In 1930 B. Schmidt invented the telescope that bears his name. Eleven years later, there were already two dozen Schmidt telescopes with primaries of 25 cm or more in diameter (Dimitroff and Baker 1945). An invention spreading so rapidly must have fulfilled a basic need – the need for a large field with good image quality over a substantial wavelength range. Here I shall briefly review the role of the large Schmidt telescopes in contemporary astronomy.

Technical aspects of the production of photographic Sky Surveys and Atlasses are discussed. The need to optimize all factors, e.g. telescope site, telescope optics, mechanics and control system, hypersensitization, calibration, processing and reproduction is illustrated by the ESO 1m Schmidt telescope and the ESO Sky Surveys.

On 1983 September 3 we will be celebrating the tenth anniversary of the formal hand-over of the UK 1.2m Schmidt Telescope (UKST) by the manufacturers, Messrs. Grubb Parsons of Newcastle-upon-Tyne, to the Science and Engineering Research Council (SERC). The completion of commissioning was marked in 1973 by the taking of the first completely successful sky-limited photograph on hypersensitised Eastman-Kodak Illa-J emulsion, plate J149 of field 416. That plate itself led very quickly to exciting new scientific results, including the discovery of extraordinary and still apparently unique jets in the barred spiral galaxy NGC 1097 by Wolstencroft & Zealey (1975), a discussion of the clustering of 3000 faint galaxies (Dodd et al., 1975; 1976) and a suggestion that faint blue stellar objects are not randomly distributed (Hawkins & Reddish, 1975).

A survey of surveys made with objective prisms since the Hamburg Schmidt Conference in 1972 is presented. Ten outstanding achievements attained by these techniques are listed. Detailed accounts of particular surveys both general and specific follow. A table listing fundamental classical studies is given plus a list of the location and properties of the largest Schmidt-type cameras.

The Burrell Schmidt-type telescope of the Warner and Swasey Observatory in its new location on Kitt Peak in Arizona is being used for a spectroscopic survey of the region b > +30°. and δ > +30°. The plates, which cover 5° x 5° are taken with the 1.8 prism which provides a dispersion of 1350 Å mm-1 at Hγ. Eastman Kodak IIIa-J plates, baked in forming gas, are used without filter to cover the spectral range 3300 to 5350 Å. The exposure times of 75 minutes reach a limiting blue magnitude of ~ 18.0 for threshold detection of an unwidened stellar continuum. The categories of objects which are being catalogued are blue and/or emission-line galaxies, probable HII regions, blue and/or mission-line stellar objects, known and probable blue stars, main-sequence late B and A-type stars, suspected field horizontal-branch stars of types A and F including RR Lyrae variables, suspected F and G-type subdwarfs showing a UV excess, faint carbon and late M suspected halo giants, and peculiar objects.

This title was first used by William Baum (1955) in his contribution to an IAU Joint Discussion on photoelectric image tubes and their astronomical applications. The general tone of the ten or so papers collected in the 1955 IAU Transactions was optimistic. It seemed quite clear that the high quantum efficiency of the electronic devices existing at the time would soon be turned to astronomical advantage and that objects much fainter than those detectable photographically were well within reach.

The UK Schmidt Telescope has taken several thousand photographs especially on fine grain Eastman Kodak IIIa-J and IIIa-F emulsions (see Cannon, 1984) which were hypersensitised before exposure in the telescope to improve their ability to record very faint signals.

Relatively little work has been done on further post-processing techniques to enhance photographically the faint signal from sources very near to the sky background level. Work is currently being undertaken by Malin (AAO) and others including staff at ROE to enhance faint photographic signals without a corresponding increase in noise, either by making high contrast enlargements from small areas of the original plate, or by making contact copies of the whole plate onto high contrast line material.

The combination of wide field high quality telescopes, fine grain emulsions and fast automated measuring machines offers an unrivalled opportunity for progress in statistical astronomy. This review illustrates, with examples, the many steps which are necessary to realise this potential.

The Automated Photographic Measuring (APM) system at Cambridge was started many years ago to analyse Schmidt plates, and its progress has been reported at a number of these conferences. Only brief details of its design will be given in this paper.

The system consists of a very accurate laser-beam scanning microdensitometer connected to a series of on-line computers. The microdensitometer (Figure 1) uses a 5500 kg x-y table to position the plate. The plate transmission is digitised to 12-bit accuracy at a rate of one sample every 4 microseconds, although the image processing hardware limits this speed in practice. Special-purpose hardware exists to convert the transmission measurements into density or intensity estimates, to smooth the spot-size digitally, to measure the background and to compute image parameters.

This paper describes the facilities available at the ESO headquarters in Munich for scanning photographic plates and for extracting information from these plates. ESO has an integrated system of three microdensitometers, a PDS 1010A, an Optronics S3000 and a Grant 800. Data analysis facilities include an HP 1000 series computer supporting an interactive data analysis system and two Vax 11/780 computers supporting general purpose computing and data analysis. Application areas currently active are: techniques for identifying objects in images and for classifying these objects, methods of analyzing extended images, and a package to reduce echelle spectra.

This paper reviews the methods and early results of work in progress at the Royal Observatory, Edinburgh that involves the machine-processing of objective-prism plates. This work is in two categories: (i) semi-automated galaxy redshifts, and (ii) automated quasar detection. The galaxy redshifts are used, for example, to determine the radial velocities of clusters, to test cluster membership, to reveal superimposed clusters, and to reveal connecting bridges between clusters. Automated quasar detection uses selection criteria that are known, pre-defined and rigidly maintained to select complete samples of quasars. Unlike the earlier visual samples of optically-selected quasars these new samples are well suited to work in cosmology and the collective properties of quasars.

This paper will review progress on the automated detection of variable objects on Schmidt plates, describe the various populations of variables which have been discovered, and summarise some of the most interesting results to date. So far, the searches have been based on sequences of a dozen or more sky limited UK 1.2m Schmidt plates taken on timescales from one day to several years. The plates were measured on the COSMOS measuring machine at the Royal Observatory, Edinburgh which typically detects between 100,000 and 200,000 images in the central 16 deg2 of each plate. The measures for each field are combined, and calibrated using deep electronographic sequences, to give data sets of some 100,000 objects, complete in the magnitude range B = 13–21. The procedure is described in some detail by Hawkins (1983a) and results in a sequence of magnitudes accurate to about 0.1m for each object in the measured area. Field effects across the plates are allowed for, and an extensive set of diagnostics allows the distribution of errors as a function of magnitude, and on each plate, to be monitored.

Deep photographs taken with the UK 1.2m Schmidt Telescope are being scanned with the COSMOS automatic plate-measuring machine in order to carry out a survey of galaxies over large areas of sky down to faint limits (B ~ 21.5). Several fields have been examined, and the data are being used to investigate the properties and largescale projected distribution of galaxies.

The requirements for a classification system for galaxy images are discussed and the advantages and disadvantages of both visual and automatic classification techniques are assessed. The results of some preliminary experiments in the automatic classification of images are also presented.

A system for detecting and classifying (faint) astronomical objects on photographic plates is presented. The system is planned to perform all the main operations, from the digitization of the plates with a PDS lOlOA microdensitometer up to the final classification, under the control of the VAX-11/750 central processor. The aim of the project is to obtain an objective classification and reliable description of large amounts of objects, in a reasonable amount of time,for specific research projects. The detection and classification procedure is organized in modules, each of which is general enough to be applied to different kinds of analyses and to different classes of objects, according to astronomical requirements. An example of application to the problem of star/galaxy discrimination is given.