2 Observational material

2.1 Schmidt plates

The basic observational material of the present study consists of 35 photographic plates from the Tautenburg 2m telescope in its Schmidt mode. Each plate covers an unvignetted area of 3 $.\!\!^\circ$3$\times$3 $.\!\!^\circ$3, centered on $\alpha = 9^{\rm h} 55\hbox{$.\!\!^{\rm m}$ }6, \ \delta = 68\hbox{$^\circ$ }04\hbox{$^\prime$ }$(J2000). The plate scale amounts to 51 $.\!\!^{\prime\prime}$4mm^-1. 27 plates were selected from the Tautenburg plate archive, further 8 plates were taken in 1997 and 1998 in preparation of the present project. For the majority (30) of the plates nearly identical emulsions were used. These emulsions, in combination with a Schott GG13 filter, closely match the standard B band. Three plates have blue-sensitive emulsions but were taken either without filter or through a Schott GG11 filter. Two other plates have a KodakIIIa-J emulsion in combination with a GG13 filter. 31 plates have a calibration wedge exposed onto.

2.2 CCD observations

CCD frames in the R band and in a 100Å wide band around H$\alpha$ were taken for some sub-areas of the field. The Tautenburg Schmidt telescope was used, equipped with a SITe chip of 2128 $\times$ 2128 pixels with pixel size of 24$\mu$m $\times$ 24$\mu$m. Total exposure times were about 1000s to 2000s in R and 2000s in H$\alpha$. The weather conditions were in general good, though not always photometric. The seeing was 2... 3 $^{\prime\prime}$, comparable with the mean seeing on the photographic plates. The image reduction was done with the IRAF package. The zero-point of the R band magnitude scale was calibrated with the magnitude scale given by Zickgraf et al. (1990).