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2. Observations and reductions

The UBVRI CCD photometric observations were obtained, using photometric CCD system at f/13 cassegrain focus of the 104-cm telescope of the Uttar Pradesh State Observatory (UPSO) on nights during October - December 1990 and November - December 1992. In order to improve the S/N ratio, the observations were taken in binning mode of tex2html_wrap_inline1223 pixels. In this setup, each pixel of tex2html_wrap_inline1225 size CCD corresponds to 0.66 arcsec and the entire chip covers a field of tex2html_wrap_inline1227 arcmin. The details of the CCD system have been given in Mohan et al. (1991).

Figure 1: Identification map for the cluster Be 64

Figure 2: Identification map for the cluster Be 69

Figure 3: The internal errors as a function of V magnitude

In case of Be 64 one field centred on the cluster was observed (Fig. 1 (click here)). In addition to it, two fields situated tex2html_wrap_inline1231 towards east and west of the cluster were also observed. For Be 69 two overlapping fields called north and south were imaged to cover the cluster region (Fig. 2 (click here)). Also, two nearby fields towards north and south of the cluster situated tex2html_wrap_inline1233 away from the cluster centre were observed. Multiple exposures with exposure times ranging from 10 s to 900 s depending on the presence of bright stars and filter used were taken. Details of observations have been given in Table 1. We have sandwiched the exposures of each field in each filter with exposure of a comparison field in the same filter. The comparison frames were in the vicinity of the cluster regions. The observations were carried out within two hours of meridian so that air mass change within a given sandwich was only marginal. The comparison fields were standardised using Landolt's (1983) stars. A number of bias and twilight flat - field frames were also taken on several nights during the observing runs.

The data analysis was carried out using the computing facilities (Vax Station and Alpha Station) available at the Observatory. The frames were cleaned employing the standard procedures using ESO MIDAS softwares running on the above systems of the Observatory. Different clean frames of same field in the same filter were co-added. Photometry of co-added frames was carried out using DAOPHOT photometric package by Stetson (1987). PSF was obtained for each frame using several uncontaminated stars and the PSF magnitudes were suitably tied to aperture photometry magnitudes. For comparison fields, standard stars and bright stars in the cluster fields, only aperture photometry was obtained.

Figure 4: a) The variation of colour - index parameter (CIP) with age. The data have been taken from Vandenberg (1985). b) Same as Fig. 4a. The data have been taken from Bertelli et al. (1994)

For each frame, differential magnitudes and colours were obtained using the observed magnitudes of the comparison field. There differential magnitudes were then standardised using the transformation equations obtained by Mohan et al. (1991) for the CCD system.

For V magnitudes no colour term could be determined. The internal errors estimated from the scatter in the individual measures of different exposures are shown in Fig. 3 (click here) as a function of V magnitude. The average value of tex2html_wrap_inline1247 for stars having tex2html_wrap_inline1249 comes out to be tex2html_wrap_inline1251, tex2html_wrap_inline1253, tex2html_wrap_inline1255, tex2html_wrap_inline1257 and tex2html_wrap_inline1259 whereas, for stars having tex2html_wrap_inline1261, tex2html_wrap_inline1263, tex2html_wrap_inline1265, tex2html_wrap_inline1267, tex2html_wrap_inline1269 comes out to be 0.04. The X and Y co-ordinates as well as photometric data of the stars measured in the clusters Be 64 and Be 69 have been given in Tables 2 and 3 respectively.

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