The majority of extra-solar planets discovered to date have been found using Doppler-shift measurements obtained with the Hamilton Spectrometer at Lick Observatory and the High Resolution Echelle Spectrometer (HIRES) at Keck Observatory. Each of these spectrometers employs an integral exposure meter which enables observers to optimize exposure times so as to achieve the required signal-to-noise and to determine the photon-weighted midpoint of each science exposure (which is needed to correct the Doppler shift to the Solar System barycenter). In both of these systems, a propeller mirror located behind the spectrometer slit picks off a few percent of the light and directs it to a photo-multiplier tube (PMT) used to measure the exposure level versus time. In late 2006, the new Automated Planet Finder (APF) Telescope and APF Spectrometer are scheduled to begin operations at Lick Observatory; both will be dedicated exclusively to the search for extra-solar planets. Like the Hamilton and HIRES Spectrometers, the APF Spectrometer will employ an integral exposure meter, but one with a significantly different design. The APF exposure meter will employ a stationary pellicle located ahead of the slit to pick off 4% of the light and direct it to the guide camera. That camera will produce images typically at a 1 Hz rate, and those images will be used both for autoguiding and for computing the exposure level delivered to the spectrometer. In each guide camera image obtained during a science exposure, the time-tagged signal from the pixels that correspond to the spectrometer slit will be integrated in software to determine the current exposure level and the photon-weighted midpoint of that science exposure. We compare these two different design approaches, and describe the significant hardware and software features of each of these systems.
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.
The San Diego State University Generation 2 CCD controller (SDSU-2)
architecture is widely used in both optical and infrared astronomical
instruments. This architecture was employed in the CCD controllers
for the DEIMOS instrument commissioned on Keck-II in June 2002.
In 2004, the CCD dewar in the HIRES instrument on Keck-I will be
upgraded to a 3 x 1 mosaic of MIT/LL 2K x 4K CCDs controlled by an
SDSU-2 CCD controller.
For each of these SDSU-2 CCD controllers, customized versions of PAL
chips were developed to extend the capabilities of this controller
architecture. For both mosaics, a custom timing board PAL enables rapid, software-selectable switching between dual- and single-amplifier-per-CCD readout modes while reducing excess utilization of fiber optic bandwidth for the latter. For the HIRES CCD mosaic, a custom PAL for the clock generation boards provides software selection of different clock waveforms that can address the CCDs of the mosaic either individually or globally, without any need to reset the address jumpers on these boards.
The custom PAL for the clock generation boards enables a method for
providing differing exposure times on each CCD of the mosaic. These
distinct exposure times can be implemented in terms of a series of
sub-exposures within a single, global mosaic observation. This allows for more effective observing of sources that have flux gradients across the spectral dimension of the CCD mosaic because those CCDs located near the higher end of the flux gradient can be read out more frequently, thus reducing the number of cosmic rays in each individual sub-exposure from those CCDs.
This paper documents the astrometric slitmask design, submission,
fabrication, control and configuration tools used for two large
spectrographs at W. M. Keck Observatory on Mauna Kea, Hawai'i.
For supplemental illustrations and documents, including an online
version of the poster and interactive demos, we refer the reader to
http://spg.ucolick.org/Docs/SPIE/2004 .
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.
This paper describes tools and methods used to manage DEIMOS removable elements (filters, gratings, and slitmasks). The iterative process of adapting and refining our basic strategy to the working conditions and requirements of Keck Observatory staff is not yet complete; hence this paper should be read as a Work In Progress report.
KEYWORDS: Computer programming, Telescopes, Control systems, Switches, Computer architecture, Systems modeling, Software development, Calibration, Analog electronics, Space telescopes
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.
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