The new deployable tertiary mirror for the Keck I telescope (K1DM3) at the W. M. Keck Observatory has been assembled, tested and shipped to the telescope site, and is currently being installed. The mirror is capable of reflecting the beam to one of six positions around the telescope elevation ring or to retract out of the way to allow the use of Cassegrain instruments. This new functionality is intended to allow rapid instrument changes for transient event observations and improve telescope operations. This paper presents the final as-built design. Additionally, this paper presents detailed information about our alignment approach in the attempt to duplicate the instrument pointing orientation of the existing M3.
Motivated by the ever increasing pursuit of science with the transient sky (dubbed Time Domain Astronomy or TDA), we are fabricating and will commission a new deployable tertiary mirror for the Keck I telescope (K1DM3) at the W.M. Keck Observatory. This paper presents the detailed design of K1DM3 with emphasis on the opto- mechanics. This project has presented several design challenges. Foremost are the competing requirements to avoid vignetting the light path when retracted against a sufficiently rigid system for high-precision and repeatable pointing. The design utilizes an actuated swing arm to retract the mirror or deploy it into a kinematic coupling. The K1DM3 project has also required the design and development of custom connections to provide power, communications, and compressed air to the system. This NSF-MRI funded project is planned to be commissioned in Spring 2017.
The University of California Observatories will design and construct a deployable tertiary mirror (named K1DM3) for the Keck 1 telescope, which will complement technical and scientific advances in the area of time-domain astronomy. The K1DM3 device will enable astronomers to swap between any of the foci on Keck 1 in under 2 minutes, both to monitor varying sources (e.g. stars orbiting the Galactic center) and catch rapidly fading sources (e.g. supernovae, flares, gamma-ray bursts). In this paper, we report on the design development during our in-progress Preliminary Design phase. The design consists of a passive wiffle tree axial support system and a diaphragm lateral support system with a 5 arcminute field-of-view mirror. The mirror assembly is inserted into the light path with an actuation system and it relies on a kinematic mechanism for achieving repeatable, precise positioning. This project, funded by an NSF MRI grant, aspires to complete by the end of 2016.
We aim to build a new tertiary mirror (M3) and its mount for the 10 m Keck I (K1) telescope at the W. M. Keck
Observatory (WMKO) to make its full observational capabilities available for time-sensitive scientific programs.. In
contrast to the existing tertiary mirror and mount, the device will rapidly deploy and rotate the mirror to any instrument
at a Nasmyth focus or, as desired, stow the mirror out of the light path to permit observations at the Cassegrain focus. In
this manner, the K1 deployable tertiary mirror (K1DM3) will enable observations with any of the K1 instruments on any
given night, and at any given time. The K1DM3 device will be integrated within the K1 telescope control system and
WMKO has committed to a new operations model that takes full advantage of this new capability.
This paper presents refinements to the design of the TMT primary mirror segment passive-support system that are
effective in reducing gravity print-through and thermal distortion effects. First, a novel analytical method is presented
for tuning the axial and lateral support systems in a manner that results in improved optical performance when subject to
varying gravity fields. The method utilizes counterweights attached to the whiffletrees to cancel astigmatic and comatic
errors normally resulting when the lateral support system resists transverse loads induced by gravity. Secondly, several
central diaphragm designs are presented and analyzed to assess lateral-gravity and thermal distortion performance: 1) a
simple flat diaphragm, 2) a stress-relieving diaphragm having a slotted outer rim and a circumferential convolution near
the outside diameter, and 3) a flat diaphragm having a slotted outer rim. The latter design is chosen based on results from
analytical studies which show it to have better overall optical performance in the presence of gravity and thermal
The Thirty Meter Telescope (TMT) project, a partnership between ACURA, Caltech, and the University of California, is
currently developing a 30-meter diameter optical telescope. The primary mirror will be composed of 492 low expansion
glass segments. Each segment is hexagonal, nominally measuring 1.44m across the corners. Because the TMT primary
mirror is curved (i.e. not flat) and segmented with uniform 2.5mm nominal gaps, the resulting hexagonal segment
outlines cannot all be identical. All segmentation approaches studied result in some combination of shape and size
variations. These variations range from fractions of a millimeter to several millimeters. Segmentation schemes for the
TMT primary mirror are described in some detail. Various segmentation approaches are considered, with the goal being
to minimize various measures of shape variation between segments, thereby reducing overall design complexity and
cost. Two radial scaling formulations are evaluated for their effectiveness at achieving these goals. Optimal tuning of
these formulations and detailed statistics of the resulting segment shapes are provided. Finally, we present the rationale
used for selecting the preferred segmentation approach for TMT.
The Thirty Meter Telescope (TMT) Project will design and build a thirty-meter diameter telescope for research in
astronomy at optical and infrared wavelengths. TMT is a partnership between the University of California, Caltech, and
the Association of Canadian Universities for Research in Astronomy (ACURA). The $80 million TMT design and
development phase is fully funded and Preliminary Design is in progress. An additional $300 million has been pledged
towards early TMT construction which will commence in 2009. We include a high level description of the design of the
telescope and its planned adaptive optics and science instrumentation. The schedule of key milestones for completing
the design and construction is summarized.
Proc. SPIE. 7017, Modeling, Systems Engineering, and Project Management for Astronomy III
KEYWORDS: Optical transfer functions, Point spread functions, Telescopes, Thirty Meter Telescope, Spatial frequencies, Error analysis, Wavefronts, Computer simulations, Space telescopes, Systems modeling
We investigate a new metric, Normalized Point Source Sensitivity (PSSN), for characterizing the seeing limited
performance of the Thirty Meter Telescope. As the PSSN metric is directly related to the photometric error of
background limited observations, it truly represents the efficiency loss in telescope observing time. The PSSN
metric properly accounts for the optical consequences of wavefront spatial frequency distributions due to different
error sources, which makes it superior to traditional metrics such as the 80% encircled energy diameter. We
analytically show that multiplication of individual PSSN values due to individual errors is a good approximation
for the total PSSN when various errors are considered simultaneously. We also numerically confirm this feature
for Zernike aberrations, as well as for the numerous error sources considered in the TMT error budget using a
ray optics simulator, Modeling and Analysis for Controlled Optical Systems. We also discuss other pertinent
features of the PSSN including its relations to Zernike aberration and RMS wavefront error.
The Thirty Meter Telescope (TMT) project has been collecting data on five candidate sites since 2003. This paper
describes the site testing portion of the TMT site selection program and the process and standards employed
by it. This includes descriptions of the candidate sites, the process by which they were identified, the site
characterization instrument suite and its calibration and the available results, which will be published shortly.
The out-of-plane degrees of freedom (piston, tip, and tilt) of each of the 492 segments in the Thirty Meter Telescope
primary mirror will be actively controlled using three actuators per segment and two edge sensors along each intersegment
gap. We address two important topics for this system: edge sensor design, and the correction of fabrication and
The primary mirror segments are passively constrained in the three lateral degrees of freedom. We evaluate the segment
lateral motions due to the changing gravity vector and temperature, using site temperature and wind data, thermal
modeling, and finite-element analysis.
Sensor fabrication and installation errors combined with these lateral motions will induce errors in the sensor readings.
We evaluate these errors for a capacitive sensor design as a function of dihedral angle sensitivity. We also describe
operational scenarios for using the Alignment and Phasing System to correct the sensor readings for errors associated
with fabrication and installation.
I describe the design of the Thirty Meter Telescope, a project to build a ground-based thirty-meter telescope. The
partners include the University of California, Caltech, the Association of Canadian Universities for Research in
Astronomy (ACURA), and NSF. The Project is currently in the design and development phase and will be ready for a
2009 construction start.
This paper describes the studies performed to establish a baseline conceptual design of the Segment Support Assembly
(SSA) for the Thirty Meter Telescope (TMT) primary mirror. The SSA uses a combination of mechanical whiffletrees
for axial support, a central diaphragm for lateral support, and a whiffletree-based remote-controlled warping harness for
surface figure corrections. Axial support whiffletrees are numerically optimized to minimize the resulting gravityinduced
deformation. Although a classical central diaphragm solution was eventually adopted, several lateral support
concepts are considered. Warping harness systems are analyzed and optimized for their effectiveness at correcting
second and third order optical aberrations. Thermal deformations of the optical surface are systematically analyzed
using finite element analysis. Worst-case performance of the complete system as a result of gravity loading and
temperature variations is analyzed as a function of zenith angle using an integrated finite element model.
The most common detector configuration for Shack Hartmann (SH) wavefront sensors used for adaptive optics (AO)
wavefront sensing is the quad cell. Advances in detectors, such as the CCDs being developed in a project on which we
are collaborators (funded by the Adaptive Optics Development Program), make it possible to use larger pixel arrays.
The CCD designs incorporate improved read amplifiers and novel pixel geometries optimized for laser guide star (LGS)
AO wavefront sensing. While it is likely that finer sampling of the SH spot will improve the ability of the wavefront
sensor to accurately determine the spot displacement, particularly for elongated or aberrated spots such as those seen in
LGS AO systems, the optimal sampling is not dependent simply on the number of pixels but must also take into account
the effects of photon and detector noise. The performance of a SH wavefront sensor also depends on the performance
of the algorithm used to find the spot displacement. In the literature alternatives have been proposed to the common
center of mass algorithm, but these have not been simulated in detail. In this paper we will describe the results of our
study of the performance of a SH wavefront sensor with a well sampled spot. We will present results for simulations of
the wavefront sensor that enable us to optimize the design of the detector for varying conditions of signal to noise and
spot elongation. We will also discuss the application of correlation algorithms to SH wavefront sensors and present
results regarding the performance and statistics of this algorithm.
The Thirty Meter Telescope (TMT) project has chosen a reference configuration with the telescope elevation axis above the primary mirror. The TMT telescope design has a segmented primary mirror, with 738 segments, nominally 1.2 m across corners, and it uses an articulated tertiary mirror to feed science light to predefined instrument positions on two large Nasmyth platforms. This paper outlines the development of the telescope structural design to meet the motion requirements related to the image quality error budget. The usage of opto-structural performance evaluation tools such as Merit Function Routine are described in addition with the optimization techniques used during the telescope structure design development.
The Thirty Meter Telescope (TMT) is a collaborative project between the California Institute of Technology
(CIT), the University of California (UC), the Association of Universities for Research in Astronomy (AURA),
and the Association of Canadian Universities for Research in Astronomy (ACURA). The Alignment and Phasing
System (APS) for the Thirty Meter Telescope will be a Shack-Hartmann type camera that will provide a variety
of measurements for telescope alignment, including segment tip/tilt and piston, segment figure, secondary and
tertiary figure, and overall primary/secondary/tertiary alignment. The APS will be modeled after the Phasing
Camera System (PCS), which performed most, but not all, of these tasks for the Keck Telescopes. We describe
the functions of the APS, including a novel supplemental approach to measuring and adjusting the segment
figures, which treats the segment aberrations as global variables.
The Thirty Meter Telescope project will design and build a thirty-meter diameter telescope for research in astronomy at optical and infrared wavelengths. The highly segmented primary mirror will use edge sensors to align and stabilize the relative piston, tip, and tilt degrees of freedom of the segments. We describe an edge sensor conceptual design and relate the sensor errors to the performance of the telescope as whole. We discuss the sensor calibration, installation, maintenance, and reliability.
The Thirty Meter Telescope (TMT) Project will design and build a thirty-meter diameter telescope for research in
astronomy at optical and infrared wavelengths. TMT is a partnership between the University of California, Caltech,
Association of Canadian Universities for Research in Astronomy (ACURA), and the Association of Universities for
Research in Astronomy (AURA). The TMT design and development phase is funded and work is underway. We
include a high level description of the design of the telescope and its planned adaptive optics and science
instrumentation. The organizational structure of the project is summarized along with the schedule of key milestones in
the design. We are carrying out key conceptual and cost reviews in 2006 and will be prepared to begin construction in
2009, with first light in 2015.
The California Extremely Large Telescope, CELT, is a proposed 30-m telescope. Choosing the best possible site for CELT is essential in order to extract the best science from the observations and to reduce the complexity of the telescope. Site selection is therefore currently one of the most critical pacing items of the CELT project. In this paper, we first present selected results from a survey of the atmospheric transparency at optical and infrared wavelengths over the southwestern USA and northern Mexico using satellite data. Results of a similar study of South America have been reported elsewhere. These studies will serve as the pre-selection criterion of the sites at which we will perform on-site testing. We then describe the current status of on-site turbulence evaluation efforts and the future plans of the CELT site testing program.
The California Extremely Large Telescope (CELT) is a joint project of the University of California and the California Institute of Technology to build and operate a 30-meter diameter telescope for research in astronomy at visible and infrared wavelengths. The current optical design calls for a primary, secondary, and tertiary mirror with Ritchey-Chretién foci at two Nasmyth platforms. The primary mirror is a mosaic of 1080 actively stabilized hexagonal segments. This paper summarizes the recent progress on the conceptual design of this telescope.
The California Extremely Large Telescope (CELT) is a project to build a 30-meter diameter telescope for research in astronomy at visible and infrared wavelengths. The current optical design calls for a primary, secondary, and tertiary mirror with Ritchey-Chretién foci at two Nasmyth platforms. The primary mirror is a mosaic of 1080 actively-stabilized hexagonal segments. This paper summarizes a CELT report that describes a step-by-step procedure for aligning the many degrees of freedom of the CELT optics.
The California Extremely Large Telescope is a study currently underway by the University of California and the California Institute of Technology, to assess the feasibility of building a 30-m ground based telescope that will push the frontiers to observational astronomy. The telescope will be fully steerable, with a large field of view, and be able to work in both a seeing-limited arena and as a diffraction-limited telescope, with adaptive optics.
We explore the issues in the control and alignment of the primary mirror of the proposed 30 meter California Extremely Large Telescope and other very large telescopes with segmented primaries (consisting of 1000 or more segments). We show that as the number of segments increases, the noise in the telescope active control system (ACS) increases, roughly as (root)n. This likely means that, for a thousand segment telescope like CELT, Keck-style capacitive sensors will not be able to adequately monitor the lowest spatial frequency degrees of freedom of the primary mirror, and will therefore have to be supplemented by a Shack-Hartmann-type wavefront sensor. However, in the case of segment phasing, which is governed by a `control matrix' similar to that of the ACS, the corresponding noise is virtually independent of n. It follows that reasonably straightforward extensions of current techniques should be adequate to phase the extremely large telescopes of the future.
The primary mirror of the proposed California Extremely Large Telescope is a 30-meter diameter mosaic of hexagonal segments. An initial design calls for about a thousand segments with a hexagon side length of 0.5 meters, a primary-mirror focal ratio of 1.5, and a segment surface quality of about 20 nanometers rms. We describe concepts for fabricating these segments.
The primary mirror of the proposed California Extremely Large Telescope is a 30-meter diameter mosaic of hexagonal segments. The primary mirror active control will be achieved using four systems: sensors, actuators, processor, and alignment camera. We describe here the basic requirements of sensors and actuators, sketch a sensor design, and indicate interesting actuator alternatives.
California Institute of Technology and University of California have begun conceptual design studies for a new telescope for astronomical research at visible and infrared wavelengths. The California Extremely Large Telescope (CELT) is currently envisioned as a filled-aperture, steerable, segmented telescope of approximately 30 m diameter. The key to satisfying many of the science goals of this observatory is the availability of diffraction-limited wavefront control. We describe potential observing modes of CELT, including a discussion of the several major outstanding AO system architectural design issues to be resolved prior to the initiation of the detailed design of the adaptive optics capability.
All Cassegrain spectrographs suffer from gravitationally- induced flexure to some degree. While such flexure can be minimized via careful attention to mechanical design and fabrication, further performance improvements can be achieved if the spectrograph has been designed to minimize hysteresis and has active compensation for any residual flexure. The Echellette Spectrograph and Imager (ESI), built at UCO/Lick Observatory for use at Cassegrain focus on Keck II, compensates for such residual flexure via its collimator mirror. The collimator is driven by three actuators that provide control of collimator focus, tip, and tilt. The ESI control software utilizes a mathematical model of gravitationally-induced flexure to periodically compute and apply flexure corrections by commanding the corresponding tip and tilt motions to the collimator. In addition, the ESI control software provides an optional, manual, closed-loop method for adjusting the collimator position to compensate for any non-repeatable errors. Such errors may result from mechanical hysteresis or from thermally-induced structural deformations of the instrument and are thus not accounted for by the gravitational flexure model. This method relies on measuring the centroid position of fiducial spots within each echellete image. The collimator is adjusted so that the positions of these spots match those in a reference image. These spots are produced by a small round hole in the slit mask located near one end of the slit. We discuss the design and calibration of this flexure compensation system and report on its performance ont he telescope.
The Echellete Spectrograph and Imager (ESI), currently being completed for use at the cassegrain focus of the Keck II telescope, employs two moderate size translating fold mirrors. These mirrors are used to shift between the three instrument modes; medium resolution echellete mode; low resolution prismatic mode; and imaging mode. In order to maintain the optical stability and calibration of these three modes the mirrors must be removed and repeatably located to within 1.3 arcsecs of tip and tilt. In addition, the mirrors must maintain a fixed orientation relative to the telescope axis under a variety of gravity and thermal loads. In this paper we describe a novel concept for moving and locating these mirrors. Analytical analysis of the mounts is presented. Optical and mechanical testing is described.
The Echellette Spectrograph and Imager (ESI) is being built at UCO/Lick Observatory for the Cassegrain focus of the Keck II telescope. The collimator mirror is optimally constrained by a space-frame structure. It will be actively moved to provide the focus and flexure (tip and tilt) control for the instrument. Careful attention to space-frame geometry has simplified the mechanical design. Analytical and Finite Element Analysis (FEA) are presented to demonstrate how a simple but very stiff structure is used to provide support, flexure control, and focus.
The Echellette Spectrograph and Imager (ESI), currently being developed for use at the Cassegrain focus of the Keck II 10-m telescope, employs two large (25 kg) prisms for cross dispersion. In order to maintain optical stability in the spectroscopic modes, these prisms must maintain a fixed angle relative to the nominal spectrograph optical axis under a variety of flexural and thermal loads. In this paper, we describe a novel concept for mounting large prisms that has been developed to address this issue. Analytical and finite element analyses (FEA) of the mounts are presented. Optical and mechanical tests are also described.
The Echellette Spectrograph and Imager (ESI) is one of several second-generation instruments for the Keck telescopes. The motivation for the f/15 Cassegrain-mounted instrument has been to provide a versatile, extremely efficient, and stable system for faint object spectroscopy and imaging, on a comparatively limited schedule and budget. In keeping with these goals, a space-frame instrument structure has been designed, analyzed, and fabricated. The mainframe structure provides the mechanical interface between the telescope and instrument, support points for all the optical, mechanical, and electronic sub-systems, and provides a rigid base for the active- collimator flexure control system. The fundamental concepts and motivation for using a space-frame are discussed, and their application to the design, analysis, and fabrication of the ESI structure is presented.
We briefly review the existing instruments at the first Keck telescope and their performance characteristics. These include the high resolution echelle spectrograph (HIRES), the low resolution imaging spectrograph (LRIS), and the near infrared camera (NIRC). Other Keck 1 instruments include the long wavelength spectrograph (LWS) and long wavelength imaging camera (LWIRC). The instruments currently being developed for the second Keck telescope are described and their expected performance characteristics are described. These include the deep imaging multi-object spectrograph (DEIMOS), the near infrared echelle spectrograph (NIRSPEC), the echelle spectrograph and imager (ESI), the diffraction limited near infrared camera (NIRC-2), and the ultraviolet side of the LRIS (LRIS-B). Keck 2 will also have a major new facility, an adaptive optics (AO) system. This system will deliver diffraction limited images in the 1 - 5 micron region and will be used in front of the NIRC-2. This AO system will contain a laser to generate an artificial sodium star, thus giving AO essentially full sky coverage. The AO system design and status is summarized. Keck Observatory is also planning an interferometer using the Keck 1 and Keck 2 telescopes, with a baseline of 85 m. We describe the plans and progress on this adaptive optics augmented infrared interferometer.
The performance goals of the telescope are reviewed and compared with the achieved values. The optical performance is close to the original goals, but our initial observations support evidence from other observatories that Mauna Kea seeing is even better than was assumed in setting the goals; so it is important not to lower our aims. The primary mirror active control system performance is summarized as well as the pointing the tracking performance. The telescope is substantially operational and since January 1994 we have been devoting the majority of nights to astronomical science.
Astronomical observations are now taking place on the Keck I telescope on a regular basis. We summarize here the status of the Keck I and II optics, and the current wavefront and image quality of the Keck I telescope as measured by in-telescope optical tests. Shack-Hartmann measurements of the individual primary mirror segments yield 80% encircled energy diameters that vary from 0.31 to 0.60 arc seconds. Full width at half maximum measurements of direct segment images obtained on a night of excellent seeing varied from 0.32 to 0.51 arcsec, and the combined image was 0.42 arcsec.
The active mirror control system of the W.M. Keck telescope maintains the optical figure of the segmented primary mirror under the changing influences of gravity and temperature. The ultimate performance of the system depends on the size of the calibration errors and on its stability. The design error budget calls for the calibrated mirror control system to contribute an image blur less than 0.1 arc seconds (80% enclosed energy) over the full range of operating conditions.
The achieved pointing and tracking performance of the telescope is presented and compared with the Keck goals. The implications of the current performance on observing are discussed, and planned remedies for deficiencies in pointing and tracking are proposed.
The segmented design of the W. M. Keck Telescope primary mirror places several unique demands upon the alignment and adjustment of the telescope optics. These include: (1) careful determination of the optical figures of individual segments (to provide input data for warping harness adjustment), (2) control of the two tilt degrees of freedom for each of the thirty-six primary mirror segments, and (3) phasing or control of the piston degree of freedom for each of these segments. In addition, (4) the proper alignment of the secondary with respect to the primary, although it is a requirement common to monolithic and segmented telescopes alike, is a more subtle and complicated task for the latter because the optic axis of the primary is not readily defined. These four tasks are performed at Keck by the Phasing Camera System.
The status and plans for a multi-phase program to build adaptive optics (AO) user facilities for one of the Keck telescopes is presented. The planned facilities include (1) fast tip/tilt correction, (2) near infrared AO with natural stars, and potentially (3) a near infrared AO facility with a single laser beacon. Description of these facilities and their implementation on the telescope are described. In addition, descriptions of the current and future suite of scientific instruments that would take advantage of adaptive optics are provided. Problems and concerns associated with implementing adaptive optics facilities at Keck (e.g., a segmented primary, a 10 meter baseline, rotation of a non-symmetric pupil, etc.) are discussed.
We discuss issues in optimizing the design of adaptive optics and laser guide star systems for the Keck Telescope. The initial tip-tilt system will use Keck's chopping secondary mirror. We describe design constraints, choice of detector, and expected performance of this tip-tilt system as well as its sky coverage. The adaptive optics system is being optimized for wavelengths of 1 - 2.2 micrometers . We are studying adaptive optics concepts which use a wavefront sensor with varying numbers of subapertures, so as to respond to changing turbulence conditions. The goal is to be able to `gang together' groups of deformable mirror subapertures under software control, when conditions call for larger subapertures. We present performance predictions as a function of sky coverage and the number of deformable mirror degrees of freedom. We analyze the predicted brightness of several candidate laser guide star systems, as a function of laser power and pulse format. These predictions are used to examine the resulting Strehl as a function of observing wavelength. We discuss laser waste heat and thermal management issues, and conclude with an overview of instruments under design to take advantage of the Keck adaptive optics system.
We have installed a high-speed camera system at the Keck Telescope, to be used for studying and monitoring atmospheric seeing as well as for telescope diagnostic purposes. This instrument, which consists of a Dalsa camera with a 64 X 64 pixel CCD, a 4 Megabyte Epix frame grabber, and a 486 computer, records sequences of 1248 frames at 181 Hz and 0.2 arcsecond resolution. We note that the Keck Telescope, by virtue of its 10 meter baseline as well as its ability to separate images formed by any or all of its 36 primary mirror segments, is ideally suited to seeing studies, in particular to those involving relatively long baselines and aperture-aperture correlations of wavefront aberrations. We present power spectra for atmospheric wavefront tilts for the primary mirror segments. In general they show the power law frequency dependance expected on theoretical grounds. However the measured segment-to-segment correlations are systematically smaller than theory predicts by a significant factor. It is possible that this effect is a manifestation of a finite outer scale of turbulence.
The W.M. Keck Observatory and its Ten-Meter Telescope are nearing completion at the summit of Mauna Kea. The 10-m diameter primary mirror has a 17.5-m focal length and is composed of 36 hexagonal segments. There will be seven Ritchey-Chretien f/15 foci: two of them at Nasmyth foci, one at Cassegrain focus, and four at bent Cassegrain foci on the elevation ring. There will also be an f/25 IR focal plane at the intersection of the optical and elevation axes, whose focus will be chopped by a beryllium secondary mirror. Image quality with a FWHM of the order of about 0.25 arcsec, and an 80-percent enclosed energy diameter of about 0.40 arcsec, are anticipated.
In order to reduce polishing costs and correct unexpected errors in fabrication and polishing, the support of very large optics can be actively enlisted in telescope mirror optical figure adjustment. A set of leaf springs is used by the Keck Ten-Meter Telescope to apply moments about the pivots of the mirror mosaics' whiffletree support. The springs successfully reduce the polished rms surface error by a factor of 6 to 15, while reducing the 80-percent enclosed energy diameter by a factor of 2.5-6.0. Additional current limitations on figure improvement include the difficulties of polishing higher spatial frequencies and predicting warping during mirror fabrication.