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2. A (VERY) BRIEF HISTORY OF SKY SURVEYS

We cannot do justice to this subject in the limited space here, and we just mention some of the more important highlights, in order to put the subject in a historical context. The interested reader may wish to start with a number of relevant, excellent Wikipedia articles and other Web resources for additional information and references.

Historically, surveying of the sky started with a naked eye (that may be looking through a telescope), and we could consider Charles Messier's catalog from the middle of the 18th century as a forerunner of the grander things to come. In the pre-photography era, the most notable sky surveying was done by the Herschel family (brother William, sister Caroline, and son John), starting in the late 18th century. Among the many notable achievements, the Herschels also introduced stellar statistics - counting of stars per unit area, that can be used to learn more about our place in the universe. Their work was continued by many others, leading to the publication of the first modern catalogs in the late 19th centrury, e.g., the still-used New General Catalogue (NGC) and Index Catalogue (IC) by John Dreyer (Dreyer 1888, 1895).

Visually compiled star catalogs culminated in the mid/late 19th century, including the Bonner Durchmusterung (BD) in the North, its Southern equivalent, the Cordoba Durchmusterung (CD), that contained nearly 900,000 stars.

The field was transformed by the advent of a new technology - photography. Surveys that may be recognized as such in the modern sense of the term started with the first photographic efforts that covered systematically large areas of the sky at the end of the 19th century. Perhaps the most notable of those is the Harvard Plate Collection, that spans over a century of sky coverage, and that is currently being scientifically rejuvenated through digitization by the Digital Access to Sky Century at Harvard project (DASCH; http://hea-www.harvard.edu/DASCH; Grindlay et al. 2009). A roughly contemporaneous, international effort, Carte du Ciel, led to the production of the Astrographic Catalogue (AC), reaching to m ~ 11 mag, that served as the basis of the more modern astrometric catalogs.

Another important innovation introduced at the Harvard College Observatory at the turn of the 19th century was systematic monitoring of selected areas on the sky, a precursor of the modern synoptic sky surveys. Photographic monitoring of the Magellanic Clouds enabled Henrietta Leavitt (Leavitt & Pickering 1912) to discover the period-luminosity relations for Cepheids, thus laying the groundwork for the cosmological distance scale and the discovery of the expanding universe by Edwin Hubble and others in the 1920's.

In the early decades of the 20th century, Edward Pickering, Annie Jump Cannon, and their collaborators at Harvard produced the Henry Draper Catalogue (HD), named after the donor, that was eventually extended to ~ 360,000 stars, giving spectral types based on objective prism plates. Around the same time, Harlow Shapley and collaborators catalogued for the first time tens of thousands of galaxies in the Southern sky, and noted the first signs of the large-scale structure in the universe.

Around the same time, roughly the first third of the 20th century, the Dutch school (Jakobus Kapteyn, Pieter van Rhijn, Jan Oort, Bart Bok, and their students and collaborators) started systematic mapping of the Milky Way using star counts, and laid foundations for the modern studies of Galactic structure. Kapteyn also introduced Selected Areas (SA), a strategically chosen set of directions where star counts can yield information about the Galactic structure, without the need to survey the entire sky. In 1966, the heavily used Smithsonian Astrophysical Observatory Catalog was published, that contained positions, proper motions, magnitudes, and (usually) spectral types for over 250,000 stars.

A key figure, emerging in the 1930's was Fritz Zwicky, who, among many other ideas and discoveries, pioneered systematic sky surveys in the ways that shaped much of the subsequent work. He built the first telescope on Mt. Palomar, the 18-inch Schmidt, then a novel design for a wide-field instrument. With Walter Baade, he used it to search for Supernovae (a term he and Baade introduced), leading to the follow-up studies and the physical understanding of this phenomenon (Baade & Zwicky 1927). Zwicky's systematic, panoramic mapping of the sky led to many other discoveries, including novel types of compact dwarf galaxies, and more evidence for the LSS. In 1960's, he and his collaborators E. Herzog, P. Wild, C. Kowal, and M. Karpowitz published the Catalogue of Galaxies and of Clusters of Galaxies (CGCG; 1961-1968), that served as an input for many redshift surveys and other studies in the late 20th century. All this established a concept of using wide-field surveys to discover rare or interesting objects to be followed up by larger instruments.

The potential of Schmidt telescopes as sky mapping engines was noted, and a 48-inch Schmidt was built at Palomar Mountain, largely as a means of finding lots of good targets for the newly built 200-inch, for a while the largest telescope in the world. A major milestone was the first Palomar Observatory Sky Survey (POSS-I), conducted from 1949 to 1958, and spearheaded mainly by Edwin Hubble, Milton Humason, Walter Baade, Ira Bowen, and Rudolph Minkowski, and was funded mainly by the National Geographic Society (Minkowski & Abell 1963).

POSS-I mapped about 2/3 of the entire sky, observable from Palomar Mountain, initially from the North Celestial Pole down to Dec ~ -30°, and later extended to Dec ~ -42° (the Whiteoak extension). The survey used 14-inch wide photographic plates, covering roughly 6.5° × 6.5° fields of view (FOV) each, but with a useful, unvignetted FOV of ~ 6° × 6°, with some overlaps, with a total of 936 fields in each of the two bandpasses, one using the blue-sensitive Kodak 103a-O plates, and one using the red sensitive Kodak 103a-F emulsion. Its limiting magnitudes vary across the survey, but are generally close to mlim ~ 21 mag. Reproduced as glass copies of the original plates and as paper prints, POSS-I served as a fundamental resource, effectively a roadmap for astronomy, for several decades, and in its digital form it is still used today.

Its cataloguing was initially done by eye, and some notable examples include the Uppsala General Catalogue (UGC) of ~ 13,000 galaxies with apparent angular diameters > 1 arcmin (Nilson 1973) and Morphological Catalog of Galaxies (MCG) containing ~ 30,000 galaxies down to m ~ 15 mag (Vorontsov-Velyaminov & Arkhipova 1974); Abell's (1958) catalog of ~ 2,700 clusters of galaxies; and many others.

The Second Palomar Observatory Sky Survey (POSS-II), conducted about 4 decades later, was the last of the major photographic sky surveys (Reid et al. 1991). Using an improved telescope optics and improved photographic emulsions, it covered the entire Northern sky with ~ 900 partly overlapping 6.5° fields spaced by 5°, in 3 bandpasses, corresponding to Kodak IIIa-J (blue), IIIa-F (red) and IV-N (far red) emulsions.

A number of other surveys have been conducted at the Palomar 48-inch Schmidt (renamed to Samuel Oschin Telescope, in honor of the eponymous benefactor) in the intervening years, including Willem Luyten's measurements of stellar proper motions, the original Hubble Space Telescope Guide Star Catalog (GSC), and a few others.

Southern sky equivalents of the POSS surveys in terms of the coverage, depth, and distribution were conducted at the European Southern Observatory's 1.0-m Schmidt telescope, and the 1.2-m UK Schmidt at the Anglo-Australian Observatory in the 1970's and 1990's, jointly called the ESO/SERC Southern Sky Survey. Andris Lauberts (1982) produced a Southern sky equivalent of the UGC catalog.

Both POSS and ESO/SERC surveys have been digitized independently by several groups in the 1990's. Scans produced at the Space Telescope Science Institute were used to produce both the second generation HST Guide Star Catalog (GSC-2; Lasker et al. 2008), and the Digital Palomar Observatory Sky Survey (DPOSS; Djorgovski et al. 1997a, 1999); the images are distributed through several Digitized Sky Survey (DSS) servers world-wide. The US Naval Observatory produced the astrometric USNO-A and USNO-B catalogs that also include proper motions (Monet et al. 2003). These digital versions of photographic sky surveys are described in more detail below. The surveys were also scanned by the Automated Plate Measuring facility (http://www.ast.cam.ac.uk/~mike/casu; APM) in Cambridge, the SuperCOSMOS group (http://www-wfau.roe.ac.uk/sss; Hambly et al. 2001), and the Automated Plate Scanner group (APS; http://aps.umn.edu; Cabanela et al. 2003). Scans of the ESO/SERC Southern Sky survey plates resulted in the APM Galaxy Survey (Maddox et al. 1990a, b). DSS scans and other surveys can be also accessed through SkyView (http://skyview.gsfc.nasa.gov; McGlynn et al. 1997).

The middle of the 20th century also saw an appearance of a plethora of sky surveys at other wavelengths, as the new regimes opened up (IR, radio, X-ray, gamma-ray, etc.). Enabled by the new technologies, e.g., electronics, access to space, computers, etc., these new, panchromatic views of the sky led to the discoveries of many previously unknown types of objects and phenomena, e.g., quasars, pulsars, various X-ray sources, protostars, or gamma-ray bursts, to name but a few.

A good account of the history of radio astronomy is by Sullivan (2009). Following the pioneering work by Karl Jansky and Grote Reber, radio astronomy started to blossom in the 1950's, fueled in part by the surplus radar equipment from the World War II. A key new development was the technique of aperture synthesis and radio interferometers, pioneered by Martin Ryle, Anthony Hewish, and collaborators. This led to the first catalogs of radio sources, the most enduring of which were the Third Cambridge (3C) and Fourth Cambridge (4C) catalogs, followed by the plethora of others. Optical identifications of radio sources by Walter Baade, Rudolph Minkowski, Maarten Schmidt, and many others in the 1950's and 1960's opened whole new areas of research that are still thriving today.

While the infrared (IR) light was discovered already by William Herschel circa 1800, the IR astronomy started in earnest with the advent of first efficient IR detectors in the 1960's. A pioneering 2 µm IR sky survey (TMASS) was done by Gerry Neugebauer and Robert Leighton (1969), resulting in a catalog of ~ 5,600 sources. Its modern successor some 3 decades later, the Two Micron All-Sky Survey (2MASS), catalogued over 300 million in three bandpasses (JHK); it is described in more detail below. Deeper surveys followed, notably the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS), and more are forthcoming.

A wholesale exploration of the mid/far IR regime required a space-borne platform, and the Infrared Astronomy Satellite (IRAS), launched in 1983, opened a huge new area of research, continued by a number of space IR missions since. Some of the milestones include the DIRBE and FIRAS experiments on the Cosmic Background Explorer satellite (COBE), launched in 1989, and, more recently the Wide-field Infrared Survey Explorer (WISE), launched in 2009.

Astronomy at higher energies, i.e., UV beyond the atmospheric cutoff limit, X-rays, and gamma-rays required access to space, that was a beneficent product of the space race and the cold war, starting shortly after the World War II.

The UV (i.e., with lambda < 320 nm or so) astronomy from space started with a number of targeted missions starting from the 1960's, the first, shallow all-sky survey was done with the TD-1A satellite in 1972, followed by the Extreme UV Explorer (EUVE, 1992-2001). The next major milestone was the Galaxy Evolution Explorer (GALEX, 2003-2011), that surveyed most of the sky in two broad bands, down to the UV equivalent of the optical POSS surveys, and a much smaller area about 2 - 3 mag deeper.

Following the birth of the X-ray astronomy with rocket-borne experiments in the 1960's, the first X-ray all-sky surveys started with the SAS-A Uhuru (1970-1973) satellite, HEAO-1 mission (1977-1979), and HEAO-2 Einstein (1978-1981), followed by many others, including Rosat (1990-1999). Many other missions over the past 3 decades followed the survey ones with pointed observations of selected targets. They led to the discoveries of many new aspects of the previously known types of sources, e.g., accreting binaries, active galactic nuclei, clusters of galaxies, etc., and provided key new insights into their nature.

Gamma-ray astronomy is still mostly done with space-borne instruments that cover most or all of the sky. This is largely because gamma-rays are hard to focus and shield from, so the instruments tend to look at all directions at once. While the first cosmic gamma-ray emission was detected in 1961 by the Explorer-11 satellite, with a total of about 100 photons collected, gamma-rays surveys really started with the SAS-2 (1972) and the COS-B (1975-1982) satellites. A major milestone was the Compton Gamma-Ray Observatory (CGRO, 1991-2000), followed by the Fermi Gamma-Ray Space Telescope (FGST), launched in 2008. These and other missions uncovered many important phenomena in the high-energy universe, but perhaps the most spectacular are the cosmic gamma-ray bursts (GRBs), discovered in 1967 by the Vela satellites, and finally understood in 1997 thanks to the BeppoSAX mission.

Descriptions of all of these fascinating discoveries are beyond the scope of this Chapter, and can be found elsewhere in these volumes.

In the 1990's, the era of fully digital sky surveys began in earnest. Aside from the digitized versions of the photographic sky surveys, a major milestone was the Sloan Digital Sky Survey (SDSS), which really helped change the culture of astronomy. Another key contribution at that time was the Two Micron All-Sky Survey (2MASS). Radio astronomy contributed the New VLA Sky Survey (NVSS) and the Faint Images of Radio Sky at Twenty centimeters (FIRST). Significant new surveys appeared at every wavelength, and continue to do so. We discuss a number of them in more detail in Sec. 4.

The early redshift observations by Vesto Melvin Slipher in the 1920's led to the discovery of the expanding universe by Edwin Hubble. Galaxy redshift surveys in a more modern sense can be dated from the pioneering work by Humason et al. (1956).

The early 1980's brought the first extensive redshift surveys, driven largely by the studies of the large-scale structure (LSS). An important milestone was the first Center for Astrophysics (CfA) redshift survey, conducted by Marc Davis, David Latham, John Huchra, John Tonry (Davis et al. 1982, Huchra et al. 1983), and their collaborators, based on the optical spectroscopy of ~ 2,300 galaxies down to mB approx 14.5 mag, selected from the Zwicky and Nilson catalogs, obtained at Mt. Hopkins, Arizona. Arecibo redshift survey, conducted by Riccardo Giovanelli, Martha Haynes, and collaborators, used the eponymous radio telescope to measure ~ 2,700 galaxy redshifts through their H I 21-cm line. These surveys gave us the first significant glimpses of the LSS in the nearby universe. They were followed in a short order by the second CfA redshift survey, led by John Huchra and Margaret Geller (Geller & Huchra 1989), that covered galaxies down to mB approx 15.5 mag; it was later combined with other survey for a total of ~ 18,000 redshifts, compiled in the Updated Zwicky Catalog (UZC; Falco et al. 1999). Additional H I surveys from Arecibo, added up to ~ 8,000 galaxies. Several redshift surveys used target selection on the basis of FIR sources detected by the IRAS satellite, in order to minimize the selection effects due to the interstellar extinction (Strauss et al. 1992, Fisher et al. 1995, Saunders et al. 2000). Many other redshift surveys of galaxies, obtained one redshift at a time, followed.

Together, these surveys added a few tens of thousands of galaxy redshifts, mainly used to study the LSS. Good reviews include, e.g., Giovanelli & Haynes (1991), Salzer & Haynes (1996), Lahav & Suto (2004), and Geller et al. (2011). As of this writing, John Huchra's ZCAT website is still maintained at https://www.cfa.harvard.edu/~dfabricant/huchra/zcat, and a good listing of redshift surveys is currently available at http://www.astro.ljmu.ac.uk/~ikb/research/galaxy-redshift-surveys.html.

Development of highly multiplexed (multi-fiber or multi-slit) spectrographs in the late 1980's and 1990's brought a new generation of massive redshift surveys, starting with the Las Campanas Redshift Survey, led by Steve Schectman, Gus Oemler, Bob Kirshner, and their collaborators (Shectman et al. 1996), that produced ~ 26,400 redshifts in sky strips covering ~ 700 deg2. The field was changed dramatically in the late 1990's and 2000's by two massive redshift surveys, 2dF and SDSS, described in more detail below. Together, they contributed more than a million galaxy redshifts, and changed the field.

This blossoming of sky surveys created the exponential data explosion discussed in Sec. 1, and transformed the way astronomy is done, both scientifically and technologically. The state of the art at the transition from the photography era to the digital era is encapsulated well in the IAU Symposia 161 (MacGillivray et al. 1994) and 179 (McLean et al. 1998).

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