The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, echelle spectrometer that specializes in the discovery and characterization of exoplanets using Doppler spectroscopy. In designing KPF, the guiding principles were high throughput to promote survey speed and access to faint targets, and high stability to keep uncalibrated systematic Doppler measurement errors below 30 cm s−1. KPF achieves optical illumination stability with a tip-tilt injection system, octagonal cross-section optical fibers, a double scrambler, and active fiber agitation. The optical bench and optics with integral mounts are made of Zerodur to provide thermo-mechanical stability. The spectrometer includes a slicer to reformat the optical input, green and red channels (445–600 nm and 600–870 nm), and achieves a resolving power of ∼97,000. Additional subsystems include a separate, medium-resolution UV spectrometer (383–402 nm) to record the Ca II H & K lines, an exposure meter for real-time flux monitoring, a solar feed for sunlight injection, and a calibration system with a laser frequency comb and etalon for wavelength calibration. KPF was installed and commissioned at the W. M. Keck Observatory in late 2022 and early 2023 and is now in regular use for scientific observations. This paper presents an overview of the as-built KPF instrument and its subsystems, design considerations, and initial on-sky performance.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development at the UC Berkeley Space Sciences Laboratory for the W.M. Keck Observatory. KPF is designed to characterize exoplanets via Doppler spectroscopy with a goal of a single measurement precision of 0.3 m s-1 or better, however its resolution and stability will enable a wide variety of astrophysical pursuits. Here we provide post-preliminary design review design updates for several subsystems, including: the main spectrometer, the fabrication of the Zerodur optical bench; the data reduction pipeline; fiber agitator; fiber cable design; fiber scrambler; VPH testing results and the exposure meter.
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.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development for the W.M. Keck Observatory. The instrument recently passed its preliminary design review and is currently in the detailed design phase. KPF is designed to characterize exoplanets using Doppler spectroscopy with a single measurement precision of 0.5 m s−1 or better; however, its resolution and stability will enable a wide variety of other astrophysical pursuits. KPF will have a 200 mm collimated beam diameter and a resolving power greater than 80,000. The design includes a green channel (445 nm to 600 nm) and red channel (600 nm to 870 nm). A novel design aspect of KPF is the use of a Zerodur optical bench, and Zerodur optics with integral mounts, to provide stability against thermal expansion and contraction effects.
The Keck Cosmic Web Imager (KCWI) is a new facility instrument being developed for the W. M. Keck Observatory and funded for construction by the Telescope System Instrumentation Program (TSIP) of the National Science Foundation (NSF). KCWI is a bench-mounted spectrograph for the Keck II right Nasmyth focal station, providing integral field spectroscopy over a seeing-limited field up to 20" x 33" in extent. Selectable Volume Phase Holographic (VPH) gratings provide high efficiency and spectral resolution in the range of 1000 to 20000. The dual-beam design of KCWI passed a Preliminary Design Review in summer 2011. The detailed design of the KCWI blue channel (350 to 700 nm) is now nearly complete, with the red channel (530 to 1050 nm) planned for a phased implementation contingent upon additional funding. KCWI builds on the experience of the Caltech team in implementing the Cosmic Web Imager (CWI), in operation since 2009 at Palomar Observatory. KCWI adds considerable flexibility to the CWI design, and will take full advantage of the excellent seeing and dark sky above Mauna Kea with a selectable nod-and-shuffle observing mode. In this paper, models of the expected KCWI sensitivity and background subtraction capability are presented, along with a detailed description of the instrument design. The KCWI team is lead by Caltech (project management, design and implementation) in partnership with the University of California at Santa Cruz (camera optical and mechanical design) and the W. M. Keck Observatory (program oversight and observatory interfaces). The optical design of the blue camera for the Keck Cosmic Web Imager (KCWI) by Harland Epps of the University of California, Santa Cruz is a lens assembly consisting of eight spherical optical elements. Half the elements are calcium fluoride and all elements are air spaced. The design of the camera barrel is unique in that all the optics are secured in their respective cells with an RTV annulus without additional hardware such as retaining rings. The optical design and the robust lens mounting concept has allowed UCO/Lick to design a straightforward lens camera assembly. However, alignment sensitivity is a strict 15 μm for most elements. This drives the fabrication, assembly, and performance of the camera barrel.
We present progress in efforts underway at the University of California Observatories to develop high performance durable silver-based mirror coatings for telescope and instruments. Silver-based coatings are extremely prone to tarnish and/or corrosion, and successful coatings depend not only on the materials used but also the deposition processes employed. Our physical vapor deposition (PVD) chamber allows both sputtering and ion-assisted e-beam depositions for head-to-head comparison of deposition processes, and we present results of these comparisons. In this paper, we review the problem and discuss our recent activities and findings. We discuss a systematic study to determine which oxides, nitrides and fluorides provide the best protection in environmental tests. We present initial results into the effects of stress in our specific thin films, and thee effects of stress on mirror coating durability. We also discuss studies using Atomic Layer Deposition (ALD) over-coating of Ag, and we describe a large ALD research chamber currently under construction that will demonstrate ALD processes on larger substrates (70 cm diameter).
KEYWORDS: Mirrors, Telescopes, Astronomy, Calibration, Sensors, Distortion, Data modeling, Spectroscopy, James Webb Space Telescope, Magnetic resonance imaging
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 Lick Observatory's Shane 3-meter telescope has been upgraded with a new infrared instrument (ShARCS - Shane Adaptive optics infraRed Camera and Spectrograph) and dual-deformable mirror adaptive optics (AO) system (ShaneAO). We present first-light measurements of imaging sensitivity in the Ks band. We compare mea- sured results to predicted signal-to-noise ratio and magnitude limits from modeling the emissivity and throughput of ShaneAO and ShARCS. The model was validated by comparing its results to the Keck telescope adaptive optics system model and then by estimating the sky background and limiting magnitudes for IRCAL, the pre- vious infra-red detector on the Shane telescope, and comparing to measured, published results. We predict that the ShaneAO system will measure lower sky backgrounds and achieve 20% higher throughput across the JHK bands despite having more optical surfaces than the current system. It will enable imaging of fainter objects (by 1-2 magnitudes) and will be faster to reach a fiducial signal-to-noise ratio by a factor of 10-13. We highlight the improvements in performance over the previous AO system and its camera, IRCAL.
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 describe the design and first-light early science performance of the Shane Adaptive optics infraRed Camera- Spectrograph (ShARCS) on Lick Observatory’s 3-m Shane telescope. Designed to work with the new ShaneAO adaptive optics system, ShARCS is capable of high-efficiency, diffraction-limited imaging and low-dispersion grism spectroscopy in J, H, and K-bands. ShARCS uses a HAWAII-2RG infrared detector, giving high quantum efficiency (<80%) and Nyquist sampling the diffraction limit in all three wavelength bands. The ShARCS instrument is also equipped for linear polarimetry and is sensitive down to 650 nm to support future visible-light adaptive optics capability. We report on the early science data taken during commissioning.
A Cassegrain mounted adaptive optics instrument presents unique challenges for opto-mechanical design. The flexure and temperature tolerances for stability are tighter than those of seeing limited instruments. This criteria requires particular attention to material properties and mounting techniques. This paper addresses the mechanical designs developed to meet the optical functional requirements. One of the key considerations was to have gravitational deformations, which vary with telescope orientation, stay within the optical error budget, or ensure that we can compensate with a steering mirror by maintaining predictable elastic behavior. Here we look at several cases where deformation is predicted with finite element analysis and Hertzian deformation analysis and also tested. Techniques used to address thermal deformation compensation without the use of low CTE materials will also be discussed.
We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to
develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously
identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and
barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed,
and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for
the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter
deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope
mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range
instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope
at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition
(ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are
smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely
promising for protecting silver from corrosion.
The Keck Cosmic Web Imager (KCWI) is a new facility instrument being developed for the W. M. Keck Observatory
and funded for construction by the Telescope System Instrumentation Program (TSIP) of the National Science
Foundation (NSF). KCWI is a bench-mounted spectrograph for the Keck II right Nasmyth focal station, providing
integral field spectroscopy over a seeing-limited field up to 20"x33" in extent. Selectable Volume Phase Holographic
(VPH) gratings provide high efficiency and spectral resolution in the range of 1000 to 20000. The dual-beam design of
KCWI passed a Preliminary Design Review in summer 2011. The detailed design of the KCWI blue channel (350 to
700 nm) is now nearly complete, with the red channel (530 to 1050 nm) planned for a phased implementation contingent
upon additional funding. KCWI builds on the experience of the Caltech team in implementing the Cosmic Web Imager
(CWI), in operation since 2009 at Palomar Observatory. KCWI adds considerable flexibility to the CWI design, and will
take full advantage of the excellent seeing and dark sky above Mauna Kea with a selectable nod-and-shuffle observing
mode. In this paper, models of the expected KCWI sensitivity and background subtraction capability are presented,
along with a detailed description of the instrument design. The KCWI team is lead by Caltech (project management,
design and implementation) in partnership with the University of California at Santa Cruz (camera optical and
mechanical design) and the W. M. Keck Observatory (program oversight and observatory interfaces).
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.
The Lick Observatory 3-meter telescope has a history of serving as a testbed for innovative adaptive optics techniques.
In 1996, it became one of the first astronomical observatories to employ laser guide star (LGS) adaptive optics as a
facility instrument available to the astronomy community. Work on a second-generation LGS adaptive optics system,
ShaneAO, is well underway, with plans to deploy on telescope in 2013. In this paper we discuss key design features and
implementation plans for the ShaneAO adaptive optics system. Once again, the Shane 3-m will host a number of new
techniques and technologies vital to the development of future adaptive optics systems on larger telescopes. Included is a
woofer-tweeter based wavefront correction system incorporating a voice-coil actuated, low spatial and temporal
bandwidth, high stroke deformable mirror in conjunction with a high order, high bandwidth MEMs deformable mirror.
The existing dye laser, in operation since 1996, will be replaced with a fiber laser recently developed at Lawrence
Livermore National Laboratories. The system will also incorporate a high-sensitivity, high bandwidth wavefront sensor
camera. Enhanced IR performance will be achieved by replacing the existing PICNIC infrared array with an Hawaii
2RG. The updated ShaneAO system will provide opportunities to test predictive control algorithms for adaptive optics.
Capabilities for astronomical spectroscopy, polarimetry, and visible-light adaptive optical astronomy will be supported.
We report on the on-going effort at University of California Observatories Astronomical Coatings Lab to develop robust
protected-silver coatings suitable for telescope mirrors. We have identified a very promising recipe based on YF3 that
produces excellent reflectivity at wavelengths of 340 nm and greater, has ~1.5% emissivity in the thermal IR, and does
not contain problematic materials for the Mid-IR, such as SiO2 and Al2O3. The recipe holds up extremely well to
aggressive environmental testing (80C and 80% RH; high-H2S atmosphere), and currently is being evaluated under real
observatory conditions. This coating may satisfy the need for telescope mirror coatings that are long-lasting (~5 years or
more) and have good reflectivity into the UV. We also evaluate and compare some other silver-based coatings developed
elsewhere that should be useful in the same role.
In addition, we describe recent upgrades to our coating facilities allowing us to deposit ion-assisted e-beam coatings on
optics up to ~1m. This novel arrangement places the e-gun and ion source on a pivoting "swing-arm", allowing the
position to move radially without changing the e-gun/ion source/ substrate geometry. Large substrates can be coated with
good uniformity using single-axis rotation only. This technique is scalable to arbitrarily large substrate sizes.
We present an update on efforts at University of California Observatories to develop improved optical coatings for
astronomical telescopes and instruments. The main thrust has been in the areas of protected silver mirror coatings and
sol-gel based anti-reflection coatings. We report on the performance of silver coatings used for several years in Keck and
Lick instruments, as well as that on the Lick 1-m telescope. We discuss process improvements, including use of reactive
ion-assisted deposition of oxides. Sol-gel based AR coatings have been exposed to cryogenic environments to test their
suitability for IR instruments, with encouraging results. Finally, we describe our plans for future work.
A mosaic of two 2k x 4k fully depleted, high resistivity CCD
detectors was installed in the red channel of the Low Resolution
Imaging Spectrograph for the Keck-I Telescope in June, 2009 replacing
a monolithic Tektronix/SITe 2k x 2k CCD. These CCDs were fabricated
at Lawrence Berkeley National Laboratory (LBNL) and packaged and
characterized by UCO/Lick Observatory. Major goals of the detector
upgrade were increased throughput and reduced interference fringing
at wavelengths beyond 800 nm, as well as improvements in the
maintainability and serviceability of the instrument. We report on
the main features of the design, the results of optimizing detector
performance during integration and testing, as well as the
throughput, sensitivity and performance of the instrument as
characterized during commissioning.
There is a continued need for efficient reflective and anti-reflection (AR) coatings for increasingly large optics in
astronomy. The requirements for these coatings differ in several respects from those developed for commercial use. In
general, they require a broad spectral coverage, high-efficiency, long life under semi-exposed conditions, and the ability
to be removed without damage to expensive substrates. UCO/Lick Observatory has undertaken an effort to develop
improved coatings for astronomical optics. In this paper, we report on progress toward (a) robust protected silver
coatings for telescopes; (b) enhanced silver and aluminum coatings for instruments; and (c) hardened sol-gel AR
coatings. Examples of some of our new coatings are in use at Lick and Keck Observatories. The problems involved in
successful coatings are multifaceted and we summarize our major findings to date. This includes our requirements, test
procedures, and performance and durability results for the three types of coatings mentioned.
We briefly describe the design, construction and performance of the recently-commissioned Atmospheric Dispersion
Corrector (ADC) for the Keck-I Cassegrain focus. This is a "longitudinal" ADC with fused silica prisms slightly over 1-
meter in diameter, designed to operate at zenith distances up to 60 degrees and over a 20 arcminute field-of-view with
negligible impact on image quality and throughput. It provides dispersion compensation from 0.31 to 1.1 microns. The
sol-gel-based antireflection coatings were a major technological challenge, and we encountered some previously
unrecognized performance consequences of the LADC design which should be considered before adopting this design.
We describe the design and construction of the Atmospheric Dispersion Corrector (ADC) for the Keck-I Cassegrain focus. This is a "linear" or "longitudinal" ADC with fused silica prisms slightly over 1-meter in diameter. It is designed to operate at zenith distances up to 60 degrees and over a 20 arcminute field-of-view with negligible impact on image quality and throughput, and to provide dispersion compensation from 0.31 to 1.1 microns. During the design phase, it was realized that the LADC design effectively displaces the optical axis of the telescope as the prisms separate, leading to (a) a tilting of the focal surface, and (b) a change in telescope pointing. Both effects can have significant consequences, particularly for off-axis instruments, and should be carefully considered in selecting this ADC design. We also discuss in some detail the broad-band anti-reflection coatings, which consist of silica Sol-gel over MgF2. The Keck ADC is currently undergoing final assembly and testing at the UCO/Lick Observatory Instrument Labs, and will be commissioned in late 2006.
The DEIMOS spectrograph is a multi-object spectrograph being built for Keck II. DEIMOS was delivered in February 2002, became operational in May, and is now about three-quarters of the way through its commissioning period. This paper describes the major problems encountered in completing the spectrograph, with particular emphasis on optical quality and image motion. The strategies developed to deal with these problems are described. Overall, commissioning is going well, and it appears that DEIMOS will meet all of its major performance goals.
Two recent Keck optical imaging spectrographs have been designed with
active flexure compensation systems (FCS). These two instruments utilize different methods for implementing flexure compensation.
The Echellette Spectrograph and Imager (ESI), commissioned at the Cassegrain focus of the Keck II Telescope in late 1999, employs an open-loop control strategy. It utilizes a mathematical model of gravitationally-induced flexure to periodically compute flexure corrections as a function of telescope position. Those
corrections are then automatically applied to a tip/tilt collimator
to stabilize the image on the detector.
The DEep Imaging Multi-Object Spectrograph (DEIMOS), commissioned at the Nasmyth focus of Keck II in June 2002, implements a closed-loop control strategy. It utilizes a set of fiber-fed FCS light sources at the ends of the slitmask to produce a corresponding set of spots on a pair of FCS CCD detectors located on either side of the science CCD mosaic. During science exposures, the FCS detectors are read out
several times per minute to measure any translational motion of the
FCS spot images. Correction signals derived from these FCS images
are used to drive active optical mechanisms which steer the spots back to their nominal positions, thus stabilizing the FCS spot images as well as those on the science mosaic.
We compare the design, calibration, and operation of these two systems on the telescope. Long-term performance results will be provided for the ESI FCS, and preliminary results will be provided for the DEIMOS FCS.
The DEep Imaging Multi-Object Spectrograph (DEIMOS) images with an 8K x 8K science mosaic composed of eight 2K x 4K MIT/Lincoln Lab (MIT/LL) CCDs. It also incorporates two 1200 x 600 Orbit Semiconductor CCDs for active, close-loop flexure compensation. The science mosaic CCD controller system reads out all eight science CCDs in 40 seconds while maintaining the low noise floor of the MIT/Lincoln Lab CCDs. The flexure compensation (FC) CCD
controller reads out the FC CCDs several times per minute during science mosaic exposures. The science mosaic CCD controller and the FC CCD controller are located on the electronics ring of DEIMOS. Both the MIT/Lincoln Lab CCDs and the Orbit flexure compensation CCDs and their associated cabling and printed circuit boards are housed together in the same detector vessel that is approximately 10 feet away from the electronics ring.
Each CCD controller has a modular hardware design and is based on the San Diego State University (SDSU) Generation 2 (SDSU-2) CCD controller. Provisions have been made to the SDSU-2 video board to accommodate external CCD preamplifiers that are located at the detector vessel. Additional circuitry has been incorporated in the CCD controllers to allow the readback of all clocks and bias voltages for up to eight CCDs, to allow up to 10 temperature monitor and control points of the mosaic, and to allow full-time monitoring of power supplies and proper power supply sequencing. Software control features of the CCD controllers are: software selection between multiple mosaic readout modes, readout speeds, selectable gains, ramped parallel clocks to eliminate spurious charge on the CCDs, constant temperature monitoring and control of each CCD within the mosaic, proper sequencing of the bias voltages of the CCD output
MOSFETs, and anti-blooming operation of the science mosaic.
We cover both the hardware and software highlights of both of these CCD controller systems as well as their respective performance.
The DEIMOS spectrograph, being designed and built by Lick Observatory, for the Keck-II telescope, requires a dewar system that can support a mosaic of eight 2K X 8K science CCDs and two 600 X 1200 CCDs for flexure control. The science CCD must be mounted on the mosaic backplane with a height difference of less than 10 (mu) . Inside the dewar the mosaic must translate in the focus direction plus or minus 1 millimeter and in the direction of the slit by 90 (mu) for flexure control. This system has now completed the design stage, with many of the subsystems having been prototyped, and is now in the later part of the fabrication stage.
DEIMOS is a dual beam, off axis, multi object spectrograph of medium resolution being designed for the Keck II Telescope on Mauna Kea in Hawaii. The difficult an advanced scientific goals of the DEIMOS project have generated many challenging design requirements. The DEIMOS team at Lick Observatory has been responding to these challenges with new and unique concepts in instrument design and fabrication. This paper is an update to the paper presented at the SPIE conference in Landskrona, Sweden in 1996.
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.
We have previously described a system that derives the pointing coordinates of an equatorial telescope by measuring the angular position of a dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell. In that system, two precision tilt-sensors aligned orthogonally and mounted in the plane of the table are used as nulling devices to close an active servo loop which holds the table level as the telescope moves. Rotary encoders measure the angle by which each tilt-table axis rotates, and a mathematical transform converts those encoder readings into telescope hour angle and declination. Recent work has indicated the feasibility of several simplifications to that system. First, by use of suitable low friction bearings on the tilt-table axes, along with non-contacting encoders, the active servo loop is no longer needed to level the tilt- table. Rather, a simple suspended weight keeps the platform almost level, with the residual small tilt error measured by the precision tilt sensors.Second, by suitable orientation of the weight and the tilt sensors relative to the telescope polar axis, the system can measure telescope hour angel and declination directly, eliminating the need for the complex mathematical transform. Experimental result using these ideas are presented.
Inexpensive optical rotary incremental encoders now available can provide resolution approaching one arcsecond. However, several factors limit the accuracy of measurement that can be obtained. We report on test results of rotary incremental encoders obtained with a test setup that compared the output of two such encoders driven by the same shaft. Although intrinsic non-linear response of the encoders tested is specified to be less than plus or minus 15 arcseconds, additional errors are often caused by the coupling of the encoder to a rotating device. Bearing runout and shaft misalignment typically require use of a flexible coupler, but tests with several types of small inexpensive flexible couplers have shown that these can contribute additional errors including windup and non-uniform rotation that is affected by small changes in alignment. An additional minor source of error is due to a reproducible periodic error of several arcseconds generated in the interpolation electronics used to provide high resolution by subdividing the analog signal from the encoder. The driving torque required by a typical Gurley encoder is larger than might be expected, and has been measured at various speeds by determining the amount of windup with a solid aluminum coupling shaft.
This paper describes the design of DEIMOS -- a dual beam, off axis, multi object spectrograph of medium resolution, being designed for the Keck II telescope on Mauna Kea in Hawaii. The difficult and advanced scientific goals of the DEIMOS project have generated many challenging design requirements. The DEIMOS team at Lick Observatory has been responding to these challenges with new and unique concepts in instrument design and fabrication.
This report describes a method for using precision tilt-sensors to measure the position of an equatorial telescope relative to the local horizontal plane. Unlike conventional systems which measure the telescope position using position encoders coupled to the telescope axes, this method avoids many sources of non-repeatable error, such as hysteresis in the telescope structure due to inelastic flexure of the fork or yoke, or random slippage in the couplings between the position encoders and the telescope axes. In this respect, it shares many of the advantages of optical gyros, but achieves these at much lower cost. We present a design for a compact and relatively inexpensive dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell. The table contains two precision tilt-sensors, aligned orthogonally with the tilt axes of the table. The sensors are used as nulling devices to close a servo loop which keeps the table level at all times. This provides a precise and stable reference against which the telescope position is measured. A high resolution incremental encoder is directly coupled to each tilt-table axis and measures the angle by which that axis rotates to keep the table level. A mathematical transform converts these two encoder readings into local hour angle and declination. Preliminary tests of the tilt sensors and of a single-axis prototype tilt-table are reported, and future plans described. The use of tilt-tables for measuring the positions of non-equatorial telescopes is also briefly examined.
The Canada-France-Hawaii-Telescope has placed in operation a servo controlled roller-screw support system for its primary mirror. This paper will address the goals of CFHT in upgrading from a fixed `bendix' style mirror supporting defining pads. The main design criteria was to have a system that would define the mirror like the original pads, but have adjustability to remove coma dependent on telescope position.
Francois Roddier, Lennox Cowie, J. Elon Graves, A. Songaila, Daniel McKenna, Jean Vernin, Max Azouit, J. Caccia, Eric Limburg, Claude Roddier, Derrick Salmon, Stephane Beland, David Cowley, S. Hill
During two short campaigns intensive coordinated measurements have been performed to determine the various contributions to image degradation on Mauna Kea. Some of the results already obtained are presented here.
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