Proc. Natl. Acad. Sci. USA Vol. 90, pp. 9751-9753, November 1993 Colloquium Paper

This paper was presented at a colloquium entitled "Images of Science: Science ofImages," organized by Albert V. Crewe, held January 13 and 14, 1992, at the National Academy of Sciences, Washington, DC. Charged-coupled detector sky surveys DONALD P. SCHNEIDER Institute for Advanced Study, Princeton, NJ 08540

ABSTRACT Sky surveys have played a fundamental role in years to prepare the equipment, years to acquire and cali- advancing our understanding of the cosmos. The current pic- brate the observations, and years to analyze the results-an tures of stellar evolution and structure and kinematics of our image that is in fact not far from the truth. This type of Galaxy were made possible by the extensive photographic and research rarely generates the excitement (professional or spectrographic programs performed in the early part ofthe 20th public) that accompanies many of the heralded discoveries century. The Palomar Sky Survey, completed in the 1950s, is still (e.g., pulsars or gravitational lenses), but it is unusual indeed the principal source for many investigations. In the past few for these breakthroughs not to have been based, at least in decades surveys have been undertaken at radio, millimeter, part, on the data base provided by previous surveys. It should infrared, and x-ray wavelengths; each has provided insights into also be noted that every time an astronomical survey has new astronomical phenomena (e.g., quasars, pulsars, and the 3° been undertaken in an unexplored wavelength band (e.g., cosmic background radiation). The advent of high quantum radio, x-ray), startling discoveries have followed. efficiency, linear solid-state devices, in particular charged- Astronomy owes a special debt to surveys; many of the coupled detectors, has brought about a revolution in optical most fundamental advances in the past century were made astronomy. With the recent development of large-format possible by the existence of accurate catalogs of the posi- charged-coupled detectors and the rapidly increasing capabili- tions, brightnesses, colors, motions, etc., ofcelestial objects. ties ofdata acquisition and processing systems, it is now feasible Table 1 presents a partial listing of large astronomical sur- to employ the full capabilities of electronic detectors in projects veys, the date they were performed, the wavelength band of that cover an appreciable fraction of the sky. This talk reviews the survey, and a sample of the important discoveries based the first "large scale" charged-coupled detector survey. This on the work. Occasionally the advances came only when program, designed to detect very distant quasars, reveals the information from two surveys were combined; quasars is a powers and limitations of charged-coupled detector surveys. classic example. This paper confines itselfto a discussion ofoptical surveys. It is fitting to open this conference with an overview of the Although optical surveys are the oldest (and until the last 50 oldest of the sciences, astronomy. This field has been at the years the only) type of astronomical survey, recent techno- forefront of image analysis for millennia, from the earliest logical advances have revolutionized our ability to perform paintings and star charts through the incredible electronic large-scale (covering an appreciable fraction of the sky) visions of the planets provided by spacecraft. Astronomy is surveys with high-efficiency solid-state detectors. also the purest (or the most primitive, depending on one's point of view) science in its form of image acquisition. Survey Efficiencies Although we have a large variety of instrumentation at our disposal, one cannot call observational astronomy an exper- Before we enter into a discussion of current and future imental science; we but passively record images of the astronomical surveys, it is desirable to develop a quantitative heavens, for we cannot manipulate the objects of interest. figure ofmerit, a relation that allows an objective comparison The goal ofthis presentation is to convey the excitement of of a range of instrumentation and observational techniques. what I call the third electronics revolution in optical astron- A simple expression for the efficiency of a given survey is: omy. The first occurred about half a century ago, when the linear response and high sensitivities ofphotomultiplier tubes Survey efficiency = D2 Qi q f, dramatically improved the accuracy of many astronomical measurements. The two-dimensional detectors of the 1970s, where De is the effective collecting diameter ofthe instrument such as silicon intensified targets and charged-coupled de- (in the case ofoptical astronomy, the diameter ofthe primary tectors (CCDs), allowed pictures of small areas of the sky to mirror), Ql is the solid angle of the field of view, q is the be taken with linear high-quantum-efficiency devices. I be- system quantum efficiency, and f is the fraction of the time lieve that the next decade will witness another quantum leap. that survey data can be acquired (1). In this paper the units This advance will not be driven by breakthroughs in detector of system efficiency will be expressed in m2-deg2. technology but will occur because data-processing capabili- The last factor,f, may seem superfluous (it is obvious that ties will permit the undertaking of large-scale surveys with the efficiency goes as the fraction of the time the shutter is electronic detectors. open; one should simply survey all ofthe time) but is actually quite important in distinguishing differences between what at Importance of Surveys in Astronomy first appear to be surveys with similar efficiencies. Without the final factor, one would assume that doubling the collect- When one thinks of large scientific survey projects, one ing area and halving the field of view will result in no change visualizes a large team of scientists and engineers laboring in survey efficiency, but this is in general not true. If one wants to survey an area to given flux limit, the exposure time The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Abbreviations: CCD, charge-coupled detector; deg, degree; SDSS, in accordance with 18 U.S.C. §1734 solely to indicate this fact. Sloan digital sky survey. 9751 Downloaded by guest on October 3, 2021 9752 Colloquium Paper: Schneider Proc. Natl. Acad. Sci. USA 90 (1993) Table 1. Astronomical surveys Survey Wavelength, ,um Date Scientific results Henry Draper 1920s Stellar evolution (spectroscopy) 0.5 Galactic structure Astrometric (stellar positions) Galactic kinematics Harvard Sky Patrol 0.5 1920s Variable stars Palomar Sky Survey 0.5 1950s Quasars, clusters of galaxies Radio (3C, etc.) 1i0 1950s Quasars, pulsars, HI gas Superluminal motion X-rays 0.001 1970s-80s Hot gas in clusters Compact objects IRAS 30 1980s Infrared cirrus, star formation Luminous IR galaxies/quasars COBE 1000 1980s-90s Shape of 30 background Deviations from isotropy IRAS, Infrared Astronomical Satellite; COBE, Cosmic Background Explorer.

on both telescopes is the same, but one must take 4 times as camera/spectrograph (2), completed in 1984, which contains many "pictures" with the larger telescope, and each image four 800 x 800 Texas Instruments CCDs in a two-by-two has an associated overhead caused by prepping and reading mosaic. (iii) The technique of continuous scanning CCDs of the detectors. (In some cases this "dead time" can be (running in time-delay-and-integrate mode) was developed. avoided; see the next section.) The final factor also incor- This process takes advantage of the geometry of the CCD porates site-dependent effects (e.g., the fraction of the time readout mechanism. To measure the accumulated signal in lost to clouds at an observatory; the fraction of the orbit each pixel on the device, the charge is shifted row by row during which a satellite can acquire data on the field). Since down the CCD to the readout amplifier. By having the sky this paper wants to emphasize the technological aspects of move across the CCD at the same rate as the electrons are surveys, we will only consider the impact of the instrumen- being shifted through the detector (each individual object tation's properties in the estimate of f. This definition of stays with the same packet of charge), one eliminates the survey efficiency omits details ofthe detectorproperties-for overhead associated with prepping and reading the detector example, the linear response, large dynamic range, and noise (the factorfin the efficiency relation becomes 1.0). (iv) Data attributes of CCDs result in improvements in data quality acquisition and analysis systems became capable of coping over that obtained photographically. with the large data rates. Table 2 shows the survey efficiencies of a range of optical In 1981 Maarten Schmidt (California Institute of Technol- instrumentation. The top line gives the parameters for the ogy), James Gunn (Princeton University), and I began tests venerable Palomar Optical Sky Survey. Although the col- to see ifCCD surveys could be effective in the search for very lecting area of the Palomar Schmidt is not particularly large high redshift quasars. At that time the highest known redshift (only about 6% of the 5-m Hale telescope, which was was 3.53, a discovery made in 1973. Despite extensive efforts completed at the same time and was situated only 200 m in the late 1970s, no larger redshift had been found, and in away), the Schmidt's enormous field of view easily compen- 1981 only about 40 quasars had been identified with redshifts sates for its aperture deficiency. Even with the detector larger than 3. Detecting quasars at z > 3.5 is quite a challenge: technology of a dozen years ago, the photographically the objects are rare, so large areas of sky must be searched; equipped Schmidt remained a far better survey instrument the objects are faint, so the survey detectors must have high than a CCD camera mounted on the Hale; the 2 orders of quantum efficiency; and the objects emit very little flux in the magnitude gain in efficiency of the solid-state device is blue region ofthe spectrum, so the survey must operate at red insufficient to overcome the miniscule sky coverage caused wavelengths. CCDs satisfied the final two criteria, and with by the small physical size of the CCD. the light-gathering power of the Hale telescope combined with the 4-Shooter camera, it became possible to perform an Palomar Transit "Grism" Survey efficient CCD survey (see Table 2). The first 4-Shooter survey data were obtained in early Four recent developments have made CCD surveys compet- 1985; within a year we were able to perform routine survey itive with wide-field photographic surveys. (i) The format of operations. The survey data were acquired by using the Hale electronic devices has grown rapidly. In 1977 the largest telescope as a transit instrument-i.e., the tracking motors of CCDs available for astronomical work were but 256 pixels on the telescope were shut down and the CCDs were continu- a side; this size grew to 800 in 1981, and detectors having a ously read at the sidereal rate (the apparent motion ofthe sky 2048 x 2048 pixel format are now in use. (ii) Cameras now due to the earth's rotation) for several hours. This technique exist that contain mosaics of CCDs; the survey described in produces survey areas that are long tapestries of the sky; a this section is based on data from the 4-Shooter CCD typical night covers an area -8.5 arc min wide (the field of Table 2. Examples of optical instrumentation Instrument De, m Ql, sq deg q f SE Palomar Sky Survey 1.2 36 0.005 0.8 0.21 KPNO 4-m (plates) 3.7 0.5 0.005 0.8 0.03 Palomar 5-m (plates) 4.7 0.2 0.005 0.8 0.02 Palomar 5-m (PF CCD) 4.7 0.01 0.2 0.8 0.04 Palomar 5-m (CCD scan) 4.7 0.02 0.4 1.0 0.17 Digital Sky Survey 2.2 1.3 0.4 1.0 2.5 PF CCD, prime focus CCD; KPNO, Kitt Peak National Observatory. Downloaded by guest on October 3, 2021 Colloquium Paper: Schneider Proc. Natl. Acad. Sci. USA 90 (1993) 9753 view of the 4-Shooter) and =75 degrees (deg) long (an areal proposed "all-sky" CCD survey. The University ofChicago, coverage of 2.2 sq deg/hr [1 sq deg = (ir/180)2 steradian] at Johns Hopkins University, Fermi National Accelerator Lab- the celestial equator). The exposure times, only 33 sec at the oratory, The Institute for Advanced Study, and Princeton celestial equator, are unusually short for survey work, but University, together with the Sloan Foundation, have begun that is all the time an object takes to drift across the 4-Shooter work on the SDSS. This project will image 10,000 sq deg (one field at the sidereal rate. quarter of the sky) in four different filters (u, g, r, and i). In The spectra of quasars possess broad, strong emission addition, high-quality spectra of approximately 1 million features; the strongest of all is the Lyman a line ofhydrogen. galaxies and 100,000 quasars will be obtained in the course of At a redshift of 3.5, this line is shifted all the way from the far operations. ultraviolet to the middle of the visual band; above a redshift The SDSS telescope is a specially designed 2.5-m telescope of 4.7, the Lyman a feature moves to the near infrared. By with a 3-deg-wide field of view (the secondary mirror of this placing low-resolution spectral dispersers (a combination of telescope is half of the diameter of the primary mirror!) that transmission gratings and prisms called a "grism"), we were will be located at Apache Point Observatory in New Mexico. able to obtain spectra of all objects in the survey strip. The The camera consists of a five-by-six array of 2048 x 2048 spectra cover the wavelength range from 4600 A to 7500 A at Tektronix CCDs. The telescope will scan in great circles a resolution of 120 A. With this instrumentation, the survey along the sky at the sidereal rate; this results in an effective can detect the Lyman a line for quasars with redshifts exposure time per CCD of 55 sec, a data rate of 5 between 2.7 and 4.8. Lines from lower redshift quasars as well as from nearby emission line galaxies are also picked up Mbyte-sec-1, and a survey data base of 8 Thytes. As you can with this technique, but because these lines are generally see in Table 2, this system is >1 order of magnitude more much weaker than the Lyman a line, the flux limit for the efficient than the Palomar Schmidt as a survey instrument. high-redshift quasars is much lower than that for the other When the atmospheric conditions do not permit imaging objects. operations (a few clouds, substandard seeing), the camera will Survey data covering a total of 61 sq deg were obtained be replaced with two fiber-fed spectrographs. Each spectro- between 1985 and 1989. The data base consists of wave- graph will produce spectrafrom 4000 A to 9000 A at a resolving length- and flux-calibrated spectra of =600,000 objects. Each power of 2000 for 300 objects simultaneously. The targets for pixel of each spectrum is examined for the presence of an this aspect of the survey will be selected from the SDSS emission line by an automated software algorithm; if the imaging data; the goal is to obtain data on enough galaxies to signal in the putative line exceeds a given signal-to-noise ratio reveal the three-dimensional distribution ofluminous matter to (SNR) threshold, the object is flagged as a possible quasar. A redshifts of approximately 0.2 and to study the spatial distri- total of 1665 candidate lines were identified. Identification of bution and luminosity function of quasars out to redshifts these lines was made possible with subsequent high SNR, beyond 4. broad-wavelength-coverage spectroscopic observations. The The primary mirror for the SDSS telescope was poured in individual observations of candidates were completed in early 1992, and preliminary construction at the site was begun 1991. Approximately 35% of the candidate lines do not in the summer of 1992. Testing of the system is scheduled to appear in the second spectra (they are noise fluctuations in start in November 1994, with the survey to commence a year the survey data), 45% are emission-line galaxies at redshifts later. The estimated duration of the SDSS observations is 5 between 0.0 and 0.4, and the remainder are quasars, 90 years. A description of the SDSS can be found in ref. 1. having redshifts above 2.7 and nine having redshifts exceed- The impact ofthe SDSS and other "electronic" surveys on ing four (the largest redshift found was 4.733). astronomy will be enormous, perhaps exceeding even that of In addition to locating a number of very distant quasars, the original Palomar Sky Survey. Not only will the new data this survey produced a set of 90 quasars found to have be of much higher quality than that of the photographic well-defined selection criteria and to form an excellent base surveys, but also the distribution of the electronic data, to undertake statistical studies of the evolution and luminos- already digitized and on compact disks (or the popular ity function of quasars at early epochs. In 1990 we initiated storage medium ofthe 21st century), will make future surveys a new CCD survey designed to find quasars out to a redshift much more accessible to the community than the expensive of5.5; only 14 months after starting the project, a quasar with glass copies of photographic plates. At this point astronomy redshift 4.897 was discovered. For details of the Palomar will truly have entered the electronic age. Transit Grism Survey, see ref. 3 and references therein. I thank James Gunn for a number of discussions about CCD Sloan Digital Sky Survey (SDSS) surveys. The encouraging results from the Palomar Transit Grism 1. Gunn, J. E. & Knapp, G. R. (1993) in Sky Surveys: Protostars Survey and a number ofother CCD surveys undertaken in the to Protogalaxies, Astronomical Society of the Pacific Confer- past few years suggest that we should raise our sights to ence Series, ed. Soifer, B. T. (Brigham Young Univ., Provo, unthinkable CCD UT), Vol. 43, pp. 267-279. something just 10 years ago-an all-sky 2. Gunn, J. E., Carr, M. L., Danielson, G. E., Lorenz, E. O., survey. Such a survey would go much deeper and have better Lucinio, R., Nenow, V. E., Smith, D. J., Westphal, J. A., image quality and photometric accuracy than the photo- Schneider, D. P. & Zimmerman, B. A. (1987) Opt. Eng. 26, graphic material that has served so well in the past. 779-787. I close my talk with a briefvision ofwhat might be available 3. Schneider, D. P., Schmidt, M. & Gunn, J. E. (1991) Astron. J. at the turn of the century and give a brief description of a 102, 837-840. Downloaded by guest on October 3, 2021