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Astron. Astrophys. 361, 73-84 (2000)

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3. Reductions

All of the reductions have been carried out with the image processing system MIDAS. For the stellar photometry, the DAOPHOT and ALLSTAR code have been used. The raw data has been corrected for bias and flat-fields. Due to technical problems with the filter wheel resulting in vignetting, the flat-fields are not only very inhomogeneous but do also vary in time. Therefore some of the data couldn't be used at all. The dimensions of the unvignetted section of each field are given in Table 1.

For the calibration, at least 10 standard stars have been measured at the beginning, the middle and the end of every night at different air-masses. They have been selected from Landolt (1992). From the measured standard stars we derive the following relation between instrumental v, (v-i) and calibrated V, (V-I) for 1 sec of exposure time:


The completeness corrections [FORMULA] and [FORMULA] defined as the ratios of recovered to injected stars in V and I frames respectively are derived by means of the usual artificial stars experiments. They are plotted for each field and filter in Fig. 2. The photometric errors are 0.01,0.05,0.12,0.20 for V=10,21,22,23 respectively and 0.01,0.02,0.05,0.2 for I=10,18,20,21.5 mag.

[FIGURE] Fig. 2. Completeness factors [FORMULA] and [FORMULA] as functions of the magnitudes for all the fields.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000