A UBV / variability / proper motion QSO survey from Schmidt plates I . Method and success rate

We describe the first combined variability-proper motion survey for QSOs on Schmidt plates. 85 Tautenburg Schmidt plates covering about 9 square degrees near the north Galactic pole were digitised by means of the APM facility in Cambridge. Variability indices have been derived from U and B data, both with a time-baseline of three decades. Structure function analysis has been applied to search for long-term variability. An almost complete set of propermotions were obtained to a mean B ≈ 20 with a mean error of about 2.0mas/yr for B̄ < 19.5. Adopting a limiting (completeness) magnitude of Blim = 19.7 the success rate of the survey is estimated to > 40 per cent. The completeness of the QSO sample detected by this survey is expected to about 90 per cent. The final sample contains 168QSO candidates; only one has a radio counterpart. The variability-proper motion survey is compared with the selection based on the two-colour index diagram.