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.
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.
We present the design and status report on the development of an Integral Field Unit (IFU) for the Echellette spectrograph and imager (ESI), a recently developed R=13000, Cassegrain spectrograph at Keck II. We have designed a family of IFU’s for the spectrograph, providing a range of field-coverages and dispersions. The optical designs are based on the Advanced Image Slicer concept of Content. We describe the completely monolithic, passive, and modular implementation of this design as an IFU head. Each IFU head resides in an ESI slit mask holder, so that it is completely selectable/deselectable as an observing mode during a nights observing run.
The Echelle Spectrograph and Imager (ESI) is a multipurpose instrument which has been delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. ESI saw first light on August 29, 1999. The optical performance of the instrument has been measured using artificial calibration sources and starlight. Measurements of the average image FWHM in echelle mode are 22 microns, 16 to 18 microns in broad band imaging mode, and comparable in the low- dispersion prismatic mode. Images on the sky, under best seeing conditions show FWHM sizes of 34 microns. Maximum efficiencies are measured to be 30 percent for echelle and anticipated to be greater than 38 percent for low dispersion prismatic mode including atmospheric, telescope and detector losses. In this paper we describe the instrument and its specifications. We discuss the testing that led to the above conclusions.
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.
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.
The Echellette Spectrograph and Imager (ESI) for Keck 2 is a versatile Cassegrain instrument which will take data in 3 independent modes. In the echellette mode, it is a medium- dispersion prism cross-dispersed spectrometer featuring a 20- arcsec slit height, 11.5 km/sec/pixel average resolution and full coverage of the (0.39 to 1.09)-micron spectral range in a single exposure. In the prism-only mode, it is a low- dispersion multi-slit spectrometer which covers a 2.0-arcmin- wide field area with an 8.0-arcmin height perpendicular to dispersion. Prismatic resolution is roughly linear with wavelength, ranging from about 62 km/sec/pixel at 0.39 microns to 285 km/sec/pixel at 0.80 microns. In direct-imaging mode, the aforementioned 16.0 sq arcmin field area is reimaged directly unto the CCD detector at a resolution of 0.153 arcsec/pixel. ESI contains an on-axis reflecting collimator which accommodates an off-axis field of view. Cross dispersion is provided by an Ohara BSL7Y prism used in double-pass, followed by a second prism of the same material used in single-pass. The camera is a 10-element all-spherical Epps lens which services a single flat (2048 by 4096 by 15-micron) CCD. The same camera and detector are used for all 3 operating modes without modification. The ESI mechanical design is based upon the 'space-frame' concept which was used successfully for the Keck telescope(s) mechanical structure(s). This results in large weight reduction relative to more typical Cassegrain spectrographs, with the added expectations of very high stiffness and sub-pixel image stability during long exposures. ESI is funded by a grant from CARA and the project has been under way for about 27 months. Most of the mechanical design work is finished and construction is in progress. Electronics, data reduction and user-interface software are nearing completion. All of the optics (including coatings) have been completed and delivered. A thinned science-grade MIT/Lincoln Laboratory CCD for ESI has also been delivered. It is anticipated the ESI may be operational toward the end of 1998.