3. Photometry

The instrumental CCD magnitudes were measured using the procedures described in Cunow (1993). The dark current was found to be negligible for this CCD chip. The instrumental CCD magnitudes are aperture magnitudes with apertures being large enough to contain the whole object. The following aperture radii were used: , , , , , , and . The aperture chosen for an individual object is the smallest one with a diameter which is at least twice as large as the object size. Checks by eye ensured that the chosen aperture is always larger than the whole object.

 
Figure 1:   Comparison of catalogue magnitudes with the CCD magnitudes for the E-region stars. a) shows the B measurements, b) the V measurements and c) the R measurements

For transformation of the instrumental magnitudes to the standard system, the following equations were used:
   

X is the airmass, , and are the instrumental magnitudes and B, V and R the magnitudes in the Kron-Cousins-System. The coefficients k were obtained from the standard stars by a fitting procedure following the guidelines by Harris et al. (1981). Small colour terms were found:

   

Abell cluster ESO/SERC

no. field no. h m

0916 567 10 03.9 -19 22

S0645 569 10 46.2 -18 27

3471 438 11 11.2 -30 09

3528 443 12 54.3 -29 01

S0726 382 13 15.2 -33 38

1732 576 13 25.0 -20 13

3558 444 13 27.9 -31 29

3576 445 13 52.8 -30 17

1996 513 14 57.4 -23 55

3621 514 15 28.7 -24 52

Figure 1 (click here) shows the CCD magnitudes versus the catalogue magnitude for the E-region stars. From the scatter the random magnitude errors for these stars were determined for the different filters: , and . These errors are similar to the ones found for our previous CCD sequences (Cunow 1993; Cunow & Wargau 1993, 1994; Cunow & Ungruhe 1995). From this similarity and from the fact that in this work observations and reductions were done in the same way as for our previous CCD sequences, the same magnitude errors as given in those papers are expected for the programme objects in this work (from for the bright objects to for the very faint objects).