The DEep Imaging Multi-Object Spectrograph (DEIMOS) was commissioned on Keck II in June 2002. It employs a closed-loop flexure compensation system (FCS) to measure and compensate for image motion resulting from gravitationally-induced flexure of spectrograph elements. The FCS utilizes a set of fiber-fed FCS light sources located at the edges of the instrument focal plane to produce a corresponding set of spots on a pair of FCS CCD detectors located on either side of the science CCD mosaic. (This FCS light follows the same light path through the instrument as the science spectra.) During science exposures, the FCS detectors are read out several times per minute. These FCS images are analyzed in real time to measure any translational motion of the FCS spots and to derive correction signals; those signals drive active optical mechanisms
which steer the spots back to their nominal positions, thus stabilizing the images on the FCS CCDs and the science mosaic. This paper describes the commissioning of the DEIMOS FCS system, its
continued evolution during its first 18 months of operation on the
telescope, and its operational performance over that period. We describe the various challenges encountered while refining the initial FCS prototype (deployed at commissioning) into a fully-operational and highly-reliable system that is now an essential component of the instrument. These challenges include: reducing stray light from FCS light sources to an acceptable level; resolving interactions between FCS acquisition and slit mask alignment; providing robust rejection of cosmic ray events in
FCS images; implementing a graphical user interface for FCS control and status.
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 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.
The DEIMOS spectrograph has now been installed on the Keck-II telescope and commissioning is nearly complete. The DEEP2 Redshift Survey, which will take approximately 120 nights at the Keck Observatory over a three year period and has been designed to utilize the power of DEIMOS, began in the summer of 2002. The multiplexing power and high efficiency of DEIMOS enables us to target 1000 faint galaxies per clear night. Our goal is to gather high-quality spectra of ≈ 60,000 galaxies with z>0.75 in order to study the properties and large scale clustering of galaxies at z ≈ 1. The survey will be executed at high spectral resolution, R=λ/Δλ ≈ 5000, allowing us to work between the bright OH sky emission lines and to infer linewidths for many of the target galaxies (for several thousand objects, we will obtain rotation curves as well). The linewidth data will facilitate the execution of the classical redshift-volume cosmological test, which can provide a precision measurement of the equation of state of the Universe. This talk reviews the project, summarizes our science goals and presents some early DEIMOS data.
Measurements of anisoplanatism from data obtained with natural guide star adaptive optics on the Lick Observatory 3m are presented. These were obtained from short exposures of binary stars with the IRCAL camera whose field of view (~20”) is generally considered isoplanatic in the K-band. However, measurable amounts of high-order anisoplanatism were present at separations of ~7” and ~12” with an isoplanatic patch size estimated to be ~26”. Within this field, there was measureable differential image motion between the binary star components. This image motion was small compared to the size of the diffraction-spot and therefore had negligible effect.
The Deep Imaging Multi-Object Spectrograph (DEIMOS)was delivered to the Keck II telescope during February 2002, and has been commissioned in the several months since then. Most of the instrument is in a barrel that rests on a cradle at the Nasmyth focus, and rotates to track field rotation. This paper describes the architecture of the rotator control software, including the communications protocols, time synchronization with the telescope control software, methods adopted for meeting the real-time control requirements, safety issues for a multi-ton rotating mass, and unusual position encoder challenges.
The DEIMOS Spectrograph Camera contains tow doublets and a triplet. Each group contains materials differing in thermal coefficient expansion, mechanical and optical properties. To mate the elements and at the same time accommodate large camera temperature changes, we will fill the space between with an optical fluid couplant. We selected candidate couplants, lens-support materials, and fluid-constraining materials based on published optical, mechanical and chemical properties. We then tested the chemical reactivity between the coupling fluids, lens-support and fluid- constraining materials. We describe here the test configurations, our criteria for reactivity, and the result for various test durations. We describe our conclusions and final choices for couplant and materials.
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.
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.
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.