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Low-Light-Level Charge-Coupled Device Imaging in Astronomy

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Low-Light-Level Charge-Coupled Device Imaging in Astronomy J. Anthony Tyson Vol. 3, No. 12/December 1986/J. Opt. Soc. Am. A 2131 Low-light-level charge-coupled device imaging in astronomy J. Anthony Tyson AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Received May 24, 1986; accepted August 7, 1986 Charge-coupled devices (CCD's) on both large and small telescopes are revolutionizing astronomy, permitting studies to be made of objects up to 10 times fainter than possible by using photographic and video camera techniques. This is due both to the high quantum efficiency and to the photometric stability of the CCD. Chopping techniques, taking advantage of this stability, permit cancellation of low-level systematics. Faint galaxies and stars of 27-V magnitude are detected in 2-h integration on a 4-m telescope, corresponding to 0.02 (photons/sec)/pixel. Automated image preprocessing and pattern recognition at high data rates permit statistical studies to be made of multispectral CCD data. In the future the efficiency of large telescopes for survey studies will be improved by the use of CCD mosaics covering the entire usable focal plane. the photographic plate is seriously inadequate in sensitivity, INTRODUCTION wavelength response, reproducibility, dynamic range, and The history of astronomy is, in large part, dominated by the linearity. history of detector development. As detector technology Much of modern optical and infrared (IR) astronomy is progressed from the unaided eye to visual telescopes, to photon starved. That is, to pursue current research in imag- photographic emulsions, and to photoelectric systems, each ing and spectroscopy, it is often necessary to integrate longer improvement brought with it answers to old questions as than several nights for sufficient signal-to-noise ratio. Yet well as a host of new questions and the discovery of new for such long integrations there are diminishing returns: types of objects. In astronomy, as in particle physics, pro- Low-level systematic error because of weather (night-sky gress is closely tied to deeper images, higher resolution, larg- variations, clouds, etc.) limits the accuracy of photometry. er statistical samples, and broader spectral response, as the Thus this fractional error in night-sky brightness translates observer effectively opens his eyes wider to the view offered into a limiting faintness for surface photometry of galaxies. by the universe. Even worse, there are limiting upper-atmosphere-generated The remarkable improvement in detector sensitivity dur- systematics on the detector itself. To solve these problems, ing the past century-from early emulsions to charge-cou- we must use the highest-quantum-efficiency detectors on pled devices (CCD's)-is sketched in Fig. 1. The flux from the largest telescopes and develop image-acquisition and the faintest galaxies detected in optical surveys is plotted as -reduction techniques that cancel the systematics. a function of time. In the CCD camera the astronomer now has at his disposal an instrument whose sensitivity is limited CHARGE-COUPLED DEVICES primarily by quantum mechanics and the nature of our at- mosphere. Perhaps the most significant gains are now being Followingthe invention of CCD's by Boyle and Smith,' they made in faint-object spectroscopy. For years it has been have been used widely as memories, delay lines, and imagers. relatively easy to record images of faint galaxies and stars for The book by Sequin and Tompsett2 reviews the early CCD which adequate spectra, at a few angstroms' resolution, were architecture. The architecture with the maximum effective beyond the reach of the largest telescopes. The recent re- quantum efficiency is the so-called "frame-transfer" CCD. duction in CCD readout noise and the virtual elimination of Charge in pixel wells is clocked down columns and read out low-level systematics in CCD's has raised the throughput of (last row) through an on-chip amplifier. The presence of spectrographs from less than 1 to more than 10%. Further surface states and the resulting trapped charge in surface- significant improvements in sensitivity can be attained only channel CCD's reduces their usefulness at low 'light levels. by the construction of larger-aperture telescopes. A solution is to store the charge in the bulk semiconductor While the observer of small objects has in the CCD camera away from the Si-SiO2 interface. CCD's used in low-light- a device nearly optimum in terms of sensitivity and re- level detection are buried channel and usually three phase. sponse, this happy state of affairs does not extend to wide- See Blouke et al.3 for a description of a modern CCD. If the field photometry and to statistical or survey observations. device is kept sufficiently cold, say, at 160 K, latent images Currently available CCD's are small and cover only about 1% may be stored for days, and the thermal phonon-induced of the good image area at the prime focus of typical large noise is negligible in a 1-h exposure. Thermally induced telescopes. The statistical astronomer simply cannot afford dark current at 160 K is about 20 e per pixel per hour. to waste 99% of the image and is thus forced to rely on Recent buried-channel CCD's have full wells of 105 e or photographic emulsions for the detector. The price paid for greater and have a dynamic range of up to 105. The charge- wider coverage is high: In comparison with CCD cameras, transfer efficiency is now typically 0.99999 at exposure levels 0740-3232/86/122131-08$02.00 © 1986 Optical Society of America 2132 J. Opt. Soc. Am. A/Vol. 3, No. 12/December 1986 J. Anthony Tyson MODERNLIMITS FOR GALAXY PHOTOMETRY graphic plates is more than a factor of 10. Photomultiplier tubes revolutionized the field of single-object photometry 30 and spectroscopy. The photocathode quantum efficiency of * LARGEAREA SURVEYS - * SMALLAREA SURVEYS up to 20%was equivalent to an order-of-magnitude increase 27 V EXPEREENTS - STATEOF ART 7% E 4 TH LIT 2R in the light-gathering power of the telescope. Even so, the -7 A, VISUALCATALOGS .; 25 phototube could measure only one wavelength at a time in a I 24 EI 23 -G scanning spectrograph. The introduction of image intensi- ao , fiers brought the quantum efficiency of photomultiplier I -5 20 tubes to imaging detectors. During the 1970's, ISIT's and 5I similar intensified detectors were introduced into spectro- -4 5 If graphs; see Robinson and Wampler and Boksenberg.6 Good reviews are given by Ford 7 and Timothy. 8 Aside from -3 PHXOTOGRAPHY '5 limitations imposed by their front-end quantum efficiency, these detectors have been limited in faint-object work be- 19A 191S 12I 1G3G 1948 15 19S0 17S 1lOS 1330 cause of systematics: YEAR Problems with the stability and repro- ducibility of the detector often preclude effective Fig. 1. The history of the faint limit of galaxy photometry has I chopping dominated by advances in detector technology. Shown is the lii ietn techniques for removal of low-level systematics in long expo- 2 ing flux in photons cm- sec-1 A-i (and also green magnitude) as a sures. Multiobject low-light-level spectroscopy and surveys function of time. The CCD has pushed the efficiency per detector for very faint objects had to wait for the CCD. area of telescopes to its theoretical maximum. The lower dasshed In cases in which images are needed more than once per line charts the large-area photographic Schmidt surveys. 100 sec or so, CCD's have a disadvantage: A noise penalty of 10 or more electrons is paid on each read, per pixel. Thus, for speckle work where millisecond sampling is required, intensifiers (discussed elsewhere in this issue) are the only 200 alternative. Since the signal-to-noise ratio (S/N) is deter- mined by the first element in the detector system, the use of an intensified detector with a quantum efficiency of 4-10% results in a decrease in S/N (per exposure time) by a factor of 3-5. However, for the vast majority of applications in low- light-level imaging and spectroscopy, longer integration times are allowable, and one can take advantage of the high quantum efficiency (QE) and dimensional stability of the CCD. In many cases, 1-2-m class telescopes instrumented with CCD's are now doing the research that was possible only on the largest telescopes in the past. , 100 200 Loh9 built one of the first CCD cameras used in astrono- / EFFECTIVEILLUMINATION PHOTONS*A E my. The care and feeding of CCD's was reviewed by Gunn and Westphal,'0 and several good reviews of CCD's applied Fig. 2. The deferred charge effect at very low light levels in CCD's. to astronomy appear in Vol. 331 of the Proceedings of the Current state-of-the-art CCD's have less than 10-30-e deferpred Society of Photo-Optical and Instrumentation Engineers.1 charge. These nonlinearities, important only in very short or s] troscopic exposures, can be reduced in image processing. New techniques leading to enhanced and long-term stable ultraviolet (UV) response of CCD's have been developed above 10-50 e/pixel. This deferred-charge effect is show]n in recently. 12 "3 Unlike photographic plates, video cameras, Fig. 2, and it can be reduced in image processing. 4 Phe deferred charge or fat-zero level varies from column toi Col- umn and is generally below 30 e in the best current CC]D's. This low-level nonlinearity is of concern only in very s tort exposures or spectroscopic applications, in which the baLck- ground in the exposure is below 100 e. z Characteristics of currently marketed CCD's suitable for L very low-light-level work vary widely.
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