The Integration and Verification Testing of the Large Synoptic Survey Telescope (LSST) Camera is described. The LSST Camera will be the largest astronomical camera ever constructed, featuring a 3.2 giga-pixel focal plane mosaic of 189 CCDs with in-vacuum controllers and readout, dedicated guider and wavefront CCDs, a three element corrector with a 1.6-meter diameter initial optic, six optical filters covering wavelengths from 320 to 1000 nm with a novel filter exchange mechanism, and camera-control and data acquisition capable of digitizing each image in two seconds. In this paper, we describe the integration processes under way to assemble the Camera and the associated verification testing program. The Camera assembly proceeds along two parallel paths: one for the focal plane and cryostat and the other for the Camera structure itself. A range of verification tests will be performed interspersed with assembly to verify design requirements with a test-as-you-build methodology. Ultimately, the cryostat will be installed into the Camera structure as the two assembly paths merge, and a suite of final Camera system tests performed. The LSST Camera is scheduled for completion and delivery to the LSST observatory in 2020.
The CCD sensors that will be used in the LSST Camera are controlled using a large number of bias and clock voltages. The objective of this project is to explore the large parameter space of operating voltages and optimize the performance of the CCDs made by ITL, one of two vendors manufacturing the LSST CCDs. The CCD voltages were adjusted from their nominal operating values in small increments and the electro-optical characteristics were measured. These measurements were performed first using a single ITL sensor and later with a 3 by 3 CCD mosaic with integrated controller electronics (Raft) of engineering grade ITL sensors, using a test stand at SLAC.
We present an overview of the Integration and Verification Testing activities of the Large Synoptic Survey Telescope (LSST) Camera at the SLAC National Accelerator Lab (SLAC). The LSST Camera, the sole instrument for LSST and under construction now, is comprised of a 3.2 Giga-pixel imager and a three element corrector with a 3.5 degree diameter field of view. LSST Camera Integration and Test will be taking place over the next four years, with final delivery to the LSST observatory anticipated in early 2020. We outline the planning for Integration and Test, describe some of the key verification hardware systems being developed, and identify some of the more complicated assembly/integration activities. Specific details of integration and verification hardware systems will be discussed, highlighting some of the technical challenges anticipated.
We employ electrostatic conversion drift calculations to match CCD pixel signal covariances observed in at field exposures acquired using candidate sensor devices for the LSST Camera.1, 2 We thus constrain pixel geometry distortions present at the end of integration, based on signal images recorded. We use available data from several operational voltage parameter settings to validate our understanding. Our primary goal is to optimize flux point spread function (FPSF) estimation quantitatively, and thereby minimize sensor-induced errors which may limit performance in precision astronomy applications. We consider alternative compensation scenarios that will take maximum advantage of our understanding of this underlying mechanism in data processing pipelines currently under development. To quantitatively capture the pixel response in high-contrast/high dynamic range operational extrema, we propose herein some straightforward laboratory tests that involve altering the time order of source illumination on sensors, within individual test exposures. Hence the word hysteretic in the title of this paper.
The LSST Camera science sensor array will incorporate 189 large format Charge Coupled Device (CCD) image sensors.
Each CCD will include over 16 million pixels and will be divided into 16 equally sized segments and each segment will
be read through a separate output amplifier.
The science goals of the project require CCD sensors with state of the art performance in many aspects. The broad
survey wavelength coverage requires fully depleted, 100 micrometer thick, high resistivity, bulk silicon as the imager
substrate. Image quality requirements place strict limits on the image degradation that may be caused by sensor effects:
optical, electronic, and mechanical.
In this paper we discuss the design of the prototype sensors, the hardware and software that has been used to perform
electro-optic testing of the sensors, and a selection of the results of the testing to date. The architectural features that lead
to internal electrostatic fields, the various effects on charge collection and transport that are caused by them, including
charge diffusion and redistribution, effects on delivered PSF, and potential impacts on delivered science data quality are
addressed.
Near-future astronomical survey experiments, such as LSST, possess system requirements of unprecedented
fidelity that span photometry, astrometry and shape transfer. Some of these requirements flow directly to the
array of science imaging sensors at the focal plane. Availability of high quality characterization data acquired
in the course of our sensor development program has given us an opportunity to develop and test a framework
for simulation and modeling that is based on a limited set of physical and geometric effects. In this paper we
describe those models, provide quantitative comparisons between data and modeled response, and extrapolate
the response model to predict imaging array response to astronomical exposure. The emergent picture departs
from the notion of a fixed, rectilinear grid that maps photo-conversions to the potential well of the channel.
In place of that, we have a situation where structures from device fabrication, local silicon bulk resistivity
variations and photo-converted carrier patterns still accumulating at the channel, together influence and distort
positions within the photosensitive volume that map to pixel boundaries. Strategies for efficient extraction of
modeling parameters from routinely acquired characterization data are described. Methods for high fidelity
illumination/image distribution parameter retrieval, in the presence of such distortions, are also discussed.
The Large Synoptic Survey Telescope instrument include four guiding and wavefront sensing subsystems called corner
raft subsystems, in addition to the main science array of 189 4K x 4K CCDs. These four subsystems are placed at the
four corners of the instrumented field of view. Each wavefront/guiding subsystem comprises a pair of 4K x 4K guide
sensors, capable of producing 9 frames/second, and a pair of offset 2K x 4K wavefront curvature sensors from which the
images are read out at the cadence of the main camera system, providing 15 sec integrations. These four
guider/wavefront corner rafts are mechanically and electrically isolated from the science sensor rafts and can be installed
or removed independently from any other focal plane subsystem. We present the implementation of this LSST
subsystem detailing both hardware and software development and status.
We describe a camera beam simulator for the LSST which is capable of illuminating a 60mm field at f/1.2 with realistic astronomical scenes, enabling studies of CCD astrometric and photometric performance. The goal is to fully simulate LSST observing, in order to characterize charge transport and other features in the thick fully-depleted CCDs and to probe low level systematics under realistic conditions. The automated system simulates the centrally obscured LSST beam and sky scenes, including the spectral shape of the night sky. The doubly telecentric design uses a nearly unit magnification design consisting of a spherical mirror, three BK7 lenses, and one beam-splitter window. To achieve the relatively large field the beam-splitter window is used twice. The motivation for this LSST beam test facility was driven by the need to fully characterize a new generation of thick fully-depleted CCDs, and assess their suitability for the broad range of science which is planned for LSST. Due to the fast beam illumination and the thick silicon design [each pixel is 10 microns wide and over 100 microns deep] at long wavelengths there can be effects of photon transport and charge transport in the high purity silicon. The focal surface covers a field more than sufficient for a 40×40mm LSST CCD. Delivered optical quality meets design goals, with 50% energy within a 5 micron circle. The tests of CCD performance are briefly described.
The design of the Large Synoptic Survey Telescope (LSST) requires a camera system of unprecedented size and complexity. Achieving the science goals of the LSST project, through design, fabrication, integration, and operation, requires a thorough understanding of the camera performance. Essential to this effort is the camera modeling which defines the effects of a large number of potential mechanical, optical, electronic or sensor variations which can only be captured with sophisticated instrument modeling that incorporates all of the crucial parameters. This paper presents the ongoing development of LSST camera instrument modeling and details the parametric issues and attendant analysis involved with this modeling.
The Large Synoptic Survey Telescope (LSST) uses a novel, three-mirror, telescope design feeding a camera system that
includes a set of broad-band filters and three refractive corrector lenses to produce a flat field at the focal plane with a
wide field of view. Optical design of the camera lenses and filters is integrated in with the optical design of telescope
mirrors to optimize performance. We discuss the rationale for the LSST camera optics design, describe the methodology
for fabricating, coating, mounting and testing the lenses and filters, and present the results of detailed analyses
demonstrating that the camera optics will meet their performance goals.
The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated
by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the highenergy
universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV.
These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3–12 keV with
high spectral resolution of ΔE ≦ 7 eV, enabled by a micro-calorimeter array located in the focal plane of
thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5–80 keV, located in the focal plane of
multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4–12 keV,
with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera
type soft gamma-ray detector, sensitive in the 40–600 keV band. The simultaneous broad bandpass, coupled
with high spectral resolution, will enable the pursuit of a wide variety of important science themes.
KEYWORDS: Cameras, Optical filters, Large Synoptic Survey Telescope, Sensors, Charge-coupled devices, Electronics, Telescopes, Control systems, Camera shutters, Imaging systems
The Large Synoptic Survey Telescope (LSST) is a large aperture, wide-field facility designed to provide deep images of
half the sky every few nights. There is only a single instrument on the telescope, a 9.6 square degree visible-band
camera, which is mounted close to the secondary mirror, and points down toward the tertiary. The requirements of the
LSST camera present substantial technical design challenges. To cover the entire 0.35 to 1 μm visible band, the camera
incorporates an array of 189 over-depleted bulk silicon CCDs with 10 μm pixels. The CCDs are assembled into 3 x 3
"rafts", which are then mounted to a silicon carbide grid to achieve a total focal plane flatness of 15 μm p-v. The CCDs
have 16 amplifiers per chip, enabling the entire 3.2 Gigapixel image to be read out in 2 seconds. Unlike previous
astronomical cameras, a vast majority of the focal plane electronics are housed in the cryostat, which uses a mixed
refrigerant Joule-Thompson system to maintain a -100ºC sensor temperature. The shutter mechanism uses a 3 blade
stack design and a hall-effect sensor to achieve high resolution and uniformity. There are 5 filters stored in a carousel
around the cryostat and the auto changer requires a dual guide system to control its position due to severe space
constraints. This paper presents an overview of the current state of the camera design and development plan.
The Large Synoptic Survey Telescope (LSST) uses a novel, three-mirror, modified Paul-Baker design,
with an 8.4-meter primary mirror, a 3.4-m secondary, and a 5.0-m tertiary feeding a refractive camera design with 3
lenses (0.69-1.55m) and a set of broadband filters/corrector lenses. Performance is excellent over a 9.6 square
degree field and ultraviolet to near infrared wavelengths.
We describe the image quality error budget analysis methodology which includes effects from optical and
optomechanical considerations such as index inhomogeneity, fabrication and null-testing error, temperature
gradients, gravity, pressure, stress, birefringence, and vibration.
Extracting science from the LSST data stream requires a detailed knowledge of the properties of the LSST catalogs and
images (from their detection limits to the accuracy of the calibration to how well galaxy shapes can be characterized).
These properties will depend on many of the LSST components including the design of the telescope, the conditions
under which the data are taken and the overall survey strategy. To understand how these components impact the nature
of the LSST data the simulations group is developing a framework for high fidelity simulations that scale to the volume
of data expected from the LSST. This framework comprises galaxy, stellar and solar system catalogs designed to match
the depths and properties of the LSST (to r=28), transient and moving sources, and image simulations that ray-trace the
photons from above the atmosphere through the optics and to the camera. We describe here the state of the current
simulation framework and its computational challenges.
The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated
by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the
high-energy universe by performing high-resolution, high-throughput spectroscopy with moderate angular
resolution. ASTRO-H covers very wide energy range from 0.3 keV to 600 keV. ASTRO-H allows a combination
of wide band X-ray spectroscopy (5-80 keV) provided by multilayer coating, focusing hard X-ray
mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3-12 keV)
provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD
camera as a focal plane detector for a soft X-ray telescope (0.4-12 keV) and a non-focusing soft gamma-ray
detector (40-600 keV) . The micro-calorimeter system is developed by an international collaboration led
by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of
ΔE ~7 eV provided by the micro-calorimeter will enable a wide variety of important science themes to be
pursued.
The Hard X-ray Imager (HXI) is one of four detectors on board the ASTRO-H mission (6th Japanese X-ray
satellite), which is scheduled to be launched in 2014. Using the hybrid structure composed of double-sided silicon
strip detectors and a cadmium telluride double-sided strip detector, the instrument fully covers the energy range
of photons collected with the hard X-ray telescope up to 80 keV with a high quantum efficiency. High spatial
resolution of 250 μm and an energy resolution of 1-2 keV (FWHM) are both achieved with low noise front-end
ASICs. In addition, the thick BGO active shields surrounding the main detector package is a heritage of the
successful performance of the Hard X-ray Detector on board the Suzaku satellite. This feature enables the
instrument to achieve an extremely high background reduction caused by cosmic-ray particles, cosmic X-ray
background, and in-orbit radiation activation. In this paper, we present the detector concept, design, latest
results of the detector development, and the current status of the hardware.
ASTRO-H is the next generation JAXA X-ray satellite, intended to carry instruments with broad energy coverage
and exquisite energy resolution. The Soft Gamma-ray Detector (SGD) is one of ASTRO-H instruments and will
feature wide energy band (40-600 keV) at a background level 10 times better than the current instruments on
orbit. SGD is complimentary to ASTRO-H's Hard X-ray Imager covering the energy range of 5-80 keV. The
SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield
where Compton kinematics is utilized to reject backgrounds. The Compton camera in the SGD is realized as
a hybrid semiconductor detector system which consists of silicon and CdTe (cadmium telluride) sensors. Good
energy resolution is afforded by semiconductor sensors, and it results in good background rejection capability due
to better constraints on Compton kinematics. Utilization of Compton kinematics also makes the SGD sensitive
to the gamma-ray polarization, opening up a new window to study properties of gamma-ray emission processes.
The ASTRO-H mission is approved by ISAS/JAXA to proceed to a detailed design phase with an expected
launch in 2014. In this paper, we present science drivers and concept of the SGD instrument followed by detailed
description of the instrument and expected performance.
KEYWORDS: Telescopes, Radon, Information operations, Sensors, Space telescopes, Astronomy, Astronomical imaging, Current controlled current source, Denoising, Stars
We report on long exposure results obtained with a Teledyne HyViSI H2RG detector operating in guide mode. The sensor simultaneously obtained nearly seeing-limited data while also guiding the Kitt Peak 2.1 m telescope. Results from unguided and guided operation are presented and used to place lower limits on flux/fluence values for accurate centroid measurements. We also report on significant noise reduction obtained in recent laboratory measurements that should further improve guiding capability with higher magnitude stars.
The Large Synoptic Survey Telescope (LSST) uses a novel, three-mirror, modified Paul-Baker design, with an 8.4-
meter primary mirror, a 3.4-m secondary, and a 5.0-m tertiary feeding a camera system that includes a set of broad-band
filters and refractive corrector lenses to produce a flat focal plane with a field of view of 9.6 square degrees. Optical
design of the camera lenses and filters is integrated with optical design of telescope mirrors to optimize performance,
resulting in excellent image quality over the entire field from ultra-violet to near infra-red wavelengths. The LSST
camera optics design consists of three refractive lenses with clear aperture diameters of 1.55 m, 1.10 m and 0.69 m and
six interchangeable, broad-band, filters with clear aperture diameters of 0.75 m. We describe the methodology for
fabricating, coating, mounting and testing these lenses and filters, and we present the results of detailed tolerance
analyses, demonstrating that the camera optics will perform to the specifications required to meet their performance
goals.
We present characterization methods and results on a number of new devices produced specifically to address LSST's
performance goals, including flatness, QE, PSF, dark current, read noise, CTE, cosmetics, and crosstalk. The results
indicate that commercially produced, thick n-channel over-depleted CCDs with excellent red response can achieve tight
PSF at moderate applied substrate bias with no evidence of persistent image artifacts. We will also report ongoing
studies of mosaic assembly techniques to achieve chip-to-chip
co-planarity, high fill factor, and thermal stability.
The LSST camera is a wide-field optical (0.35-1μm) imager designed to provide a 3.5 degree FOV with 0.2
arcsecond/pixel sampling. The detector format will be a circular mosaic providing approximately 3.2 Gigapixels per
image. The camera includes a filter mechanism and shuttering capability. It is positioned in the middle of the telescope
where cross-sectional area is constrained by optical vignetting and where heat dissipation must be controlled to limit
thermal gradients in the optical beam. The fast f/1.2 beam will require tight tolerances on the focal plane mechanical
assembly. The focal plane array operates at a temperature of approximately -100°C to achieve desired detector performance. The
focal plane array is contained within a cryostat which incorporates detector front-end electronics and thermal control.
The cryostat lens serves as an entrance window and vacuum seal for the cryostat. Similarly, the camera body lens serves
as an entrance window and gas seal for the camera housing, which is filled with a suitable gas to provide the operating
environment for the shutter and filter change mechanisms. The filter carousel accommodates 5 filters, each 75 cm in diameter, for rapid exchange without external intervention.
We present the first astronomical results from a 4K2 Hybrid Visible Silicon PIN array detector (HyViSI) read out
with the Teledyne Scientific and Imaging SIDECAR ASIC. These results include observations of astronomical
standards and photometric measurements using the 2.1m KPNO telescope. We also report results from a test
program in the Rochester Imaging Detector Laboratory (RIDL), including: read noise, dark current, linearity,
gain, well depth, quantum efficiency, and substrate voltage effects. Lastly, we highlight results from operation of
the detector in window read out mode and discuss its potential role for focusing, image correction, and use as a
telescope guide camera.
The Large Synoptic Survey Telescope (LSST) is a unique, three-mirror, modified Paul-Baker design with an 8.4m primary, a 3.4m secondary, and a 5.0m tertiary feeding a camera system that includes corrector optics to produce a 3.5 degree field of view with excellent image quality (<0.3 arcsecond 80% encircled diffracted energy) over the entire field from blue to near infra-red wavelengths. We describe the design of the LSST camera optics, consisting of three refractive lenses with diameters of 1.6m, 1.0m and 0.7m, along with a set of interchangeable, broad-band, interference filters with diameters of 0.75m. We also describe current plans for fabricating, coating, mounting and testing these lenses and filters.
The LSST camera is a wide-field optical (0.35-1um) imager designed to provide a 3.5 degree FOV with better than 0.2 arcsecond sampling. The detector format will be a circular mosaic providing approximately 3.2 Gigapixels per image. The camera includes a filter mechanism and, shuttering capability. It is positioned in the middle of the telescope where cross-sectional area is constrained by optical vignetting and heat dissipation must be controlled to limit thermal gradients in the optical beam. The fast, f/1.2 beam will require tight tolerances on the focal plane mechanical assembly.
The focal plane array operates at a temperature of approximately -100°C to achieve desired detector performance. The focal plane array is contained within an evacuated cryostat, which incorporates detector front-end electronics and thermal control. The cryostat lens serves as an entrance window and vacuum seal for the cryostat. Similarly, the camera body lens serves as an entrance window and gas seal for the camera housing, which is filled with a suitable gas to provide the operating environment for the shutter and filter change mechanisms. The filter carousel can accommodate 5 filters, each 75 cm in diameter, for rapid exchange without external intervention.
Science studies made by the Large Synoptic Survey Telescope will reach systematic limits in nearly all cases. Requirements for accurate photometric measurements are particularly challenging. Advantage will be taken of the rapid cadence and pace of the LSST survey to use celestial sources to monitor stability and uniformity of photometric data. A new technique using a tunable laser is being developed to calibrate the wavelength dependence of the total telescope and camera system throughput. Spectroscopic measurements of atmospheric extinction and emission will be made continuously to allow the broad-band optical flux observed in the instrument to be corrected to flux at the top of the atmosphere. Calibrations with celestial sources will be compared to instrumental and atmospheric calibrations.
The LSST project has embarked on an aggressive new program to develop the next generation of silicon imagers for the visible and near-IR spectral regions. In order to better understand and solve some of the technology issues prior to development and mass-production for the huge LSST focal plane, a number of contracts have been written to imager firms to explore specific areas of technology uncertainty. We expect that these study contracts will do much toward reducing risk and uncertainty going into the next phase of development, the prototype production of the final large LSST imager.
KEYWORDS: Sensors, Point spread functions, Large Synoptic Survey Telescope, Silicon, Quantum efficiency, Diffusion, Absorption, Electric field sensors, Signal to noise ratio, Reflection
Sensors for the LSST camera require high quantum efficiency (QE) extending into the near-infrared. A relatively large thickness of silicon is needed to achieve this extended red response. However, thick sensors degrade the point spread function (PSF) due to diffusion and to the divergence of the fast f/1.25 beam. In this study we examine the tradeoff of QE and PSF as a function of thickness, wavelength, temperature, and applied electric field for fully-depleted sensors. In addition we show that for weakly absorbed long-wavelength light, optimum focus is achieved when the beam waist is positioned slightly inside the silicon.
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.
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 remarkable sensitivity of depleted silicon to ionizing radiation is a nuisance to astronomers. 'Cosmic rays' degrade images because of struck pixels, leading to modified observing strategies and the development of algorithms to remove the unwanted artifacts. In the new-generation CCD's with thick sensitive regions, cosmic-ray muons make recognizable straight tracks and there is enhanced sensitivity to ambient gamma radiation via Compton-scattered electrons ('worms'). Beta emitters inside the dewar, for example high-potassium glasses such as BK7 , also produce worm-like tracks. The cosmic-ray muon rate is irreducible and increases with altitude. The gamma rays are mostly by- products of 40K decay and the U and Th decay chains; these elements commonly appear as traces in concrete and other materials. The Compton recoil event rate can be reduced significantly by the choice of materials in the environment and dewar and by careful shielding. Telescope domes appear to have significantly lower rates than basement laboratories and Coude spectrograph rooms. Radiation sources inside the dewar can be eliminated by judicious choice of materials. Cosmogenic activation during high-altitude fights does not appear to be a problem. Our conclusion are supported by tests at the Lawrence Berkeley National Laboratory low-level counting facilities in Berkeley and at Oroville, California (180 m underground).
DEIMOS is a large multi-object spectrograph with an imaging mode that is being built for the W. M. Keck 2 Telescope. The detector is a 2 X 4 mosaic of eight 2048 X 4096 pixel CCDs. The mosaic assembly process must position the CCDs to be planar to within 5 microns rms. We describes the CCD support design and the measurements used to achieve this.
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.
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$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.
Lick Observatory has recently developed a near IR camera for astronomical imaging. The new camera has been built around the Rockwell International Inc. NICMOSIII 256 x 256 HgCdTe focal plane array. The dewar and optics were manufactured by IR Laboratories in Tuscon AZ, while the electronics, telescope interface, and data system were built at Lick Observatory. The Lick IR camera was designed primarily as a 1 to 2.5 micron Cassegrain imager, however, the dewar can also be operated as a high resolution spectrographic camera for the Lick Coude' Spectrograph in the 1 micron spectral region. At the Cassegrain focus the instrument is used to image faint galaxies, diffuse IR sources in star formation regions, and other astronomical objects. The camera operates at f/17 with focal plane scale reductions of 1: 1.25, 1: 2.5, and 1: 3.75. Thus the optics provide 0.2, 0.4, and 0.6 arcseconds per pixel when used in the Cassegrain mode at the 3M telescope, and 0.6, 1.2, or 1.8 arcseconds per pixel when the camera when used on the 1M telescope. The 1: 1.25 scale of the camera gives a resolving power greater than 60,000 when used with the Coude spectrograph at 1.5 microns. Standard H, J, K as well as narrow band special purpose filters, for spectroscopy, are contained in two independent motorized filter wheels.
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.
Results are presented on the fabrication and characterization of a 4 K X 2 K three-side buttable CCD produced by Orbit Semiconductor. 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 2 K X 2 K devices. Similar devices have been fabricated for us previously 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 4 K X 2 K CCDs, including measurements of charge transfer efficiency, low-temperature dark current, on-chip amplifier readout noise, localized charge traps, full well, and responsive quantum efficiency.
Results obtained in fabricating and testing of large CCD image sensors are reported. The emphasis is on high quantum efficiency, excellent charge transfer efficiency at low signal level, large pixel count, low readout noise, and very low dark current. The focus is on the use of the devices for optical astronomy where these parameters are most important. Test results for CCDs fabricated by Ford Aerospace and by EG Experiments to demonstrate the feasibility of a reproducible biased-gate using transparent indium tin oxide as a conducting layer over a silicon oxide insulating layer are discussed. Quantum efficiency of bias-gate thinned CCDs is compared with results obtained from a phosphor-coated front- illuminated CCD.
Results of charge transfer efficiency and dark current tests for large Reticon and Ford CCDs are given, as well as a description of some experiments to improve the UV quantum efficiency, using a biased flash-gate for thinned CCDs and phosphor overlay for front-illuminated CCDs. The results indicate that large affordable CCDs suitable for astronomical work are now in reach, with excellent charge-handling characteristics, low dark current and readout noise and high quantum efficiency over the 300 nm to 1000 nm wavelength range. With buttable CCDs, whose production is planned for the future, even larger arrays of CCDs will be possible.
Recent advancements in fabrication of near-infrared focal plane arrays are beginning to revolutionize near-infrared astronomy. The University of California Lick Observatory is undertaking a program to exploit these arrays by starting with a singlefilter (K band at two microns)imaging camera to be used at the Sane 3-meter telescope. The detector is a Rockwell 128x128-pixel device, sensitive from 1 to 2.5 microns and employing the NICMOS design. Camera specifications, controlling electronics, calibrations procedures, and device performance in the lab and at the telescope will be presented. Some images of astronomical targets will also be shown.
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