3.1. Photographic surveys
Photographic surveys performed with Schmidt telescopes  had a great impact on the development of astronomy. In the 1950s, a photographic survey of the sky available for observations from California ( > -33°) was performed using the 1.2-m telescope of the Palomar Observatory. Almost a thousand plates 6.5° × 6.5° each were obtained in the blue and red spectral bands. Copies of the Palomar sky survey (in the form of glass or printed copies of the plates) were spread over most astronomical institutes in the world and played a very important role in the development of all fields of astronomy, from solar system studies to remote galaxies and quasars . Objects down to B ~ 20m can be distinguished in the Palomar prints, and the structure of tens of thousands of galaxies with B 15m can be studied. In particular, based on the Palomar survey (its official name is the Palomar Observatory Sky Survey I, or POSS-I), catalogs of galaxies by Zwicky  and Vorontsov-Velyaminov (MCG in Fig. 2a)  were compiled. It is by inspecting copies of this survey that systematic studies of galaxies, from interacting  and double ones  to galaxy clusters [12, 16], began.
In the 1970s, the success of the Palomar survey stimulated carrying out similar surveys of the southern sky by the 1.2-m British Schmidt telescope (the Anglo-Australian Observatory (AAO), Australia) and the 1.0-m Schmidt telescope of the European Southern Observatory in Chile. Due to great progress in constructing telescopes and improving quality of photographic emulsions, the limiting apparent magnitude of these surveys (ESO/SERC) is by about 1.5m smaller than in POSS-I. This, in turn, initiated, at the end of the 1980s, re-surveying the northern sky with the modified Palomar Schmidt telescope using improved emulsions, but this time with three filters, including the near infrared band centered on eff 8500 Å. This survey was named POSS-II . The limiting magnitude in POSS-II for star-like objects is B 22.5m.
One photographic plate taken by a large Schmidt telescope can have 105 - 106 images of stars and galaxies. This restricted earlier works by visual inspection of only small areas of the original plates. The effective reading of information from the Schmidt plates became possible only after high-speed measuring machines were designed that allowed image digitizing and subsequent computer processing. It is in this way that the first digital sky surveys APM and DSS appeared at the beginning of the 1990s.
In this project, the microdensitometer APM (Automatic Plate Measuring machine) in Cambridge, England, was used to scan 185 plates (the scan step was 0.5") obtained with the 1.2-m Schmidt telescope of the Anglo-Australian Observatory (Australia) near the southern galactic pole . The plates cover ~ 4300 sq. deg. on the sky. Around 20 × 106 objects with B 22m were discovered on these plates. For each object, the coordinates, apparent magnitude, and a dozen other parameters characterizing the brightness distribution and shape were determined. By analyzing photometric brightness profiles, the objects were classified to form a virtually complete sample of extragalactic objects containing ~ 2 × 106 galaxies with B 20.5m.
The galaxy distribution of the APM survey over an area of ~ 50° × 100° is shown in Fig. 3. Obviously, the projected galaxy distribution is far from being homogeneous. Figure 3 clearly shows the presence of regions with an enhanced and low concentration of galaxies and elongated structures. On the basis of the APM survey, some important studies of the large-scale distribution of galaxies were carried out (see, e.g., ) and the first objective catalog of galaxy clusters  was compiled.
Figure 3. The visual distribution of APM galaxies.
The results of the APM survey and its extensions (in particular, to the northern hemisphere) were used as the basis for one of the most interesting projects of recent years - the 2dF survey of galactic redshifts (see Section 3.5 below).
In addition to APM, one can note several later projects aimed at scanning the Palomar and ESO/SERC plates: for example, APS (USA)  and COSMOS and SuperCOSMOS (England) [22, 23]. These surveys are distinguished by a large sky coverage and using plates in different colors.
3.3. DSS and DPOSS
DSS (Digitized Sky Survey) is the first high-quality and freely available digitized image of the entire sky in the optical range. This survey stemmed from the Space Telescope Institute (STScI) project on creating a star catalog that can be used to precisely point the Hubble Space Telescope (HST) to a required object and guide it during observations . To compile such a catalog, the scanning of blue photographic plates of the POSS-I and SERC surveys was initiated. The scan step was 1.7". Soon, it was understood that the importance of the digitized images is far beyond the original purpose and it was decided to open them to the wider scientific community. The total amount of the original version of the survey (DSS-I) reached 600 GB and, clearly, such a huge amount of data was not easily transportable at the beginning of the 1990s. However, after specially designed tenfold compression of the data, the survey was fit into 120 CD-roms, which have been distributed through the Astronomical Society of the Pacific (USA) starting in 1994 . Later on, free access to DSS-I was open through the web pages of STScI (http://archive.stsci.edu/dss/). Now, this survey can be remotely accessed through several cites in the USA and Canada, as well as in some European countries and Japan.
The DSS-II survey was the natural extension of DSS-I using data from POSS-II . The POSS-II plates of the northern sky, as well as the SERC plates and other surveys of the southern sky, were scanned with the step 1.0". Plates in three color bands were digitized, which allowed a comparison of sky areas in different spectral bands. The total volume of DSS-II attains ~ 5 TB and remote access to it is available, as a rule, through the same web pages as for DSS-I.
From the very beginning, DSS became one of the most useful and required tools of modern astronomy. It allowed obtaining an image of any area of the sky in several seconds, which strongly facilitated the preparation and planning of observations. Using the DSS images, a great number of papers devoted to study of individual galaxies, galaxy groups, their large-scale distribution and geometrical characteristics, the optical identification of objects observed at other wavelengths, etc. have been published.
The DPOSS project (Digitized Palomar Observatory Sky Survey - http://www.astro.caltech.edu/~george/dposs/) is also based on scanning the POSS-II plates in three colors; however, the subsequent data processing and calibration are different from those used in DSS . This survey covers all the northern sky with > -3°. Extensive CCD-observations were performed at the Palomar Observatory to provide photometric calibrations of this survey. DPOSS includes a database of images scanned with the step 1.0" (~ 3 TB) and several catalogs based on these data. The ultimate goal of DPOSS is the creation of the PNSC catalog (Palomar Norris Sky Catalog) including all objects found in the survey up to the limiting magnitude B 22m. More than a hundred measured parameters will be provided for each source in the catalog, and objects with B 21m will be classified as stars or galaxies. It is expected that PNSC will provide information on > 50 × 106 galaxies (including ~ 105 quasars) and > 109 stars.
The surveys mentioned above were based on photographic observations and, naturally, suffer from all the standard shortcomings of photo emulsions, such as low sensitivity, restricted dynamical range, and nonlinearity. All projects (both surveys and deep fields) that we discuss below are truly digital, because CCD detectors are used to perform them.
One of the best known digital surveys in a wavelength range close to the optical one is 2MASS (Two Micron All Sky Survey), which is the result of the collaborative efforts of the University of Massachusetts and the Infrared Processing and Analysis Center at Caltech (http://www.ipac.caltech.edu/2mass). 2MASS is a purely photometric survey covering the whole sky in filters J (1.25 µm), H (1.65 µm), and Ks (2.17 µm) . Observations were carried out from June 1997 to February 2001 with two robotic 1.3-m telescopes in Arizona, USA, and Chile. Each instrument was equipped with a three-channel camera imaging the sky simultaneously in the three spectral bands using 256 × 256 IR CCD-detectors with the pixel size 2.0".
The survey consists of 10 Tbt of images and catalogs of objects. Calibrated images of any area of the sky in the J, H, and Ks-bands are available through several sites (for example, http://irsa.ipac.caltech.edu/). An atlas of near-IR images of 864 galaxies has been published . The point source catalog (PSC) lists coordinates and photometric data for about 500 million objects (mostly Milky Way stars). The extended source catalog (XSC) includes data for ~ 1.65 million objects with angular sizes 10"-15" . More than 98% of these objects are galaxies, others are HII regions, stellar clusters, planetary nebulae, etc. The limiting magnitudes of the extended objects from the XSC are 13.5m (2.9 mJy) and 15.0m (1.6 mJy) in the Ks and J-bands, respectively. Figure 4 shows the distribution of objects from the XSC in galactic coordinates. The letters in this figure mark clusters and superclusters of galaxies, the extended image in the center is the Milky Way stellar disk seen `edge-on' .
Figure 4. The distribution of the 2MASS survey objects in the sky (in galactic coordinates).
Over the few years since completion, the 2MASS survey has already greatly impacted the development of astronomy. For example, according to the Astrophysics Data System (ADS: http://adsabs.harvard.edu) , the number of published papers that used the 2MASS data approached a thousand by the beginning of 2005. The main areas of study benefiting most from using the 2MASS data include the large-scale structure of the Milky Way and distribution of galaxies in the nearby Universe (in the optical range, such studies are strongly restricted by galactic interstellar absorption), as well as searches for and explorations of new types of astronomical objects (for example, low-mass stars, brown dwarfs, `red' quasars, etc.).
3.5. 2dF and 6dF
The 2dF (2 degree Field Galaxy Redshift Survey, or 2dFGRS) represents a spectroscopic survey of ~ 5% (~ 2000 sq. deg.) of the sky performed by British and Australian astronomers with the 3.9-m telescope of AAO [33, 34]. (The coverage of this survey is rather small to classify it as a survey, according to Section 2, and clearly demonstrates the conditionality of dividing modern projects into surveys and deep fields.) Objects for this survey were sampled using the extended APM source catalog (see Section 3.2) and included galaxies brighter than B 19.5m near the North and South galactic poles. A specially designed multi-object spectrograph allowing simultaneously obtaining spectra of 400 objects within the 2° field of view was used. Observations included 272 nights in the period between 1997 and 2002.
The openly accessible results of the project (http://www.mso.anu.edu.au/2dFGRS/) include: a photometric catalog of objects selected for spectroscopic studies; a spectroscopic catalog of 245,591 objects listing redshifts z and spectral types (221,414 galaxies in this catalog have reliable redshift measurements); and special software to fetch both fits-files with spectra and subsamples of objects according to selected criteria.
Upon its completion, the 2dF survey became the largest galaxy redshift survey, with the number of measurements exceeding 105 for the first time. It allows investigating the three-dimensional large-scale structure of the surrounding Universe with an unprecedented accuracy. The median redshift of the survey is z = 0.11, which corresponds to the photometric distance ~ 500 Mpc. As an example, in Fig. 5, we show the distribution of 63,000 galaxies from the survey located within narrow 3°-bands around the north (to the left) and south (to the right) galactic poles . In contrast to Figs 3 and 4, which show the galaxy distribution projected on the sky, this figure plots the galaxy distribution along the line of sight. The decrease in the number of galaxies with increasing z is the result of sampling by apparent magnitude biasing only bright remote galaxies. The principal elements of the large-scale structure of the Universe - clusters and superclusters, filaments, and voids - are clearly seen in Fig. 5.
Figure 5. The distribution of galaxies in the 2dF survey field.
The 2dF survey plays a major role in modern astronomy as one of the main sources of information on the spatial distribution of galaxies and the density of matter in the Universe. For example, the data from the survey allowed the upper limit of the total mass of the neutrino m,tot < 1.8 eV to be derived  and, in combination with data on the cosmic microwave background, the values of the main cosmological parameters to be improved .
An autonomous subproject within the 2dF is the 2dF QSO Redshift Survey (2QZ), a survey of redshifts of quasars located near the north and south galactic poles (the total coverage is ~ 700 sq. deg.) . The 2QZ contains spectra of 23,338 quasars, 12,292 stars of our Galaxy (including around 2,000 white dwarfs), and 4,558 emission-line galaxies. The data, including photometry, redshifts, and fits-files with spectra, are available via http://www.2dfquasar.org/.
The 6dF (6dF Galaxy Survey, or 6dFGS) represents a survey of redshifts and peculiar velocities of galaxies selected mainly from the XSC 2MASS survey catalog (see Section 3.4) . Selecting galaxies in the IR spectral range, where the effect of interstellar absorption inside the Milky Way is much smaller than in the optical range, allows much better studies of the object distribution at low galactic latitudes. The 1.2-m Schmidt telescope of the Anglo-Australian Observatory is used, equipped with a multi-object spectrograph simultaneously taking spectra of 150 objects inside the telescope's 6-degree field of view. Redshifts of around 150,000 galaxies are planned to be measured. The survey will cover almost the entire southern sky with < 0° (the survey coverage is 17000 sq. deg.) and will give detailed information on the distribution of galaxies within the nearby (z 0.05) volume of the Universe. At the beginning of 2005, about half of the input galaxy sample was made available (see http://www-wfau.roe.ac.uk/6dFGS/.)
The principal goal of the 6dF is to study large-scale deviations in the velocity of galaxies from the homogeneous Hubble expansion. The distribution of such deviations provides the unique means to study mass distribution in the Universe independent of the assumptions that galaxies follow the true mass distribution. For about 15,000 early-type galaxies evenly distributed over the southern sky, z-independent distances will be determined using the Fundamental Plane method (the Fundamental Plane is a three-parameter relation between photometric and kinematic characteristics of galaxies, see, e.g., ). Then, by comparing these distances with those derived from the observed values of z, it will be possible to estimate the peculiar velocities of galaxies arising due to inhomogeneities in mass distribution. (In this way, the Great Attractor with the mass ~ 5 × 1016 M in a relatively nearby region of the Universe was found ).
The Sloan Digital Sky Survey (http://www.sdss.org/) is often referred to as one of the most grandiose astronomical projects in history. Starting at the end of the 1980s, it is being carried out by more than a hundred scientists from the USA, Japan, and some European countries [42, 43].
The purpose of the SDSS is to perform a photometric and spectral study of a quarter (10000 sq. deg.) of the sky. The survey covers one large area near the Northern Galactic Pole and three bands (with a total coverage of 740 sq. deg.) in the southern hemisphere. Observations are carried out with a specially designed 2.5-m telescope (the modified Ritchey-Chretien system, 3° field of view) in New Mexico (USA). The telescope is equipped with a CCD-camera and a couple of identical multi-object fiber-optic spectrographs to simultaneously take spectra of 640 objects. Auxiliary works are also performed with several other telescopes.
The main goal of the photometric observations is to construct a database for ~ 108 galaxies and ~ 108 stars, containing the precise ( 0.1") coordinates and photometric and other characteristics. The observations are carried out in five broadband filters in the spectral range from 3500 Å to 9100 Å. The limiting magnitude of the photometric survey is B 23m for point-like objects.
The spectral observations should provide spectra of about 106 galaxies, 105 quasars, and 105 stars selected from the photometric survey. Two samples of galaxies have been selected: ~ 900,000 galaxies with B 19m and ~ 100,000 objects with large color indices (`red' galaxies) and B 20.5m. The quasar candidates are selected using the observed color indices. Stellar-like objects with radio emission are also included in the quasar candidate sample.
Systematic observations for this project started in April 2000 and are to be completed by summer 2005. In the course of the observations, the processed data have become available via the web pages of the survey. By the beginning of 2005, around half of the survey was made available. These data include ~ 6 TB of images, the photometric and astrometric catalogs for 1.41 × 108 objects, and the spectra of 528,640 objects, including 374,767 galaxies, 51,027 quasars, and 71,174 stars . The final results of the SDSS will be presented in 2006.
The SDSS is not yet completed, but a great number of papers (more than a thousand, according to ADS) have already been published. These studies include all fields of optical astronomy, from asteroids (more than several dozen thousand of them have been already discovered by the SDSS) to quasars (quasar redshifts measured in the survey first exceeded the z = 6 barrier) . The SDSS and 2dF data increased by hundreds of times the observational information on the structure, spectral characteristics, and spatial distribution of galaxies in the nearby (z 0.2) volume of the Universe. At present, almost all characteristics of galaxies known earlier are being revised and improved. This relates in particular to their spatial distribution, the luminosity function, and the dependence of galactic properties and the star formation rate on the environment (see Section 5 below).