1 Introduction

"Quasars cannot be studied until they are found'' (Weedman 1984). Optical surveys for QSOs can yield very high completeness rates over a large redshift range - yet they are hampered by the small fraction of QSOs among all objects visible in this wavelength range, with most of the latter are foreground stars and galaxies. A straightforward identification of all QSOs in a given survey field, which requires spectroscopic observations of all objects up to an adequate limiting magnitude with sufficient spectral resolution and signal-to-noise ratio, would be a voluminous task with low efficiency. Hence, optical QSO surveys are conducted in two steps: 1) selection of QSO candidates from all objects in the field, based on criteria that are supposed to discriminate QSOs from non-QSOs, and 2) spectroscopic follow-up observations of all selected candidates. The properties of the resulting QSO samples are constrained by the selection criteria of the survey.

Most selection criteria are based on the different spectral energy distribution (SED) of QSOs compared to stars and galaxies. The following properties have been proven to be particularly suited for the identification of QSO candidates: peculiar optical colours (e.g., intrinsic UV-excess, blue continuum), Lyman break (for QSOs with redshifts z>3), and prominent emission lines. Surveys based on these criteria are known to be biased in several ways (for an overview, see Wampler & Ponz 1985; Véron 1993; Hewett & Foltz 1994). Here we only note that their completeness depends (among others) on the QSO redshifts, colour indices, and emission line equivalent widths. It is widely believed that these conventional QSO surveys can reach a very high degree of completeness. However, such a claim can only be verified by means of alternative QSO surveys which are not based on the same or similar selection criteria. In fact, it is still a matter of debate whether conventional QSO surveys systematically overlook hitherto unknown and possibly substantial QSO populations (e.g., Webster et al. 1995; Drinkwater et al. 1997; Kim & Elvis 1999).

Due to their cosmological distances, QSOs have non-detectable proper motions for existing observation techniques. Therefore, the search for zero proper motion objects is expected to provide a bias-free QSO candidate sample (Sandage & Luyten 1967; Kron & Chiu 1981). However, a QSO search which is essentially based on the zero proper motion constraint is not very efficient, since the resulting sample will be dominated by faint galaxies and galactic foreground objects having insignificantly small proper motions by chance. Optical variability is a further general property of quasars (Ulrich et al. 1997; Netzer 1999), and the identification of the variable objects in a given field is a further, independent QSO search method (van den Bergh et al. 1973; Heckman 1976; Usher & Mitchell 1878; Hawkins 1983; Trevese et al. 1989; Véron & Hawkins 1993; Hook et al. 1994; Trevese et al. 1994; Meusinger et al. 1994; Véron & Hawkins 1995; Cristiani et al. 1996; Bershady et al. 1998). The combination of these two constraints, i.e. the search for variable objects with zero proper motion (VPM search = Variability and Proper Motion search), should therefore provide an alternative QSO search strategy which does not explicitely rely on the SEDs of QSOs. It has been speculated that "a search for objects which are both variable and stationary is a powerful technique for efficiently finding QSOs with no selection bias with regard to colour, redshift, spectral index, or emission line equivalent widths'' (Majewski et al. 1991; Véron 1993).

Apart from the experimental and comparably small survey by Majewski et al. (1991), the only VPM QSO survey so far is being performed by Meusinger, Scholz and Irwin on 85 Tautenburg Schmidt plates of a field near the North Galactic Pole (Meusinger et al. 1995; Scholz et al. 1997). According to a priori estimates, a high survey completeness of about 90%, in combination with a success rate of about 40%, is expected, which is confirmed by the preliminary results from spectroscopic follow-up observations (Meusinger et al. 1999).

Here, we present a new VPM QSO survey, which investigates a $10\,\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty... ...p{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi^\circ$ field centred on the globular cluster M 92. This is a more ambitious project since a quasar search in this field faces the problem of a stronger contamination by galactic foreground stars than a search at high galactic latitudes, even though the direction of the M 92 field is well off the galactic plane ( $b=35\hbox{$^\circ$ }$). On the other hand, the field is one of the "Tautenburg Standard fields'', characterized by a very large number of available plates. Further, this area has never been surveyed for QSOs before. Our search is based on 208 selected, deep photographic Schmidt plates covering epoch differences of up to 34 years. With regard to this large quantity of observational data, the present project is the largest QSO survey based on variability and/or proper motion criteria performed so far. The main aims of this project are to improve the statistics of VPM-selected QSOs and to enlarge the number of known QSOs with well-sampled light-curves measured over a time baseline of several decades. The combined sample from both VPM fields is expected to contain more than hundred QSOs with $B\le19.5$ and will be well-suited both for the comparison with QSO samples from more traditional methods and for statistical studies of quasar variability on timescales of days to decades. In addition, the present study is aimed at the detailed discussion of the selection effects of the VPM search.

The present paper is concerned with the description of the observational material (Sect. 2), the photometric and astrometric data reduction (Sect. 3), the definition of suitable indices for proper motion and variability, and the selection of the QSO candidates based on these indices (Sect. 4). The selection effects will be discussed in Sect. 5, and conclusions are summarized in Sect. 6. The identification of the QSOs among the candidates of high and medium priority by means of spectroscopic follow-up observations has been completed. The resulting QSO sample will be presented in a forthcoming paper along with the detailed discussion of the statistical properties of the VPM QSOs and the comparison with conventional optical QSO samples.