A QSO survey via optical variability and zero proper motion in the M 92 field - I. QSO candidates and selection effects

The combination of variability and proper motion constraints (VPM search) i s believed to provide an unconventional yet efficient search strategy for Q SOs with selection effects quite different from conventional optical QSO su rveys. Previous studies in a field of high galactic latitude have shown tha t the VPM method is indeed an efficient search strategy. In the present pap er, we describe a further variability-proper motion (VPM) QSO survey on Tau tenburg Schmidt plates. The survey is based on an exceptionally large numbe r of 162 B plates centred on M 92 with a time-baseline of more than three d ecades. Further U and V plates are used to measure time-averaged colour ind ices, and morphology is evaluated on a deep R plate. Proper motions with Hi pparcos-like accuracies as well as variability indices are derived for abou t 35000 star-like objects down to B = 20.5. With regard to both the number of plates and the number of objects to be investigated, this is the largest VPM survey performed so far. The present paper is focused on the descripti on of the observational material, the data reduction, the definition of the selection parameters, and the construction of the QSO candidate sample. Fu rthermore, the selection effects of the VPM-method are discussed a priori. For the present survey, the selection effects are shown to be dominated by the magnitude-dependence of the photometric accuracy. Down to the limiting magnitude of the survey B-lim = 19.7, we identify 62 high-priority QSO cand idates and a further 57 candidates of medium priority. Spectroscopic follow -up observations have been performed for all these candidates as well as fo r additional selected candidates of lower priority; the confirmed QSOs will be presented and discussed in a forthcoming paper.