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4. Reductions

The data were reduced using computing facilities available at the Anglo-Australian Observatory, Epping and at the Indian Institute of Astrophysics, Bangalore. The flat-field and data CCD frames were bias subtracted and trimmed using the FIGARO software. The flat-field frames were summed for each colour band. The evenness of flat fields is better than a few percent in all the filters.

The magnitude estimate of a star on the data frames has been done using DAOPHOT software (Stetson 1987, 1992). Further processing and conversion of these raw instrumental magnitudes into the standard photometric system have been done using the procedure outlined by Stetson (1992). The image parameters and errors provided by DAOPHOT were used to reject poor measurements. About 10% of the stars were rejected in this process. The DAOMASTER program was used for cross identifying the stars measured on different frames of a cluster region. In those cases where brighter stars are saturated on deep exposure frames, their magnitudes have been taken only from the short exposure frames. Most of the stars brighter than tex2html_wrap_inline2604 could not be measured because they are saturated even on the shortest exposure frames. The magnitudes of fainter stars have been taken only from the deep exposure frames as they are either not detected or have very small S/N ratio on short exposure frames.

The CCD instrumental magnitudes have been calibrated using the colour equations given by Sagar & Cannon (1995) for NGC 4755, as the present observations were carried out with the same equipment during the same period. For evaluating the zero-points for the data frames, we have used the mean values of atmospheric extinction for the site. For establishing the local standards, we selected about 30 isolated stars in each field and used the DAOGROW program for construction of the aperture growth curve required for determining the difference between aperture and profile-fitting magnitudes. These differences, together with the differences in exposure times and atmospheric extinction, were used in evaluating zero-points for local standards in the cluster frames. The zero-points are uncertain by tex2html_wrap_inline2608 in U and B and by tex2html_wrap_inline2614 in V, R and I. The internal errors in the case of NGC 4103, estimated from the scatter in the individual measures on different exposures, are listed in Table 5 (click here) as a function of magnitude for each filter. The errors become large (more than 0.10 mag) for stars fainter than V = 17. They can be considered as representative of the accuracy of our photometry in the other clusters.

  table334
Table 5: Internal photometric errors as a function of brightness. tex2html_wrap_inline2624 is the standard deviation per observation in magnitude

The X and Y pixel coordinates as well as V, (U-B), (B-V), (V-R) and (V-I) magnitudes of the stars observed in NGC 3228, NGC 4103, NGC 5662 and NGC 6087 are listed in Tables 6, 7, 8 and 9 respectively. They are generally average of at least two measurements. Stars observed by others have been identified in the last column. A few bright stars in each cluster are identified in the corresponding chart (see Figs. 1 (click here)-4 (click here)).

  figure366
Figure 1: Identification map for the two imaged regions of NGC 3228, reproduced from the ESO(B) sky survey. The size of a CCD frame is 3.5 tex2html_wrap_inline2652 5.4 arcmin and the coordinates are in pixel units. A few bright stars from Table 6 are identified

  figure371
Figure 2: Identification map for the imaged region of NGC 4103. Otherwise as for Fig. 1. A few bright stars from Table 7 are identified

  figure376
Figure 3: Identification map for the two imaged regions of NGC 5662. Otherwise as for Fig. 1. A few bright stars from Table 8 are identified

  figure381
Figure 4: Identification maps for NGC 6087 are given in a) and b) for the imaged regions 1 and 2, 3 respectively. Otherwise as for Fig. 1. A few bright stars from Table 9 are identified


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