2 Observations and data reduction

The observations of IC1613 were carried out on the 2-m Ritchey-Chretien telescope of the Bulgarian National Astronomical Observatory. A set of CCD frames in the UBV system was taken on October 9, 1997 with a Photometrics $1024 \times 1024$ camera.

The seeing during the observations was about 1 arcsec with stable and very good photometric conditions. The scale at the cassegrain-focus CCD was 0.33 arcsec/pixel and the observing area was a $5.6\times 5.6$ arcmin field centered on the H II regions in the northeast sector of the galaxy. The frames were obtained with total exposure times of 2700 s, 1800 s and 1800 s in the U, B and V filters, respectively. The mean airmass over the duration of each exposure was between 1.2 and 1.3. Two standard fields of NGC1502 (Reimann & Pfau 1987) were taken before and after the observations.

The standard IRAF data reduction package was used to carry out the basic CCD bias reduction and the flat field correction. The stellar photometry of the frames was done using DAOPHOT II (Stetson 1993). This package is widely used and besides Stetson's own description there are numerous references describing its application in detail (see for example Gallart et al. 1996a). The total number of measured stars is 1500 in U, B and V filters and 3100 in B and V respectively.

Fifteen to twenty relatively bright and isolated stars were chosen for each frame in order to obtain the aperture corrections and to transform the PSF magnitudes to the instrumental ones.

The atmospheric extinction for the observation night was calculated separately for each filter. The instrumental values were then corrected for extinction and transformed to the standard UBV system using standard stars in the field of NGC1502. The zero-point errors of the transformation were 0.01 mag in U, 0.02 in B and 0.04 in V. The magnitude residuals of the photometric transformation for the standard stars as a function of the U, B and V magnitudes are shown in Fig. 1.

  

Figure 1: The magnitude residuals of the photometric transformation at the standard stars as a function of the U (upper panel), B (medium panel) and V (lower panel) standard magnitudes. The photoelectric photometric standards from Sandage (1971) are represented by crosses

The four photometric standard stars from the list of Sandage (1971) available in the field were transformed to the standard system so that we could check the zero point of our calibration on in-field data. One of them - 18c - displayed large differences of about 0.2 mag in both B and V and taking into account its relative faintness (Sandage's magnitude in B is 23.18) we removed it from the calculations. The average differences for the other three stars are 0.02 in V, -0.02 in B and 0.05 in U. In Fig. 1 these stars are marked by crosses. This inaccuracy was however estimated to be within the standard photometry error and thus could not seriously affect the results of our study.

We have compared our data with the photometry of Freedman (1988a) in order to check for the presence of systematic effects. The residuals in the measurements of stars in common between the two studies are plotted in Fig. 2 as a function of the magnitudes and colors derived in the present investigation. As can be seen by inspection of these diagrams, no systematic trend is present in the V magnitudes and B-V colors. The mean differences are 0.03 in B-V and 0.01 in V.

  

Figure 2: Comparison of the V magnitudes and B-V colors derived here with the photometry of Freedman (1988a). The comparison is in the sense of this work minus Freedman (1988a)