Major astronomical discoveries of the past have been closely linked with the implementation of strikingly powerful new observational techniques in astronomy. Surveys, in general, were not as likely to lead to the discovery of new phenomena. In recent years, however, many far-reaching instrumental advances have been incorporated into observatories in space, whose mission, in part, was to conduct unbiased surveys. Here, we provide a review of some of the successes of earlier surveys, with the purpose of identifying the relative extent to which increased instrumental capabilities, as contrasted to increased sky coverage or numbers of sources observed, may be expected to lead to the discovery of new phenomena. A similar comparison can be made of the extent to which these two approaches—emphasis on instrumental capabilities, as contrasted to emphasis on sky coverage— also contribute to an increase in astrophysical understanding. Here, the distinction needs to be made between discovering a new phenomenon and understanding its underlying processes.

A respected astronomer recently suggested, because the whole spectrum has been explored, that the heroic days of astronomy are now over. I will try, by referring to the narrow optical and 21-cm windows with which I am familiar, to argue that the he was quite wrong. We still haven't covered the whole spectrum, and in any case prejudices, biases and selection effects could still veil much of the truth from sight.

Radio surveys have an important role in astronomy, one that has changed with technology and scientific requirements. Most objects studied by radio astronomers today are the unexpected discoveries of early surveys. The survey “discovery” phase began with Jansky's detection of Galactic radio emission and Reber's 160 MHz maps showing that this emission is non-thermal. Surveys made just after World War II revealed strong discrete sources which were later identified with supernova remnants, radio galaxies, and quasars. Pulsars were discovered during a sky survey for scintillating sources. BL Lac objects were recognized in early high-frequency surveys. The first gravitationally lensed quasar appeared in the extensive Jodrell Bank 960 MHz survey, and the first measurement of gravitational radiation came from the binary pulsar serendipidously found in a pulsar survey.

The next decade promises an explosion of information about the infrared sky from wavelengths from 1 μm to 1,000 μm, in the continuum and in spectral lines. Measurements of objects ranging from comets in the Kuiper Belt to newly-forming galaxies will be useful for almost every branch of astrophysics. The number of catalogued infrared sources will increase a thousand-fold from the 600,000 presently known from IRAS. This talk addresses the compelling scientific interest of the infrared and describes why, years from now, astronomers will view this decade as a golden era for the exploration of the infrared sky.

Photographic surveys have played a vital role in virtually every area of astronomical research over the last 50 years. Indeed, one can make a strong case that Schmidt telescopes in general, and the Palomar 48-inch Schmidt in particular, have made the most substantial contribution to our understanding of the Universe—at least at optical wavelengths. The all-sky atlases compiled since the initiation of the POSS I O/E survey in 1949 will continue to provide fundamental reference catalogues and potent research tools for many years to come (for our children's children's children), particularly when combined with the second epoch surveys currently under way. However, as far as undertaking new, large-scale sky surveys are concerned, it is clear that we have reached the end of an era. Alternative detector technology and new instrumentation (i.e., CCD mosaics) can now match and, indeed, exceed the capabilities of the photographic Schmidt telescopes in undertaking effective wide-field photometric surveys.

In this review, I report on the progress being made in completing the all-hemisphere photographic surveys currently being undertaken from Siding Spring with the UK Schmidt and from Palomar with the Oschin Schmidt. I also provide a summary of the availability of machine scans and/or object catalogues derived from the various surveys. Photographic survey work is not confined to these atlases, however, and section 3 provides discussion of other more specialised projects currently being undertaken. Finally, I consider the future possibilities for photographic Schmidt surveys.

The introduction of of Charge Coupled Devices (CCDs) in the middle 1970s provided astronomy with nearly perfect (linear, high-sensitivity, low-noise, high dynamic-range, digital) optical detectors. Unfortunately, restrictions imposed by CCD production and cost has typically limited their use to observations of relatively small fields. Recently a combination of technical advances have made practical the application of CCDs to survey science. CCD mosaic cameras, which help overcome the size restrictions imposed by CCD manufacture, allow electronic access to a larger fraction of the available focal plane. Multi-fiber spectrographs, which couple the low-noise, high QE performance of CCDs with the ability to observe spectra for many objects at once, have improved the spectroscopic efficiency of telescopes by factors approaching half a million. An improved understanding of image distortion gives us telescopes on which we expect sub-arcsecond images a large fraction of the time. Finally, and perhaps most important, the performance of computer hardware continues to advance, to the point where analysis of multi-terabyte datasets, while still daunting, is at least conceivable.

Among all spectral bands, the ultraviolet has long been neglected, despite the advantage of small space experiments: the sky is very dark, thus detection of faint objects does not compete against an enhanced background (O'Connell 1987) and the telescope construction techniques are very similar (at least longward of ∼50 nm) to those of optical astronomy.

Prior to the current Compton Gamma Ray Observatory (Compton) mission, no comprehensive all-sky gamma-ray surveys had been performed. There were, however, some surveys performed over limited energy bands and/or over portions of the sky. These include the HEAO-A4 hard X-ray survey and the COS-B and SAS-2 high-energy gamma-ray surveys. The early work forms a basis for understanding and appreciating the Compton results, and so is reviewed in Section 2.

Hipparcos and the Hubble Space Telescope have demonstrated the astrometric capabilities in space. SIM and GAIA are being studied for future missions. There have been many proposals for new astrometry missions from many different countries, but most of them have not been funded.

The best possibility for a mission within the next five years would be through a collaborative effort for a small, fast, cheap spacecraft which could be a precursor for future, larger, more accurate missions, which are under study.

The 330 MHz observing system at the VLA is a potentially powerful survey system. It can map fields many degrees in size quickly and at high sensitivity. However imaging characteristics unique to this data pose excessive computational burdens on conventional mapping systems. Hence while data acquisition is quick and efficient, data reduction has been difficult and slow. Here we describe how powerful new scalable processing algorithms have been used to generate the first full resolution 330 MHz images, demonstrating also that lower resolution survey work is now a tractable problem.

A meter-wave sky survey of the region north of declination +30°, excluding only four fields straddling the Galactic plane (each 8° × 8° in angular size), has been conducted with the Miyun Synthesis Radio Telescope operating at 232 MHz. The reduced Miyun General Catalogue contains 34,426 radio sources in total. To distinguish which sources were not observed by other sky surveys, we first identify sources in the MGC with those in the 6C (151 MHz) catalogue, using a matching radius of 100 arcseconds. The remaining MGC objects were then matched with the B2/B3 (408 MHz), 4C (178 MHz), Texas (365 MHz, 1400 MHz) and 87GB (4.85 GHz) catalogues. 6850 MGC sources were found to lack any radio counterpart in these reference catalogues. It means 19.9% of sources listed in the MGC are probably new.

One way to realize the Large radio Telescope with a collecting area approaching one square kilometer, continuously covering a frequency range between 200 MHz and 2 GHz, is to construct a passive spherical reflector array of about 30 individual unit telescopes, each ∼ 300 m diameter(Butcher 1995). Valleys amid the hills of southwest China would be ideal for such LT concept. We refer to this effort as the Kilometer-square Area Radio Synthesis Telescope project. Site surveying and Radio Interference monitoring looks promising. Engineering considerations are summarized.

The Rhodes/HartRAO 2326 MHz radio continuum survey was observed at HartRAO over a 13 year period (Jonas et al. 1985, Mountfort et al. 1987, Jonas & Baart 1995, Jonas et al. 1996). This is the highest frequency and highest angular resolution (HPBW=20′) all-sky radio continuum survey made using a ground-based telescope. The 1σ noise level is better than 30 mK. A high-pass filtered image of the data is shown in Figure 1.

The observation of an area of 120° × 56° centered on RA=8h DEC=20° at 408 MHz was the first astronomical use of the MPIfR 100-m telescope (1970) and was designed to compile a complete sky survey using also data from Jodrell Bank and Parkes (Haslam et al., 1982). The observation of the northern sky at 1420 MHz started in 1972 using the Stockert 25-m telescope and was finished in 1976 (Reich and Reich 1986). This survey has been completed to an all sky survey using data from Villa Elisa (Argentina). The two surveys are absolutely calibrated. The angular resolutions are 0.8° and 0.59°, respectively. A number of surveys of the Galactic plane have been made with the 100-m telescope at arc minute angular resolution. Surveys at 2695 MHz (|b| ≤ 5°) (Reich et al. 1990, Fürst et al. 1990) and at 1410 MHz (|b| < 4°) (Reich et al. 1990) are public.

At medium Galactic latitudes (up to |b| = 20°) the emission consists mainly of faint extended ridges or arcs superimposed on the still dominating, about 10 times stronger, diffuse Galactic emission. They have never been investigated in a systematic way although they provide important clues for the understanding of the “disk-halo connection”. This region is covered by new observations at 1400 MHz with the 100-m telescope.

During the past decade there has been a dramatic increase in the amount of high-frequency (v > 1000 MHz) data currently underlying the studies of bright compact sources (Tb ∼ 1011–12K, l ≤ 1 kpc) with flat spectra. But in order to determine physical conditions inside extragalactic radio sources at different stages of their evolution the studies of old extended (l ∼ 100 kpc) sources with low surface brightness and steep spectra which constitute the dominant radio source population at very low frequencies (v ≪ 100 MHz) are also needed. These sources are known to represent the final stage in the evolution of extragalactic objects.

Now in radio continuum surveys more than 10,000 radio sources have discovered in the Milky Way plane but the Galactic origin only of a small part of them has been determined. The problem comes from the absence of estimates of source distance and the optical identification even for bright radio sources, and the most of sources have not spectral data at 2-3 frequencies. As followed some hundreds of sources have not classified as thermal or non-thermal. Now we don't know the full number of supernova remnants (SNRs) in the Galaxy. The simple estimates show that a sample of Galactic SNRs is not full as for weak and extended (> 15′) as for bright and compact (< 3′) SNRs (Trushkin 1993).

The aim of DENIS is to map the southern sky in three near-infrared photometric bands (I, J, Ks) at the 1mJy sensitivity level (14th magnitude in Ks ) and arcsecond resolution using the ESO 1 meter telescope at La Silla, Chile. The DENIS operations have started in December 1995 after a protosurvey period of one year mainly dedicated to technical tests and pilot surveys of selected regions of high astronomical interest. In September 1996, approximately 3000 square degrees of the sky have been covered, and the survey should be completed in 1999.