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5. Comparison with previous photometry

In this section, we compare the present CCD photometry with all available previous photoelectric and photographic observations. We have found no relevant published data for NGC 3228, while in the other three clusters the comparison is restricted to the U, B and V pass bands. The differences (tex2html_wrap_inline2670) between the present data and data obtained by others are plotted in Figs. 5 (click here), 6 (click here) and 7 (click here) for NGC 4103, NGC 5662 and NGC 6087 respectively, while the statistical results are given in Table 10 (click here). In the table we have also listed the results of intercomparing the various previous photoelectric and photographic data available for each cluster. Figures 5 (click here)-7 (click here) and Table 10 (click here) show that:

   Table 10: Statistical results of the photometric comparison. The difference (tex2html_wrap_inline2672) is always in the sense present minus comparison data. The mean and standard deviation (tex2html_wrap_inline2674) are based on N stars. A few points discrepant by more than 3.5 tex2html_wrap_inline2676 have been excluded from the analysis

(i)
 the UBV photoelectric data obtained by different groups agree fairly well with each other. In contrast, the photographic data generally show large systematic differences compared with the photoelectric data.
(ii)
 in general the UBV photoelectric photometric data are in fairly good agreement with the present CCD data. Except for a few outliers, the distribution of photometric differences seems to be random with no dependence on either brightness or colour (see Figs. 5 (click here)-7 (click here)). The largest discrepancies (tex2html_wrap_inline2924), in each of tex2html_wrap_inline2926 and tex2html_wrap_inline2928, are for the star Haug 156 in NGC 5662, between our measurements and the data of Clariá et al.  (1991). This seems to be due to misidentification by the latter observers since our data agree fairly well with the photographic measurements of the same star by Haug (1978).
(iii)
 the agreement between the UBV photographic data and CCD data is satisfactory for NGC 5662 and also for the brighter (tex2html_wrap_inline2932) stars in NGC 4103 and NGC 6087. However, the scatter is more than in the comparison of CCD with photoelectric data. For fainter stars in NGC 4103 and NGC 6087, the photographic data show systematically increasing differences with the CCD data in both V magnitudes and (B-V) colours. The differences become very large near the limiting magnitude of the photographic measurements. The errors are most likely to lie in the photographic observations since the CCD, unlike the photographic plate, is an intrinsically linear detector and the present data compares well with the photoelectric data over a wide range of brightness as well as colour.

  figure548
Figure 5: A comparison of the present UBV photometry in NGC 4103 with data given by Wesselink (1969), Becker et al.  (1976) and Pedreros (1984). The differences (tex2html_wrap_inline2940) are in the sense present minus others' data, plotted against the present CCD photometry. Photoelectric data by Wesselink (1969) and Pedreros (1984) are denoted by open circles and triangles respectively, while photographic data by Wesselink (1969) and Becker et al.  (1976) are denoted by filled circles and crosses respectively

  figure553
Figure 6: A comparison of the present UBV photometry in NGC 5662 with data given by Moffat & Vogt (1973), Haug (1978) and Clariá et al.  (1991). Other details are the same as in Figure 5. Photoelectric data by Moffat & Vogt (1969), Haug (1978) and Clariá et al.  (1991) are denoted by open circles, triangles and filled circles respectively, while photographic data by Haug (1978) are denoted by crosses

  figure559
Figure 7: A comparison of the present UBV photometry in NGC 6087 with data given by Fernie (1961), Landolt (1964), Breger (1966), Pedreros (1984) and Turner (1986). Other details are the same as in Fig. 5. Photoelectric data by Fernie (1961) and Landolt (1964) are denoted by triangles, while those by Breger (1966), Pedreros (1984) and Turner (1986) are denoted by open circles. Photographic data by Landolt (1964) and Pedreros (1984) are denoted by crosses and filled circles respectively


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