2 Observations

An efficient search for variable objects with zero proper motion requires a large number of homogeneous (e.g., the same colour system) observations of a large number of faint objects with high astrometric and photometric accuracy, spanning a time-baseline of decades. These requirements can be matched if a substantial number of deep archival plates from a large wide-field imaging telescope is available. The archive of the Tautenburg Schmidt telescope (134 cm free aperture, 4 m focal length, $3\hbox{$.\!\!^\circ$ }3\times3\hbox{$.\!\!^\circ$ }3$ unvignetted field of view) contains more than 9000 plates taken between 1960 and 1997. For several "standard fields'', more than hundred archival plates are available. With epoch differences of three decades and more, this observational material is particularly well suited for a VPM QSO search, since all plates were taken with the same telescope, through the same filters, and onto very similar emulsions. Moreover, thanks to its large focal length, compared to other large Schmidt telescopes, the Tautenburg Schmidt has less problems with distortions due to plate bending and has a better scale for astrometric work (e.g., Schilbach et al. 1995; Scholz et al. 1993, 1994, 1996, 1997; Meusinger et al. 1996).

For the present VPM survey, the field centred on the globular cluster M 92 was chosen (Table 1). In preparation for this project, 56 plates were taken in the years 1992 to 1997. Combined with the archival plates, a total number of 332 plates of the M 92 field are available. We selected 208 sufficiently deep plates in the U, B, V or R band, among them 162 B plates (Table 1). Only the B plates are used to measure variabilities and proper motions; the measurements in the other bands only provide additional colour information. The plates were taken in the years 1963 to 1997. Compared with our first VPM survey in the M 3 field, the present survey comprises about three times more B plates with a better time coverage and a slightly longer time baseline.

Figure 1: Individual limiting magnitudes of all 208 selected Schmidt plates of the M92 field versus epoch. Different symbols represent different colour bands in the Johnson system (open circle: U, filled circle: B, plus sign: V, cross: R).

 

 

Table 1: Data on the M 92 survey field and the selected Schmidt plates.

plate centre:	$\alpha_{2000} = 17\mbox{${}^{\rm h}$ }17\mbox{${}^{\rm m}$ }7\mbox{${}^{\rm s}$ }$
	$\delta_{2000} = 43\hbox{$^\circ$ }8\hbox{$.\mkern-4mu^\prime$ }2$
	( $l=68\hbox{$^\circ$ }$, $b=+35\hbox{$^\circ$ }$)
field size:	$3\hbox{$.\!\!^\circ$ }3\times3\hbox{$.\!\!^\circ$ }3$
	minus $0\hbox{$.\!\!^\circ$ }4\times1\hbox{$.\!\!^\circ$ }3$ due to
	calibration wedge
plate scale:	51 $.\!\!^{\prime\prime}$4 mm-1
number of plates:	162 B (epochs 1963-1997)
	18 U (epochs 1966-1997)
	18 V (epochs 1966-1989)
	10 R (epochs 1966-1968)

Figure 2: Histogram of the epoch differences $\Delta \,t$ for all combinations between each two out of the 162 selected B plates.

The limiting magnitudes and epochs of the observations are summarized in Fig. 1. Figure 2 shows the histogram of the epoch differences between all combinations of two individual B plate epochs. The frequency space is almost entirely and quite homogeneously covered for epoch differences larger than one day. The large maximum epoch difference of 34 years, in combination with the huge number of plates, allows the certain detection and subsequent thorough investigation of variable objects with variability timescales of days to decades.