David Kirk Gilmore

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.

Characterization of a fully depleted CCD on high-resistivity silicon

Most scientific CCD imagers are fabricated on 30-50 (Omega) - cm epitaxial silicon. When illuminated form the front side of the device they generally have low quantum efficiency in the blue region of the visible spectrum because of strong absorption in the polycrystalline silicon gates as well as poor quantum efficiency in the far red and near infrared region of the spectrum because of the shallow depletion depth of the low-resistivity silicon. To enhance the blue response of scientific CCDs they are often thinned and illuminated from the back side. While blue response is greatly enhanced by this process, it is expensive and it introduces additional problems for the red end of the spectrum. A typical thinned CCD is 15 to 25 micrometers thick, and at wavelengths beyond about 800 nm the absorption depth becomes comparable to the thickness of the device, leading to interference fringes from reflected light. Because these interference fringes are of high order, the spatial pattern of the fringes is extremely sensitive to small changes in the optical illumination of the detector. Calibration and removal of the effects of the fringes is one of the primary limitations on the performance of astronomical images taken at wavelengths of 800 nm or more. In this paper we present results from the characterization of a CCD which promises to address many of the problems of typical thinned CCDs. The CCD reported on here was fabricated at Lawrence Berkeley National Laboratory (LBNL) on a 10-12 K

High-performance CCD on high-resistivity silicon

OMega-cm n-type silicon substrate.THe CCD is a 200 by 200 15-micrometers square pixel array, and due to the very high resistivity of the starting material, the entire 300 micrometers substrate is depleted. Full depletion works because of the gettering technology developed at LBNL which keeps leakage current down. Both front-side illuminated and backside illuminated devices have been tested. We have measured quantum efficiency, read-noise, full-well, charge-transfer efficiency, and leakage current. We have also observed the effects of clocking waveform shapes on spurious charge generation. While these new CCDs promise to be a major advance in CD technology, they too have limitations such as charge spreading and cosmic-ray effects. These limitations have been characterized and are presented. Examples of astronomical observations obtained with the backside CCD on the 1-meter reflector at Lick Observatory are presented.

Packaging design for Lawrence Berkeley National Laboratory high-resistivity CCDs

In this paper we present new results from the characterization of a fully depleted CCD on high resistivity silicon. The CCD was fabricated at Lawrence Berkeley National Laboratory on a 10-12 K(Omega) -cm n-type silicon substrate. The CCD is a 200 by 200 15-micrometers square pixel array. The high resistivity of the starting material makes it possible to deplete the entire 300 micrometers thick substrate. This results in improved red and near IR response compared to a standard CCD. Because the substrate is fully depleted, thinning of the CCD is not required for backside illumination, and the result presented here were obtained with a backside illuminated device. In this paper we present measured quantum efficiency as a function of temperature, and we describe a novel clocking scheme to measure serial charge transfer efficiency. We demonstrate an industrial application in which the CCD is more than an order of magnitude more sensitive than a commercial camera using a standard CCD.

A comparison of open versus closed loop flexure compensation systems for two Keck optical imaging spectrographs: ESI and DEIMOS

The Lawrence Berkeley National Laboratory has been developing fully-depleted high resistivity CCDs. These CCDs exhibit very high red quantum efficiency, no red fringing, and very low lateral charge diffusion, making them good candidates for astronomical applications that require better red response or better point spread function than can typically be achieved with standard thinned CCDs. For the LBNL 2Kx4K CCD we have developed a four-side mosaic package fabricated from aluminum nitride. Our objectives have been to achieve a flatness of less than 10 micrometers peak-to-valley and a consistent final package thickness variation of 10 micrometers or less in a light-weight package. We have achieved the flatness objective, and we are working toward the thickness variation objective.

CCD imaging systems for DEIMOS

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.

Design and Fabrication of Large CCDs for the Keck Observatory Deimos Spectrograph

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.

Characterization of a 4Kx2K three-side buttable CCD

The Keck II Deep Imaging Multi-Object Spectrograph (DEIMOS) is a general purpose, faint object, multi-slit, double-beam spectrograph which offers wide spectral coverage, high spectral resolution, high throughput, and long slit length on the sky. This powerful instrument will be the principal optical spectrograph on the Keck II telescope. DEIMOS is optimized for faint-object spectroscopy of individual point sources, low-surface-brightness extended objects, or widely distributed samples of faint objects on the sky. To obtain high resolution (∼1 A) and wide spectral coverage (up to 5000 A) the spectrograph uses wide angle cameras and large CCD detectors with many pixels.

Radiation events in astronomical CCD images

Results are presented on the fabrication and characterization of a 4Kx2K three-side buttable CCD produced by Orbit Semiconductor, a silicon foundry in San Jose, California. This first run of wafers was produced to test the ability of Orbit to produce high quality scientific CCDs with the characteristics required for detectors to be used in optical instruments of the Keck Observatory. Also on the wafer are two 2Kx2K devices. Similar devices have been fabricated for us by Loral/Fairchild. Extensive characterization of the Loral devices has taken place over the past few years, so interest is high about the possibility that Orbit might become a second source for similar detectors. This paper presents the first results on the 4Kx2K CCDs, and those preliminary results include measurements of charge transfer efficiency, low-temperature dark current, on-chip amplifier read- out noise, localized charge traps, full well, and responsive quantum efficiency.