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.
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 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.
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 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) 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 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.
Aspherical surfaces on lenses are difficult to produce and test. There are many innovative approaches used today to address the complexities of this process. The size, quality, and quantity of the lens to be produced dictate the best approach. The UCO/Lick Observatory Optical Lab has fabricated aspheric lenses to diameters over 12 inches using fabrication and testing techniques developed specifically for high quality, one-of-a-kind lenses, with axi-symmetric profiles and significant departures from sphere. This paper describes the manufacturing procedures used at UCO/Lick to fabricate aspheric lenses typical for today's astronomical applications.
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.
DEIMOS is a large multi-object spectrograph with an imaging mode that is being built for the W. M. Keck 2 Telescope. The detector is a 2 X 4 mosaic of eight 2048 X 4096 pixel CCDs. The mosaic assembly process must position the CCDs to be planar to within 5 microns rms. We describes the CCD support design and the measurements used to achieve this.
DEIMOS is a large multi-object spectrography with an imaging mode that is being built for the W. M. Keck 2 Telescope. The camera contains nine lens elements in five groups. The overall length of the camera and detector assembly is 0.67 meters, and the largest element is 0.33 meters in diameter. Typical centration and spacing tolerances are at the level of 25 microns. We describe the error budget, the design of the lens-supporting structure, and the assembly procedures.
Fabrication of the secondary mirrors for the W. M. Keck Telescopes required advances in techniques and tools for grinding, polishing, and testing. We describe the development and performance of those techniques and tools.
Instruments for large telescopes often require cameras with large, deeply-curved, and temperature-sensitive lenses. The instrument error budgets require each lens to be supported so that excellent performance is maintained in the face of gravitational and thermal perturbations. We describe here elastomeric mounts that address these requirements. We first describe the general design principles, the effects of errors in design and fabrication, and the performance under static and dynamic loads. We describe specific examples; the elastomer RTV560 and the lens supports for the camera of the W. M. Keck Observatory DEIMOS spectrograph.
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 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.
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 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.
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 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.
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.
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.
The ten meter diameter primary mirror of the W. M. Keck Telescope is a mosaic of thirty-six hexagonal mirrors. An active control system stabilizes the primary mirror. The active control system uses 168 measurements of the relative positions of adjacent mirror segments and 3 measurements of the'primary mirror position in the telescope structure to control the 108 degrees of freedom needed to stabilize the figure and position of the primary mirror. The components of the active control system are relative position sensors, electronics, computers, actuators that position the mirrors, and software. The software algorithms control the primary mirror, perform star image stacking, emulate the segments, store and fit calibration data, and locate hardware defects.
The Primary Mirror of the Keck Observatory Telescope is made up of an array of 36 hexagonal mirror segments under active control. The measurement of the relative orientations of the mirror segments is fundamental to their control. The mechanical and electronic design of the sensors used to measure these relative positions is described along with the performance of the sensors under a variety of tests. In use, the sensors will measure relative positions with a resolution of a few nanometers. This resolution and the low noise, drift and thermal sensitivity of the sensors are adequate to stabilize the primary mirror figure to the precision require for optical and infrared astronomy.