The oncoming meeting and the big group of specialists gathered here reminds me of the late sixties, when I started the search for Markarian galaxies at the Byurakan Observatory. We carried out the survey with a 1 m Schmidt telescope (f/2.1). The First Byurakan Survey (FBS) consisted of more than two thousand plates with an exposure time of 30–60 minutes per plate. While guiding the telescope I therefore had plenty of time to think on future survey techniques.

Optical wide-field sky surveys have long been basic tools of astronomical investigations. Their extension to the near infrared spectral range, unattainable with photographic techniques, is now possible thanks to the considerable improvements in infrared array technology that have been achieved in the past few years. The astronomical motivations for all-sky 2 μm surveys are summarized, and a brief technical description of the on-going projects, in particular DENIS (Deep Near Infrared Southern Sky Survey) that will be carried out at the European Southern Observatory in 1994–97, is presented.

The Ford Aerospace 2048 × 2048 CCD with UV coating is used at the BAO 60/90 cm f/3 Schmidt telescope focal plane, giving a field-of-view of about one square degree and spatial resolution of 1.67 arcsecond per pixel. The paper reviews the following topics:

1. 1) basic performance of the system;

2. 2) anti-blooming technique for a large field CCD;

3. 3) flat fielding for a large field CCD;

4. 4) photometric properties of compressed images.

Observational astronomy can be roughly split into two categories of study: surveys, designed to detect and categorize objects, and detailed observations that provide physical insight into individual objects. The application of large format CCDs makes it possible to conduct surveys which yield quantitative information for large samples of objects.

There is a need for very wide-field monitoring of the sky in several branches of astronomy and astrophysics, ranging from meteor research to monitoring of optical transients and counterparts to gamma ray burst sources. The photographic very wide-field monitoring of the sky is run at the Ondřejov Observatory and other stations of the Czech Meteor Network and is sensitive enough to record stars up to 11 mag on 3–4 h exposures or 1-s flashes up to 3.0 mag over a field-of-view of 180 deg. diameter. A CCD device for very wide-field monitoring is also in development with sensitivity of 5.0 mag for 1-s flashes (and 10 mag for stars with 100 s exposure) over a field-of-view of 50 × 60 degrees. Both systems are briefly described and discussed.

The 48-inch Oschin Schmidt telescope at Palomar Observatory was used to obtain the first deep, optical, wide-field survey, covering almost two-thirds of the sky between November 1949 and December 1958 (with plates for the Whiteoak extension, to δc = −42°, taken in 1964/5). The original survey was taken in two passbands — wide blue and narrow red — on 103a emulsion, and covers one hemisphere in 643 fields with a spacing of 6 degrees between centres. In 1987, plate-taking started for the second Palomar Sky Survey, POSS II, which is intended to cover the northern hemisphere in three passbands, blue (IIIa-J + GG385 filter), red (IIIa-F + RG610) and near-infrared (IVN + RG9). Given the extremely restricted overlap between fields in the old survey, we have followed the example of the UK Schmidt ESO/SERC surveys and have adopted a 5-degree spacing between fields. To date (31.7.93), we have acquired plates of acceptable quality for 597 fields (67%) of the IIIa-J survey, 668 fields (75%) of the IIIa-F and 253 fields (28%) of the IVN.

In 1990, following an agreement with Caltech (Pasadena), the production of the copies of the POSS II was started at ESO. In total, 894 fields, each in three colours (B, R, I), will be copied on-film and on-glass.

A sky patrol aims to record, as often as possible, the whole sky visible from an observing site down to a certain magnitude in one or more optical or near-optical wavelength regions.

The name of Zwicky is attached to the first photographic Palomar Survey and to the History of SNe study. An exciting study resumed by the striking image (M = −19) of a transient source appearing at extragalactic distance, as bright as the total magnitude of its parent galaxy. Expected to explode once a century, such an event is rare. But the more galaxies are checked the more SNe are found. That is why the wide-field is a good tool. The Palomar team SNe activity was followed by a quiet period of the whole SNe discovery (Fig. 1); from those the Cero El Robles search emerged in the early eighties. Several important patrols continued with Schmidt telescopes of size about 0.5 m (Bern, Hungary, Urss…) in spite of the low level of SNe found. Important because knowing that no SN exploded in a galaxy (especially in the sample of nearby galaxies) is the basis of the statistics. The SNe production increased in the late eighties. At the same time, the closest modern SN (1987A) led for the first time to the use of a number of instruments complementing each other to increase our understanding of SNe. But more important than the production of new SNe, is the increase of their spectroscopic classification which is necessary to the SNe work.

The Sonneberg Sky and Field Patrols have been the suppliers of the Sonneberg plate collection for 6 decades. They have led to the detection of more than 10,000 variable stars and the discovery of spectacular objects.

The Schmidt telescope at Calar Alto Observatory, Spain, was installed there in 1980 by the Max-Planck-Institut für Astronomie, Heidelberg, and has been operated since then by this institute; originally it had been operational at the Hamburg Observatory since 1955. The corrector plate and mirror have 80 cm and 120 cm diameter respectively; focal length is 240 cm. A field 5.5 deg by 5.5 deg is photographed on 24 cm by 24 cm curved plates. Two objective prisms of 1.7 deg and 4.0 deg refractive angle are available (Heckmann 1955; Birkle 1984). Thousands of direct and spectral plates have been taken, mainly for the Hamburg Quasar Survey (Engels et al. 1988). Other observing programs referred e.g. to astrometry of comets, giving support to ESA's Giotto space encounters with comets Halley and Grigg-Skjellerup (Roeser et al. 1986; Morley 1992). Gradually, however, the number of plates increased showing image quality which did not correspond to the excellent performance of the telescope optics. Several technical improvements have therefore been installed to remove some obvious mechanical instability.

The fundamental goals of the international astronomical community are the simultaneous use of telescopes for all different wavelength ranges supplemented by cosmic ray and neutrino research.

The small orbiting observatory MOUSE is described in its essential features. Insights in the optical solution (a 2 mirrors 3 reflections telescope using pure hyperbolical surfaces, no aspherics) are given which fit such type of optical layout to scientific requirements. An in-flight recentering of the photons collected by a solar-blind photon counter device will allow for an equivalent angular resolution of 2–3 arcsec over a field-of-view exceeding one square degree. The capabilities of the proposed satellite (imaging, low resolution grisms, polarizers) are described. The choice of the selected sky areas (the two Galactic Poles, the galactic equator and some Bulge Windows) is justified in terms of forefront astrophysical scientific objectives requiring imaging in UV over large fields. The science comprises UV analysis of a complete sample of QSOs, stellar population in galaxies, White Dwarfs, Globular and Open Clusters and OB Associations. In addition a plan for the early UV detection of Supernova events is described.

In 1987 the UV imaging telescope Glazar (Tovmassian et al. 1988) was launched to the Mir Space Station. Its sensitivity was lower than was expected. For this reason the UV survey of the sky was not done and only observations of OB stellar associations were made. The fields of about 15 stellar associations and of the Large Magellanic Cloud were observed. Only images of O, B or early A type stars were obtained on the Glazar photographs at 1640 å. The observations allowed the detection of an average ∼ 15 % new, unknown OB type stars in each of the observed fields, the discovery of hot unknown components of some late type stars and a detailed study of the distribution of stars in space. Some new, unknown OB stellar associations were discovered and more correct distances of stellar associations were determined. The distribution of the absorbing matter was also studied. New, B-type stellar associations, named B were discovered.

To search for optical counterparts of γ-ray bursters, a collaborative project has been initiated together with the γ-ray burst instrument team (BATSE) on the Compton Gamma-Ray Observatory (GRO) which provides times and locations of γ-ray bursts. On the optical part of this project, representatives of 10 observatories are presently involved.

The Galaxy is abundant in red dwarf stars. According to statistical analyses, their majority is unstable. Simultaneously, more than 60% of the variable stars known in the solar vicinity (r < 20 pc) are flare stars and taking into account the number of BY Dra stars too the proportion of red dwarfs amongst local variables is superior to three quarters. Their absolute visual magnitudes range from +6 to +17.5 or even more (Szécsényi-Nagy 1986a). During flare events their brightness may reach a maximum corresponding to an enhancement of 1000–10,000 times. The amplitude of a large flare in the U band may be as high as 8–10 magnitudes. The events take place nonperiodically in unpredictable moments; no unquestionable periodicity has been found in the time distribution of stellar flares. However the activity level of various flare stars is considerably different. Some of them show observable flare ups with a mean frequency of 1 per hour whilst others produce only one event per annum. For the scarcity of the phenomenon, traditional photoelectric photometry of individual stars is not an adequate means to a better understanding of flare stars.