Richard Reed

What is better than an 8192x8192 CCD Mosaic imager: two Mosaic wide-field imagers, one for KPNO and one for CTIO

A new generation wide-field imager is being developed and will be put into service at sites in North and South America. Driven by the requirement for larger imaging areas and more pixels but limited by manufacturing process constraints, manufacturers are developing 2, 3, and 4-side buttable CCDs that can be tiled to achieve large imaging areas as opposed to developing a single large CCD. NOAO has designed, fabricated, and tested a wide-field imager called Mosaic that tiles 8 CCDs to produce an imaging area slightly greater than 123 mm X 123 mm. Several successful science observation runs have been completed using Mosaic at the KPNO Mayall 4 m and .9 m telescopes. A second Mosaic Wide-Field Imager is presently being manufactured and will be deployed at the CTIO Blanco 4 m telescope early next year. This report will focus on the mechanical design aspects of the Mosaic Imager and the upgrade path to achieve the scientific requirements will be discussed.

New wide-field corrector for the Kitt Peak Mayall 4-m telescope

The Kitt Peak Mayall 4-m telescope required a new prime focus corrector having a flat focal plane covering 36 arcmin on a side (51 arcmin diagonal) to accommodate the Mosaic 8K X 8K CCD system. The scientific requirements for the new corrector included atmospheric dispersion compensation (ADC), excellent near-UV efficiency, excellent image quality, and extremely low scattered light and ghosting. The optical system designed to meet these demands exhibits excellent and stable performance through its first year of operation. This paper describes the innovative design and engineering aspects of the corrector. Science verification data are presented to demonstrate some of the attributes of the new corrector.

CCD detector for the space telescope imaging spectrograph

The space telescope imaging spectrograph (STIS) is currently being developed for in-orbit installation onto the Hubble Space Telescope in 1997, where it will cover the wavelength range from 115 to 1000 nm in a variety of spectroscopic and imaging modes. For coverage of the 305 - 1000 nm region (and backup of the 165 - 305 nm) region, STIS will employ a custom CCD detector which has been developed at Scientific Imaging Technologies (SITe; formerly Tektronix CCD Products Group). This backside-illuminated device incorporates a proprietary SITe backside treatment and anti-reflective coating to extend the useful quantum efficiency shortward of 200 nm. It also features low noise amplifiers, multi-pinned-phase implants, mini-channel implants, and four quadrant readout. The CCD is thermo-electrically cooled to an operating temperature of -80 degree(s)C within a sealed, evacuated housing with its exterior at room temperature to minimize the condensation of absorbing contaminants in orbit. It is coupled to a set of low noise, flexible, fault-tolerant electronics. Both housing and electronics are being developed by the STIS prime contractor, Ball Aerospace & Communications Group. We describe here the design features, performance, and fabrication status of the STIS CCD and its associated subsystem, along with results of radiation testing.

Development of a 8192x8192 CCD mosaic imager

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.

Recent advances in short-wavelength AR coatings for thinned CCDs

This paper discusses the development of two materials as AR coatings for thinned backilluminated charge-coupled devices. The first material is the heavy metal oxide Ta205 deposited as a spin on layer using sol-gel technology. The second material is Si3N4. Both these films have the high index of refraction and low absorption coefficients needed to produce good AR coatings in the near UV down to 300 nm. The goal of the program was to produce a coating which would yield devices with quantum efficiencies of greater than 50 at 300 nm. Both these materials satisfy this goal. Data on test devices will be reported. . 1.

CCD detector performance for NOAO's wide-field MOSAIC cameras

In July of 1998 the National Optical Astronomy Observatories (NOAO) successfully upgraded MOSAIC 1, an 8192 by 8192 pixel array using eight Scientific Imaging Technologies, Inc. (SITe) St-002A thinned backside 2k by 4k charge coupled devices (CCDs). In July of 1999 MOSAIC II, a clone of MOSAIC I was commissioned also using eight SITe ST-002A CCDs. Additionally in December of 1998 NOAO implemented Mini- MOSAIC a 4096 by 4096 pixel array using two SITe ST-002A thinned CCDs. This report will discuss the performance, characterization and capabilities of the three wide field imagers now in operation at NOAOs Kitt Peak Observatory, Cerro Tololo Inter-American Observatory and at the WIYN Consortium 3.5-Meter telescope on Kitt Peak.

Space Telescope Imaging Spectrograph detectors and ultraviolet signal-to-noise capabilities

The space telescope imaging spectrograph (STIS) was designed as a versatile spectrograph capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph and the Faint Object Spectrograph (FOS) over the broad bandpass extending from the UV through the visible. STIS achieves performance gains over the aforementioned first generation Hubble Space Telescope instruments primarily through the use of large a real detectors in both the UV and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long slit or slitless spectroscopy. This paper will review the detector design and in-flight performance. Attention will be focussed on the key issue of S/N performance. Spectra obtained during the first few months of operation, illustrate that high signal-to-noise spectra can be obtained while exploiting STISs multiplexing advantage. From analysis of a single spectrum of GD153, with counting statistics of approximately 165, a S/N of approximately 130 is achieved per spectral resolution element in the FUV. In the NUV a single spectrum of GRW + 70D5824, with counting statistics of approximately 200, yields a S/N of approximately 150 per spectral resolution element. An even higher S/N capability is illustrated through the use of the fixed pattern split slits in the medium resolution echelle modes where observations of BD28D42 yield a signal-to-noise of approximately 250 and approximately 350 per spectral resolution element in the FUV and NUV respectively.

Large-area edge-buttable CCDs for mosaic focal plane applications

This paper will describe in some detail tow new large area CCD image sensors designed specifically to be used either as a single imager or assembled in mosaics of CCDs. The devices have 2048 X 4096, 15 micrometers pixels; the difference being the orientation of the serial register. Performance data are presented on both front- and back-illuminated parts. In addition, a new production camera test system will also be described which is being used to screen test the Advanced Camera CCDs for the wide field and high resolution channels.

SIOO2A: a three side buttable 2048 x 4096 CCD image sensor

This paper will describe in some detail a new large area CCD image sensor designed specifically to be used either as a single imager or assembled in large, tightly configured mosaics of CCDs. The device has 2048 X 4096, 15 micrometers pixels. Performance data are presented on both front- and back-illuminated parts.

Space telescope imaging spectrograph 2048 CCD and its characteristics

Development of a 20482 CCD for a second-generation space telescope instrument has produced some very encouraging devices. The first experimental lot of 10 devices have very few defects, dark currents of less than 12 electrons/pixel/hour at -80 degree(s), readout noise levels of less than 4 electrons rms, and excellent charge transfer efficiency at signal levels of less than 10 electrons.