, Gary Mosier, Max Nein, Timothy Page, Kenneth Pitalo, David Redding, Steven Sutherlin, Gary Wilkerson
Advanced space telescopes which will eventually replace the Hubble Space Telescope will have 8 - 20 m diameter apertures. Primary mirrors of these dimensions will fold to fit into the space launcher. By necessity, these mirrors will be extremely lightweight and flexible. The historical approaches to mirror designs, where the mirror is made as rigid as possible to maintain figure and to serve as the anchor for the entire telescope, can no longer be applied. New design concepts and verification will depend entirely on analytical methods to predict optical performance. Integrated modeling of the structural, thermal, and optical performance of such mirrors is becoming the tool for advanced space mirror designs. This paper discusses some of the tasks and study results which are currently the basis for the design and integrated modeling studies of the Next Generation Space Telescope.
Future space telescopes rely on advances in technology to enable fabrication of primary mirrors with orders of magnitude more area, yet similar mass as current mirrors. This requires a shift of paradigm from the concept of the mirror as a rigid, stable unit, to the idea of the mirror as a system that uses active control to maintain the figure of a flexible surface. We discuss issues for this new class of optics and present status on a 2-m prototype mirror for NGST.
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.
A 2-m diameter mirror is being manufactured as a demonstration for NASA's Next Generation Space Telescope (NGST). This mirror meets the challenging requirements of cryogenic operation and very low mass using an active control system. The mirror system consists of an aluminized glass membrane, 2 mm thick. This membrane is supported and controlled based on wavefront measurements with 169 remotely driven actuators. The system rigidity is provided by a lightweight carbon fiber composite structure. This entire mirror system, 2 meters across weighs less than 40 kg, and will demonstrate 20 nm surface quality in a cryogenic test facility at 35 K.
A description is given for the geometry of the first generation adaptive-optical (AO) system being developed for the 6.5m single-mirror telescope that will replace the Multiple Mirror Telescope on Mt. Hopkins. The AO system consists of an adaptive secondary mirror with 330-actuators and a wave front senor composed of a 13 by 13 square subaperture array. The deformable mirror actuator vibrational modes were determined from a finite element model of the adaptive-secondary mirror. Numerical simulations in based on the finite element results were carried out to determine the expected performance of the system. Two reconstruction algorithms were compared - a least squares reconstructor and a modal equalization technique developed for this AO geometry. Strehl ratios are reported for the two algorithms for various guide star magnitudes and number of corrected wave front modes.
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.
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.
This document discusses material selection, design, and analysis of a composite gimbal for use on a high precision inertial guidance test table with active magnetic bearing suspension. The test table's system performance goals of 0.1 arc second angular pointing accury and one part per million angular rate stability, can only be achieved by using a gimbal with high specific stiffness, highly symmetric elastic properties, and high dimensional stability. These characteristics are achieved by proper selection of the ginthal's construction material, configuration, and fabrication processes. Both traditional and advanced composite materials are considered and evaluated for specific stiffness, coefficient of thermal expansion, thermal conductivity, dimensional stability, fabrication problems, and cost. Using the candidate materials, several gimbal configurations are evaluated with respect to the test table's system performance goals for angular pointing accuracy and angular rate stability. Specific gimbal design parameters affecting the system performance goals for angular pointing accuracy and angular rate stability include: the angular payload deflections due to torsional wind-up and asymmetrical stiffness; the linear payload deflections that cause torque disturbances and shaft wobble; and the natural frequencies affecting the control system bandwidths. Detailed finite element models of each configuration are used to predict the performance charteristics and demonstrate the advantages of the graphite/epoxy composite design.
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