David J. Cowley

The DEIMOS spectrograph for the Keck II Telescope: integration and testing

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.

DEIMOS: a wide-field faint-object spectrograph

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.

Evaluation of precision tilt sensors for measuring telescope position

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 telescopes 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.

ShaneAO: an enhanced adaptive optics and IR imaging system for the Lick Observatory 3-meter telescope

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.

Tests of a precision tiltmeter system for measuring telescope position

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 telescopes 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.

Design update of DEIMOS: a wide-field faint-object spectrograph

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.

Commissioning ShARCS: the Shane adaptive optics infrared camera-spectrograph for the Lick Observatory Shane 3-m telescope

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.

Elastomeric lens mounts

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.

Tests of incremental rotary encoders

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.

Opto-mechanical design of ShaneAO: the adaptive optics system for the 3-meter Shane Telescope

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.