We report on the upgraded One Degree Imager (ODI) at the WIYN 3.5 meter telescope at the Kitt Peak Observatory after the focal plane was expanded by an additional seventeen detectors in spring 2015. The now thirty Orthogonal Transfer Array CCD detectors provide a total field of view of 40’ x 48’ on the sky. The newly added detectors underwent a design revision to mitigate reduced charge transfer efficiency under low light conditions. We discuss the performance of the focal plane and challenges in the photometric calibration of the wide field of view, helped by the addition of telescope baffles. In a parallel project, we upgraded the instrument’s three filter arm mechanisms, where a degrading worm-gear mechanism was replaced by a chain drive that is operating faster and with high reliability. Three more filters, a u’ band and two narrow band filters were added to the instrument’s complement, with two additional narrow band filters currently in procurement (including an Hα filter). We review the lessons learned during nearly three years of operating the instrument in the observatory environment and discuss infrastructure upgrades that were driven by ODI’s needs.
Quantum Efficiency (QE) is one of the most important parameters when either evaluating or using an imaging sensor for scientific applications. For back illuminated CCD and CMOS imagers, QE is determined by temperature, antireflection (AR) coatings, backside charging mechanisms, and silicon thickness. The accurate and precise measurement of QE requires careful consideration of illumination, temperature, calibration standards, optics, electronic equipment and components, and scattered light. QE is also closely related to the reflectance from the sensor surface. We present in this paper a study of the QE and reflectance from a variety of sensors used for astronomical imaging. Particular attention is given to precise calibration, temperature effects, models vs. measurements, and measurement techniques. We discuss all these issues and how they relate to the measurement and actual performance of sensors with different areas, thicknesses, and AR coatings.
The Large Binocular Telescope (LBT) has eight Acquisition, Guiding, and wavefront Sensing Units (AGw units). They provide guiding and wavefront sensing capability at eight different locations at both direct and bent Gregorian focal stations. Recent additions of focal stations for PEPSI and MODS instruments doubled the number of focal stations in use including respective motion, camera controller server computers, and software infrastructure communicating with Guiding Control Subsystem (GCS). This paper describes the improvements made to the LBT GCS and explains how these changes have led to better maintainability and contributed to increased reliability. This paper also discusses the current GCS status and reviews potential upgrades to further improve its performance.
VIRUS is the massively replicated fiber-fed spectrograph being built for the Hobby-Eberly Telescope to support
HETDEX (the Hobby-Eberly Telescope Dark Energy Experiment). The instrument consists of 156 identical
channels, fed by 34,944 fibers contained in 78 integral field units, deployed in the 22 arcminute field of the
upgraded HET. VIRUS covers 350-550nm at R ≈ 700 and is built to target Lyman α emitters at 1.9 < z < 3.5 to
measure the evolution of dark energy. Here we present the assembly line construction of the VIRUS spectrographs,
including their alignment and plans for characterization. We briefly discuss plans for installation on the telescope.
The spectrographs are being installed on the HET in several stages, and the instrument is due for completion
by the end of 2014.
The One Degree Imager (ODI) was deployed during the summer of 2012 at the WIYN 3.5m telescope, located on Kitt Peak near Tucson, AZ (USA). ODI is an optical imager designed to deliver atmosphere-limited image quality (≤ 0.4” FWHM) over a one degree field of view, and uses Orthogonal Transfer Array (OTA) detectors to also allow for on-chip tip/tilt image motion compensation. At this time, the focal plane is partially populated (”pODI”) with 13 out of 64 OTA detectors, providing a central scientifically usable field of view of about 24′ x 24′; four of the thirteen detectors are installed at outlying positions to probe image quality at all field angles. The image quality has been verified to be indeed better than 0.4′′ FWHM over the full field when atmospheric conditions allow. Based on over one year of operations, we summarize pODIs performance and lessons learned. As pODI has proven the viability of the ODI instrument, the WIYN consortium is engaging in an upgrade project to add 12 more detectors to the focal plane enlarging the scientifically usable field of view to about 40′ x 40′. A design change in the new detectors has successfully addressed a low light level charge transfer inefficiency.
Observations in seeing limited imaging conditions with an extremely large telescope - such as the European Extremely
Large Telescope (E-ELT) - will require large detectors and very fast cameras (around F/1.0). The correction of field
curvature is a complex task, requiring numerous optical elements operating with high incidence angles. Large format (60
to 90 mm square) concave detectors with a curvature radius between 500 and 250 mm would considerably simplify the
optical design, while improving image quality and cutting cost of optical components. Potential applications are not
limited to astronomy exclusively. The associated advantages of curved image sensors inside (mosaicked) focal planes
have been described in our paper “The challenge of highly curved monolithic imaging detectors”, presented at SPIE
This paper compares in a first step important developments in the area of curving CCD and CMOS detectors using
different technical approaches linked to specific thinning processes with a novel approach followed after ESO’s initial
feasibility study: First results of the latter are described with a report on the chosen curving technology aimed at
producing 500 to 250 mm radius of curvature silicon detectors of approximately 60 mm square format (typical
astronomical 4k × 4k CCDs). The curvature technique has been developed for front-illuminated devices with the goal of
extending the process to back-illuminated sensors in the near future. We will discuss the fabrication process of curving
the devices as well as the difficulties encountered during development. Characterization results from a curved detector,
including metrology, and electrical performance before and after curvature are presented.
The University of Arizona Imaging Technology Laboratory (ITL) has been developing back illuminated detectors and
detector technologies for several astronomical projects in recent years. These projects include the WIYN telescope One
Degree Imager (ODI) mosaic of Orthogonal Transfer Array CCDs, the VIRUS detectors for the University of Texas'
Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), detector and packaging development for the Large
Synoptic Survey Telescope (LSST), and 10kx10k and 4kx4k CCDs for several instruments. In this paper we discuss
these projects with an emphasis on backside processing issues and detector characterization results which may be
relevant to other groups. We will also focus packaging techniques and metrology for achieving very flat and stable focal
planes. Results will include device flatness at cryogenic temperatures, process yield, photo-response non-uniformity and
cosmetics, quantum efficiency, read noise, linearity, charge transfer efficiency, and photon transfer data.
The demand from the astronomical community for high resolution low noise CCDs has led to the development of the
STA1600LN, a 10560 × 10560 pixel, 95mm × 95mm, full-frame CCD imager with 9×9 μ2 pixels. The device
improvements include noise reduction to below 3ē at 100kHz, improved quantum efficiency, as well as packaging
developments for improved fill factor in mosaic systems. We provide test results from production devices, along with
updates on scientific systems utilizing the STA1600 for astronomy.
The Geostationary Lightning Mapper (GLM) instrument selected to fly on the National Oceanic and Atmospheric
Administration (NOAA) GOES-R Series environmental satellites has very unique requirements as compared to an
imaging array. GLM's requirements to monitor lightning on a continental scale will provide new insight into the
formation, distribution, morphology and evolution of storms.
A 500 frame per second backside illuminated frame transfer CCD imager (STA3900A) with variable pixel size has
been developed to meet these requirements. A variable pixel architecture provides a near uniform mapping of the curved
surface of the earth, while 56 outputs running at 20 MHz yield greater than a 1.1 Gigapixel per second data rate with low
RMS noise and high MTF. This paper will provide detailed information on design trades required. We will report CCD
read noise, dark current, full well capacity, and quantum efficiency (QE).
The WIYN One Degree Imager (ODI) will provide a one degree field of view for the WIYN 3.5 m telescope located on
Kitt Peak near Tucson, Arizona. Its focal plane consists of an 8x8 grid of Orthogonal Transfer Array (OTA) CCD
detectors. These detectors are the STA2200 OTA CCDs designed and fabricated by Semiconductor Technology
Associates, Inc. and backside processed at the University of Arizona Imaging Technology Laboratory. Several lot runs
of the STA2200 detectors have been fabricated. We have backside processed devices from these different lots and
provide detector performance characterization, including noise, CTE, cosmetics, quantum efficiency, and some
orthogonal transfer characteristics. We discuss the performance differences for the devices with different silicon
thickness and resistivity. A fully buttable custom detector package has been developed for this project which allows
hybridization of the silicon detectors directly onto an aluminum nitride substrate with an embedded pin grid array. This
package is mounted on a silicon-aluminum alloy which provides a flat imaging surface of less than 20 microns peakvalley
at the -100 C operating temperature. Characterization of the package performance, including low temperature
profilometry, is described in this paper.
This paper presents preliminary evaluation results of a test sensor of the backside-illuminated ISIS, an ultra-high
sensitivity and ultra-high speed CCD image sensor. To achieve ultra-high sensitivity, the CCD image sensor employs the
following three technologies: backside illumination, cooling and Charge Carrier Multiplication (CCM). The test sensor
has been designed, fabricated and evaluated. At room temperature without cooling, the video camera has about ten-time
higher sensitivity than the previous one, which was supported by a conventional front side illumination technology.
Furthermore, the video camera can detect images at very low signal level, less than 5 e-, by using CCM at -40 degree C.
The WIYN One Degree Imager (ODI) will provide a one degree field of view for the WIYN 3.5 m telescope located on Kitt Peak near Tucson, Arizona. Its focal plane will consist of an 8x8 grid of Orthogonal Transfer Array (OTA) CCD detectors with nearly one billion pixels. The implementation of these detectors into the focal plane has required the development of several novel packaging and characterization techniques, which are the subject of this paper. We describe a new packaging/hybridization method in which the CCD die are directly bonded to aluminum nitride ceramic substrates which have indium bump on one side and brazed pins on the other. These custom packages allow good thermal conductivity, a flat imaging surface, four side buttability, and in situ testing of the devices during backside processing. We describe these carriers and the backside processing techniques used with them. We have also modified our cold probing system to screen these OTA die at wafer level to select the best candidates for backside processing. We describe these modifications and characterization results from several wafer lots.
We present the status of PEPSI, the bench-mounted fibre-fed and stabilized "Potsdam Echelle Polarimetric and
Spectroscopic Instrument" for the 2×8.4m Large Binocular Telescope in southern Arizona. PEPSI is under construction
at AIP and is scheduled for first light in 2009/10. Its ultra-high-resolution mode will deliver an unprecedented spectral
resolution of approximately R=310,000 at high efficiency throughout the entire optical/red wavelength range 390-1050nm without the need for adaptive optics. Besides its polarimetric Stokes IQUV mode, the capability to cover the
entire optical range in three exposures at resolutions of 40,000, 130,000 and 310,000 will surpass all existing facilities in
terms of light-gathering-power times spectral-coverage product. A solar feed will make use of the spectrograph also
during day time. As such, we hope that PEPSI will be the most powerful spectrometer of its kind for the years to come.
The latest CCD detectors destined for advanced astronomical instruments are larger and have more output amplifiers
than previous devices. Examples are the Semiconductor Technology Associates, Inc. 16-output STA1600 and STA1900
devices and the 8-output STA2200 Orthogonal Transfer Array CCDs. Back illuminated versions of these devices have
been processed and evaluated at the University of Arizona Imaging Technology Laboratory and are the subject of this
paper. Characterizing these devices has required new optical testing equipment and optimized techniques to efficiently
evaluate device performance. This is especially true when even limited volume production is required. In this paper we
discuss the hardware related to characterization of the large format (135 mm diagonal) and 8- and 16- output CCDs at
cold temperatures, including quantum efficiency, charge transfer efficiency, noise, full well, cross-talk, and operating
parameters. We also discuss related developments in dewar construction and operation, including a hybrid closed cycle
and liquid nitrogen cooling system used for long-term testing, the characterization optical system, and related device
packaging. We also describe the equipment for wafer level probe testing of the same devices.
A full-wafer, 10,580 × 10,560 pixel (95 × 95 mm) CCD was designed and tested at Semiconductor Technology
Associates (STA) with 9 μm square pixels and 16 outputs. The chip was successfully fabricated in 2006 at DALSA
and some performance results are presented here. This program was funded by the Office of Naval Research
through a Small Business Innovation in Research (SBIR) program requested by the U.S. Naval Observatory for
its next generation astrometric sky survey programs. Using Leach electronics, low read-noise output of the 111
million pixels requires 16 seconds at 0.9 MHz. Alternative electronics developed at STA allow readout at 20
MHz. Some modifications of the design to include anti-blooming features, a larger number of outputs, and use
of p-channel material for space applications are discussed.
Due to aggressive scientific specifications, Semiconductor Technology Associates and
the University of Arizona's Imaging Technology Laboratory have collaborated to
develop a fully depleted back illuminated CCD for scientific imaging. These devices are
designed to target increased quantum efficiency into the near-infrared, without reduction
in the modulation transfer function, charge transfer efficiency, or rms noise. The
STA1700 series imagers are back illuminated 100 micron thick devices with a 10 micron
pixel pitch targeted to meet the requirements of the Large Synoptic Survey Telescope
(LSST). Recent characterization results will be presented including the point spread
function of a 2 micron spot. Also discussed will be the thinning and packaging
developments for the STA1700. These efforts include the addition of a backside bias
contact, invar package design with high density connectors, as well as etching and
backside coating optimization for high resistivity silicon.
Large area focal planes for the next generation of astronomical instruments require very flat imaging surfaces (< 10 μm
peak-valley) over significant sizes (20 - 100 cm), accurate alignment of detector height, stable operation at low
temperature, and fully-buttable packaging with large I/O requirements to connect multiple amplifiers per detector. These
requirements are often mutually exclusive and therefore difficult to obtain in a single focal plane. In this paper we
discuss the hybridization or flip chip bonding technique and associated focal plane mounting methods to achieve these
goals. Specifically, we describe a technique to hybridize CCD detectors onto high thermal conductivity ceramic with
vias that lead to the I/O signals underneath the detectors. Packaging methods to mount such devices with a total flatness
non-uniformity of less than 10 microns are presented. The requirements of achieving sub-5 microns flatness are also
Steward Observatory is currently commissioning a prime focus wide-field imager for the 90-inch telescope located at Kitt Peak. The camera's focal plane array is populated with a mosaic of four thinned
Lockheed 4096 x 4096 pixel CCDs. The f/2.98 system provides a plate scale of 0.45"/pixel and a total field-of-view of 1.16° x 1.16°. The optical design includes a four element corrector
and six position filter wheel. The first science run was conducted in
November 2003. We will describe the design of the "90prime" instrument and results from the commissioning runs.
Large format imaging detectors are required in many modern astronomical optical systems. With the increase in aperture size of large telescopes, the associated cameras for imaging and spectroscopy are much larger than those of a decade ago. Large physical format detectors are required to make full use of these cameras. The detector of choice has been the charge coupled devices (CCDs), although large format 4kx4k CMOS imagers have also been fabricated. We discuss recent developments in 4kx4k pixel imagers, typically with 15 micron pixels, which are over 60 mm per side. Several companies have produced such devices with the characteristics required for astronomy. Backside processing issues are discussed, including results from optimization efforts at the University of Arizona Imaging Technology Laboratory. We also discuss the use of such imagers in the 8kx8k 90Prime prime focus mosaic camera now in operation at the Steward Observatory 2.3 m telescope.
The orthogonal-transfer array (OTA) is a new CCD architecture designed to provide wide-field tip-tilt correction of astronomical images. The device consists of an 8x8 array of small (~500x500 pixels) orthogonal-transfer CCDs (OTCCD) with independent addressing and readout of each OTCCD. This approach enables an optimum tip-tilt correction to be applied independently to each OTCCD across the focal plane. The first design of this device has been carried out at MIT Lincoln Laboratory in support of the Pan-STARRS program with a collaborative parallel effort at Semiconductor Technology Associates (STA) for the WIYN Observatory. The two versions of this device are functionally compatible and share a common pinout and package. The first wafer lots are complete at Lincoln and at Dalsa and are undergoing wafer probing.
The use of spectrographs with telescopes having high order adaptive optics (AO) systems offers the possibility of achieving near diffraction-limited spectral resolution on ground-based telescopes, as well as important advantages for instrument design. The small stellar image diameters obtained with adaptively corrected systems allow high resolution without a large loss of light at the spectrograph entrance slit, as well as greater spectral coverage per exposure. The adaptively corrected echelle spectrograph (ACES), designed at Steward Observatory for a spectral resolution R ≈ 200,000, couples the telescope pupil to the instrument with a 10 mm diameter near single-mode optical fiber. Initial observations at the 2.5m telescope on Mt. Wilson validated the concept of achieving high spectral resolution with an adaptively corrected telescope and fiber coupled spectrograph. However the transmission of multiple modes in the fiber lead to a wavelength-dependent variation in illumination that made flat fielding impossible. In this paper we describe instrument design improvements, the installation and testing of a new CCD detector, and testing aimed at understanding and eliminating the fiber-related transmission problems to permit science quality imaging.
We present an overview of the ACS on-orbit performance based on the calibration observations taken during the first three months of ACS operations. The ACS meets or exceeds all of its important performance specifications. The WFC and HRC FWHM and 50% encircled energy diameters at 555 nm are 0.088" and 0.14", and 0.050" and 0.10". The average rms WFC and HRC read noises are 5.0 e- and 4.7 e-. The WFC and HRC average dark currents are ~ 7.5 and ~ 9.1 e-/pixel/hour at their operating temperatures of - 76°C and - 80°C. The SBC + HST throughput is 0.0476 and 0.0292 through the F125LP and F150LP filters. The lower than expected SBC operating temperature of 15 to 27°C gives a dark current of 0.038 e-/pix/hour. The SBC just misses its image specification with an observed 50% encircled energy diameter of 0.24" at 121.6 nm. The ACS HRC coronagraph provides a 6 to 16 direct reduction of a stellar PSF, and a ~1000 to ~9000 PSF-subtracted reduction, depending on the size of the coronagraphic spot and the wavelength. The ACS grism has a position dependent dispersion with an average value of 3.95 nm/pixel. The average resolution λ/Δλ for stellar sources is 65, 87, and 78 at wavelengths of 594 nm, 802 nm, and 978 nm.
A 512×512 CMOS Active Pixel Sensor (APS) imager has been designed, fabricate, and tested for frontside illumination suitable for use in astronomy specifically in telescope guider systems as a replacement of CCD chips. The imager features a high-speed differential analog readout, 15 μm pixel pitch, 75 % fill factor (FF), 62 dB dynamic range, 315Ke- pixel capacity, less than 0.25% fixed pattern noise (FPN), 45 dB signal to noise ratio (SNR) and frame rate of up to 40 FPS. Design was implemented in a standard 0.5 μm CMOS process technology consuming less than 200mWatts on a single 5 Volt power supply.
CMOS Active Pixel Sensor (APS) imager was developed with pixel structure suitable for both frontside and backside illumination holding large number of electron in relatively small pixel pitch of 15 μm. High-speed readout and signal processing circuits were designed to achieve low fixed pattern noise (FPN) and non-uniformity to provide CCD-like analog outputs. Target spectrum range of operation for the imager is in near ultraviolet (300-400 nm) with high quantum efficiency. This device is going to be used as a test vehicle to develop backside-thinning process.
The baseline design for the Large Synoptic Survey Telescope (LSST) requires a detector mosaic of over 2 Gigapixels covering a 55 cm diameter focal plane with 0.2 arcsec sampling. The camera and detector package for this telescope will benefit greatly by utilizing advanced concepts not normally required for astronomical telescope instrumentation. For the detector assembly, these concepts include low-cost, back illuminated CMOS or CCDs detectors with integrated electronic modules, curved detectors which would allow fewer but larger individual sensors, small pixels which maintain high MTF and full well capacity, anti-blooming techniques, fully-buttable packaging, and near room temperature operation. The camera may require a low thermal conductance gas-filled dewar to reduce atmosphere loading on the window, interchangeable and compact optical filters, and a flexible internal shutter. In this paper we discuss these issues relating to LSST focal plane technology.
The LSST Instrument is a wide-field optical (0.3 to 1um) imager designed to provide a three degree field-of-view with better than 0.2 arcsecond sampling. The image surface of the LSST is approximately 55cm in diameter with a curvature radius of 25 meters to flat. The detector format is currently defined to be a circular mosaic of 568 2k × 2k devices faceted to synthesize this surface within the constraints of LSST's f/1.25 focal ratio. This camera will provide over 2.2 Gigapixels per image with a 2 second readout time. With an expected typical exposure time of as short as 10s, this will yield a nightly data set on order of 3 terapixels. The scale of the LSST Instrument is equivalent to a square mosaic of 47k × 47k. The LSST Instrument will also provide a filter mechanism, as well as optical shuttering capability. Imagers of this size pose interesting challenges in many design areas including detectors, interface electronics, data acquisition and processing pipelines, dewar construction, filter and shutter mechanisms. Further more, the LSST 3 mirror optical system places this instrument in a highly constrained volume where these challenges are compounded. Specific focus is being applied to meeting defined scientific performance requirements with an eye to total cost, system complexity, power consumption, reliability, and risk. This paper will describe the current efforts in the LSST Instrument Concept Design.
The Advanced Camera for Surveys (ACS) is a third generation science instrument scheduled for installation into the Hubble Space Telescope (HST) during the servicing mission 3B scheduled for late February 2002. The instrument has three cameras, each of which is optimized for a specific set of science goals. The first, the Wide Field Camera, is a high throughput (43% at 700 nm, including the HST OTA), wide field (200' X 204'), optical and I-band optimized camera. The second, the High Resolution Channel (HRC) has a 26' X 29' field of view, it is optimized for the near-UV (a peak throughput of 24% at 500 nm) and is critically sampled at approximately 630 nm. The third camera, the Solar-Blind Camera is a far-UV, photon counting array that has a relatively high throughput over a 26' X 29' field of view. Two of the three cameras employ CCD detectors: the WFC a mosaic of two SITe 2048 X 4096 pixel CCDs and the HRC a 1024 X 1024 CCD based on the Space Telescope Imaging Spectrograph 21 micrometers pixel CCD. In this paper we review the performance of the flight detectors selected for ACS.
The University of Arizona Imaging Technology Laboratory has processed 4096 X 4096 15-micron Charge Coupled Devices (CCDs) fabricated at Lockheed Martin Fairchild Systems for back illuminated scientific applications. The devices have been optimized for astronomical observations in a direct imaging mode. Three types of back illuminated devices have been developed. The oldest devices are CCD4096JJ detectors which were custom fabricated for astronomical applications. The CCD485 devices are commercial sensors, originally fabricated for digital photography and medical applications. Because no frontside ground contact was included on either device, a backside contact was developed and applied as part of the backside processing. With this addition, very high quality back illuminated sensors have been developed. The CCD486 is a newer version of the 4k by 4k CCD with low noise amplifiers and a backside contact. These sensors have now been produced back illuminated with > 90 percent QE and read noise under 4 electrons. The devices show CTE of > 0.999998. Back illuminated versions CCDs have been fabricated with peak-valley flatness non-uniformity of less than 10 microns. A new epoxy underfill technique was developed to achieve this flatness and to avoid underfill voiding during epoxy application and curing. The new method applies a contact force on the CCD during the entire 48 hour cycle.
The high QE and large variety of formats make modern back illuminated Charge-coupled devices (CCDs) nearly ideal detectors for most scientific imaging applications. In the ultraviolet (UV), however, quantum efficiency (QE) instability with temperature and with environmental conditions has limited their widespread use, especially for space applications. We have developed several techniques to achieve stable and high QE in the 200 - 300 nm wavelength range with back illuminated CCDs fabricated by various manufacturers. In this paper we report peak QE of over 90% at 240 nm (uncorrected from quantum yield). We describe a series of tests which demonstrate stability of these devices with temperature, humidity, and UV illumination. These results are all based in the chemisorption backside coating processes developed at the Steward Observatory CCD Laboratory.
The Advanced Camera for Surveys (ACS) is a third generation science instrument scheduled for installation into the Hubble Space Telescope (HST) during the servicing mission 3B scheduled for June 2001. The instrument has three different cameras, each of which is optimized for a specific set of science goals. The first, the Wide Field Camera, will be a high throughput, wide field optical and I-band optimized camera that is half-critically sampled at approximately 570 nm. The second, the High Resolution Channel (HRC) has a 26 inch by 29 inch field of view, it is optimized for the near- UV and is critically sampled at approximately 630 nm. The third camera, the Solar-Blind Camera is a far-UV, photon counting array that has a relatively high throughput over a 26 inch by 29 inch field of view. Two of the three cameras employ CCD detectors: the WFC a mosaic of two SITe 2048 by 4096 pixel CCDs and the HRC a 1024 by 1024 CCD based on the Space Telescope Imaging Spectrograph 21 micrometers pixel CCD. IN this paper we review the performances of the devices baselined as flight candidates.
The Advanced Camera for Surveys (ACS) is a third generation instrument for the Hubble Space Telescope (HST). It is currently planned for installation in HST during the fourth servicing mission in Summer 2001. The ACS will have three cameras.
This paper describes the construction and testing of the Shack-Hartmann wavefront sensor camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 X 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 by 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 by 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 micrometers rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5-5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec squares field stop matched to the quall-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 micrometers /arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10 percent compared to an ideal quad-cell with perfectly sharp boundaries.
The advanced camera for surveys (ACS) will be installed in the Hubble Space Telescope during the third servicing mission in May 2000. The ACS has three cameras, each of which is optimized for a specific set of science goals. The wide field camera, is a high throughput, wide field, optical and I-band camera that is half critically sampled at 500 nm. The high resolution camera (HRC) is optimized for the near- UV, has a 26 inch by 29 inch field of view and is critically sampled at 500 nm. The solar-blind camera, is a far-UV, photon counting camera that has a relatively high throughput over a 26 inch by 29 inch field of view. The WFC employs a mosaic of two SITe 2048 by 4096 CCDs with 15 micrometers pixels and a SITe backside treatment, while the HRC channel is designed around a 1024 by 1024 CCD with 21 micrometers pixels, and a near-UV backside treatment developed at the Steward Observatory. In this paper we review the performance of the devices currently selected for flight, and discuss the design of their flight packages.
The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HST's capability for surveys and discovery by a factor of approximately 10 at 800 nm.
Back illuminated CCDs have been the detectors of choice for most astronomical imagers and spectrographs during the past decade. In recent years, we have developed processes to improve the performance of these detectors. Recent work has resulted in improved absolute quantum efficiency (QE), QE stability with temperature, QE stability with environmental contamination, and enhanced near-IR response. We demonstrate that QE near 100% can be achieved which is stable against hydrogen, dewar outgassing, and water contamination. We show that QE decrease with temperature can be eliminated for blue/visible optimized CCDs using a backside passivation layer, and significantly reduced for UV optimized CCDs which require very thin backside films. We also show that a 20% QE increase at 900 nm can be obtained by coating the frontside of back illuminated CCDs with a reflective metal film, without increasing interference fringing.
During the ground calibration of the Space Telescope Imaging Spectrograph (STIS) large scattered light haloes were identified in images of point sources and long slit spectral images at long wavelengths (greater than 750 nm). The long wavelength scattering was traced to the SITe 1024 X 1024 CCD and its header package, raising concerns for the performance of the Advanced Camera for Surveys (ACS) CCD detectors. ACS is a third generation axial instrument for the Hubble Space Telescope (HST) and will be installed during the 1999 Servicing Mission. Two of the ACS imaging channels employ SITe CCDs, so the ACS team have conducted a study of the long- wavelength scattering, in collaboration with SITe, to assess the impact to the ACS science program and develop a solution. In this paper we discuss our solution, its implementation on ACS CCDs, and describe the results of initial tests.
The Advanced Camera for the Hubble Space Telescope will have three cameras. The first, the Wide Field Camera, will be a high throughput (45% at 700 nm, including the HST optical telescope assembly), wide field (200' X 204'), optical and I-band camera that is half critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is critically sampled at 500 nm, and has a 26' X 29' field of view and 25% throughput at 600 nm. The HRC optical path will include a coronagraph which will improve the HST contrast near bright objects by a factor of approximately 10. The third camera is a far ultraviolet, Solar-Blind Camera that has a relatively high throughput (6% at 121.6 nm) over a 26' X 29' field of view. The Advanced Camera for Surveys will increase HST's capability for surveys and discovery by at least a factor of ten.
Characterization of CCDs is extremely important when developing scientific detectors. If CCD foundries are used to produce the devices, the foundries require feedback to maintain a quality process. In this case, the users require fairly automated testing to evaluate the large number of devices obtained from even a single lot run. We have developed a CCD characterization facility which is used to evaluate these foundry devices as well as commercial scientific images. Our test capabilities include automated QE measurements, X-ray CTE and gain calibration, optical illumination from 200 nm - 1200 nm, and dark current and read noise characterization. We can also make interferometric flatness measurements of the devices. A cryogenic probe station for wafer testing is being developed to extend some of these tests to the wafer level. We discuss in this paper our facilities and techniques to measure the quantum efficiency (QE) of scientific CCDs. QE (along with read noise) is perhaps the most important parameter for many classes of astronomical observations when working at very low light levels. It is also the most useful parameter for evaluating the quality of backside processing when developing back illuminated CCDs.
One of the drawbacks of using charge-coupled devices (CCDs) for scientific imaging is their relatively small size compared to many optical systems in which they are used. Telescopes, large format cameras, and medical imaging often require detectors much larger than the few cm dimensions of modern CCDs. One solution to this problem is to closely butt several CCDs together in the focal plane of the optical system, creating a focal plane mosaic. We have developed techniques to produce back illuminated CCDs from commercial front illuminated devices for enhanced quantum efficiency and spectral coverage. In this paper we discuss our development of packages and packaging techniques to butt back illuminated CCDs together, creating mosaics of up to 64 million pixels. We have discovered several critical issues during our development of back illuminated edge-buttable CCDs which we discuss in this paper. These include the development of proper chip carriers and packages, the ability to uniformly heat the devices in the required oxidation process, the ability to uniformly match antireflection coatings for all devices in a mosaic, and the development of alternative bonding methods which allow easy bonding of edge-buttable CCDs, especially as they approach whole wafer size.
Delta-doped CCDs, developed at JPL's Microdevices Laboratory, have achieved stable 100% internal quantum efficiency in the visible and near UV regions of the spectrum. In this approach, an epitaxial silicon layer is grown on a fully-processed commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited on the surface, followed by a 15 $angstrom silicon layer for surface passivation. The boron is nominally incorporated within a single atomic layer at the back surface of the device, resulting in the effective elimination of the backside potential well. The measured quantum efficiency is in good agreement with the theoretical limit imposed by reflection from the Si surface. Enhancement of the total quantum efficiency in the blue visible and near UV has been demonstrated by depositing antireflection coatings on the delta-doped CCD. Recent results on antireflection coatings and quantum efficiency measurements are discussed.
A design for an advanced camera (AC) third-generation Hubble Space Telescope scientific instrument is discussed. The AC is a three-channel spectrophotometric camera with wavelength sensitivity from 115-1000 nm. The AC, if selected, would be launched in 1999 for installation on HST. The axial bay design incorporates optical correction for the aberrated HST primary mirror and evolutionary advances in imaging capability.
The enhancement of the quantum efficiency of charge-coupled devices (CCDs) can be accomplished by several methods. The most dramatic increase comes from thinning the CCD for use in the back illuminated mode. Techniques to improve the QE of back illuminated CCDs include the deposition of backside thin film coatings to reduce reflection losses, surface charging to eliminate the backside potential well, and several aspects of device packaging. We have developed processes in these areas which have led to the post-manufacture optimization of devices for astronomical observations. WE describe our backside oxidation results and the interaction of the oxide with backside charging, the effect of ion absorption on backside charging, the development of one and two-layer antireflection coatings, and a new packaging method to improve near-infrared quantum efficiency.
The next generation of CCD imagers will undoubtedly be mosaics, in order to overcome (1) the low yield when fabricating large devices, (2) the limited number of readout channels that can be put easily on a single CCD, and (3) the physical limitation of the 4-inch silicon wafer. As a first step towards a 8192 x 8192 CCD mosaic, we have recently fabricated a 2 x 2 array of Loral 2048 x 2048 CCDs. The two-side buttable design of these chips (by John Geary of SAO) allows us to achieve gaps of about 0.6 mm (40 pixels). This prototype mini-mosaic imager, using unthinned, front-illuminated CCDs has been used at the KPNO 0.9 m and 4 m telescopes. As we are constructing a number of scientific-grade mosaics with thinned chips for use at KPNO and CTIO, we are beginning the design and fabrication work for an 8192 x 8192 imager. This will be a 2 x 4 array of Loral 4096 x 2048 CCDs with interchip spacing of less than 0.5 mm. Such a device will have a physical size of approximately 5 inches square and will cover an area of 38.6 (59.1) arcminutes on an edge at the 4 m (0.9 m) telescope with a pixel size of 0.28 (0.43) arcseconds per pixel. This paper discusses results obtained with the 4096 x 4096 minimosaic and design strategies/progress on the larger 8192 x 8192 imager. Specifically, we present designs of the Dewar and mechanical interface for the large mosaic, a physical mounting scheme which will achieve better than 5 micron RMS flatness, and a discussion of the electronics and controller (the CTIO transputer-based ARCON), which will allow us to read out the entire array in less than two minutes. Some strategies for dealing with the large amount of data (128 Megabytes per image) will be presented.
A thinned-CCD mosaic was fabricated from four Loral 2048 X 2048 edge-buttable CCDs. Thinning was performed on the whole wafer with subsequent dicing and handling facilitated by bonding the thinned wafer to a glass plate. Packaging of the devices involved a `flip-chip' wire-bonding technique followed by chemical dissolution of the glass support. The fabrication process was designed to minimize the gaps between devices and retain a high degree of flatness in the finished CCDs.
The optimization of back illuminated CCDs for low-light-level applications requires many process steps. One such step is the deposition of thin films on the freshly thinned backside surface. These films may consist of many layers depending on both the desired properties of the detector and on the backside charging mechanism. We describe our backside coating process which has been optimized for astronomical applications. After thinning, we first grow a thin silicon oxide film in a steam environment. Following oxidation we deposit an antireflection coating optimized for a particular wavelength. We may also deposit a thin film of platinum between these layers that acts to charge the backside. Using these thin film coatings we have been able to produce CCDs which reach silicon's theoretical maximum quantum efficiency over the 300 - 1000 nm wavelength region.
Ongoing experiments using thin electrically conducting transparent layers of Indium Tin Oxide to control the surface potential of thinned CCDs are described. The results are very encouraging with good uniform ultraviolet sensitivity being obtained from CCDs of different types and thinned by different processes. The enhanced response is stable in air and in vacuum for periods longer than a year. 2.
We have developed a thinning and packaging process which allows the conversion of front-illuminated charge-coupled devices (CCDs) into back illuminated sensors. This process does not depend on any special processing by the manufacturer and can therefore be used with any type of CCD. The process consists of several major steps which include: 1) making a silicon substrate with conductive traces and indium bumps which mate to the CCD wire bonding pads 2) placing indium bumps on the CCD wire bonding pads 3) bump bonding the substrate and CCD together 4) thinning 5)packaging 6) oxidizing the backside surface 7) applying antireflection coatings and 8) backside charging. Using this process with Loral 1200x800 and 3072x1024 CCDs we have produced devices with quantum efficiency in excess of 80 in the near-UV and visible wavelength regions. The surface flatness of these devices has been measured interferometrically to deviate from a plane by less than 1 um rms for the 1200x800 pixel sensors. 2.
Charge-coupled devices (CCDs) have become extremely important detectors for the entire astronomical community. We discuss their properties in relation to astronomical imaging and spectroscopy. We also consider some of the improvements we hope to see to further their use in astronomy and other scientific fields. These include larger area detectors and mosaics of detectors with flat and stable packaging, antireflection coatings on back illuminated devices, and extremely low read noise for spectroscopic applications. We discuss some of our research into these areas.
The authors use charge-coupled devices (CCDs) from a lot of Loral wafers containing 3072 X 1024 and 800 X 1200 pixel detectors. These CCDs were specifically designed for astronomical spectrographic applications with the intention of thinning them at Steward Observatory for backside illumination. The thinning procedure and how it relates to the packaging method developed to keep the devices flat for use in fast optical beams are described. Initial results with a new oxidation procedure to allow better backside charging is also described. This oxidation method is especially effective with biased gate-charging techniques which apply a voltage directly over the backside oxide.
The design of aluminization systems for the MMT Conversion 6.5 m mirror and the Columbus
Project 8 m mirror has led us to reconsider many of the design issues and tradeoffs for such systems.
Coating of the large honeycomb mirrors will be done in situ on the telescope with a portable bell jar
forming the front half of a two-stage vacuum system. The mirror cell forms a "dirty" vacuum behind
the mirror to eliminate excess force on the glass. A multi-ring source geometry has been proposed to
allow a 1.0 m spacing between the mirror surface and the sources thereby minimizing the size of the
vacuum chamber. Evaporation source models have been developed to optimize the number of
sources, ring spacing, and high incidence angle emission to achieve better than 5% rms deviation in
coating thickness over the diameter. Code results are compared to empirical thickness profiles
measured at the University of Arizona's (UA) Sunnyside 2.0 m coating facility. Cryoadsorption
pumps are considered for reasons of economy, quality of vacuum, pumping speed, and reliability.
The interaction of the cryopumps and getter pumping with the pumping/cleaning/deposition cycle is
studied. Glow discharge cleaning is discussed and the results of deposition tests in 10' Torr residual
argon are given. Electrical requirements are estimated and a novel transformer design may decrease
the current entering the chamber from 12,000 A to less than 600 A.
Recent research efforts aimed at optimizing charge-coupled devices (CCDs) after their manufacture to achieve maximum quantum efficiency, wide spectral bandpass, and excellent cosmetics and surface flatness are discussed. We present results of a new acid thinning agitation technique which produces very uniform, high quality surfaces on large area square and rectangular CCDs and 4" silicon wafers for back illuminated operation. In particular we present thinning results of Ford Aerospace 2048x2048 pixel CCDs. A method of cleaning thinned CCDs before antireflection coating for increased QE is also discussed. The results of initial experiments with a new packaging method to mount thinned CCDs while maintaining a very flat imaging surface are presented. This bump bonding mounting technique increases yield due to reduced handling and robust packaging and is expandable to tightly packed large area focal plane mosaics.
Recent research efforts aimed at optimizing charge-coupled devices (CCDs) after their manufacture to achieve maximum quantum efficiency, wide spectral bandpass, and excellent cosmetics and surface flatness are discussed. Results are presented of a new acid thinning agitation technique which produces very uniform, high-quality surfaces on large area square and rectangular CCDs and 4-in silicon wafers for back illuminated operation. A method of cleaning thinned CCDs before antireflection coating for increased quantum efficiency is also discussed. The results of initial experiments with a new packaging method to mount thinned CCDs while maintaining a very flat imaging surface are presented. This bump bonding mounting technique increases yield due to reduced handling and robust packaging and is expandable to tightly packed large area focal plane mosaics.