3 Comparison of various photometries

In order to provide the best set of UBVRI magnitudes for the stars in the cluster region, we inter-compare the present CCD photometry and the earlier photoelectric, photographic and CCD photometric observations. For this, we used the open cluster database compiled by Mermilliod (1995). This database cross identifies the stars among the available observations with respect to the star number given by Phleps & Janes (1994). We also have therefore cross identified the stars common between present and Phleps & Janes (1994) measurements. For photometric comparison, only those sources in the cluster database are used where number of observed stars is at least 10. There are four such sources with UBV photoelectric data; one with UBV CCD data; two with BVR CCD data; one with I CCD data and three with UBV photographic data. The differences ($\Delta$) between the various data sets are plotted in Fig. 3 and the statistical results are listed in Table 3. These show that in CCD observations, the distribution of the photometric differences seems fairly random with a constant zero-point offset, except for a few outliers, which appear to be mostly stars that were treated as single in one CCD study and as blended doubles in others. As expected, the scatter increases with decreasing brightness and becomes more than $\sim$ 0.1 mag at fainter levels. Considering these and the uncertainties present in photometric measurements, we conclude that,

(i)

in general UBV photoelectric data are in fairly good agreement with the CCD data. Except for a few outliers, the distribution seems to be fairly random with a constant zero offset but no dependence on brightness;

(ii)

the photographic data show systematically increasing or decreasing differences with the CCD data in all magnitudes. The scatter is larger in comparison to the photoelectric and CCD data. The difference in V increases with faintness and become $\sim$ 0.2 mag at $V \sim 19$ mag i.e. near the limit of photographic magnitudes;

(iii)

the agreement is satisfactory between the present and other CCD data.

Figure 3: A comparison of photometric data. The present (gssg) B and V data have been compared with the corresponding CCD magnitudes given by Romeo et al. (1989) (r89), Odewahn et al. (1992) (o92) and Phelps & Janes (1994) (pj94) in A), B), C), D), E) and F) diagrams. The U, B and V CCD data of pj94 are compared with the corresponding photoelectric data given by Sandage (1958) (open circles); by Alcalá & Ferro (1988) (filled circles); by Turner (1994) (open triangles) and by Christian et al. (1985) (asterisked) in G), H) and I) diagrams. In J) diagram, the R data provided by r89 have been compared with CCD R magnitudes given by Odewahn et al. (1992) (filled circles) and with the photoelectric R magnitudes given by Alcalá & Ferro (1988) (open triangles). Present I data have been compared with the CCD I data given by r89 (filled circles) and with the photoelectric I data given by Alcalá & Ferro (1988) (open triangles) in K) diagram

3.1 Compiled catalogue

In order to provide the best UBVRI photometric measurements available for the stars in the region of NGC 7790, we transferred various observations on a common photometric system and prepared a compiled catalogue using the results of photometric comparisons carried out in the last section. For this, photographic measurements have not been considered, as they have larger errors. However, this does not introduce any data incompleteness since almost all of them also have photoelectric and/or CCD measurements. For the compiled catalogue, we considered the present BVI data, Phelps & Janes (1994) U data and Romeo et al. (1989) R data as the reference and converted all other photometric data on these systems taking due care for systematic differences, observed in the last section. As the accuracy of photoelectric and CCD measurements is comparable at a given brightness, all measurements of a star in a passband are given equal weight in determining the mean values, in cases, where more than one measurements are available. The compiled catalogue prepared in this way contains U, B, V, R and I measurements for 716, 2099, 2417, 312 and 1315 stars respectively. The stars are spread over an area of $\sim 13' \times 13'$ and have brightness range from V = 10 to 22 mag. For brighter stars (V < 12), the data are photoelectric only as CCD observations are saturated for them. While for fainter (V > 15) stars, the data are mostly from CCD observations.

  

Table 3: Statistical results of the photometric comparison are presented here. The mean and standard deviation ($\sigma$) of the differences ($\Delta$) are based on N stars given inside bracket. Photoelectric and photographic data given by Sandage (1958) are denoted by s58pe and s58pg respectively. Alcalá & Ferro (1988); Frolov (1977); Christian et al. (1985); Turner (1994); Pedreros et al. (1984); Odewahn et al. (1992); Pheleps & Janes (1994) and Romeo et al. (1989) have been abbreviated as a88pe, f77pg, c85pe, t94pe, p84pg, o92ccd, pj94ccd and r89ccd respectively

$$\begin{tabular}{ccr@{$\;\pm\;$}lr@{$\;\pm\;$}lr@{$\;\pm\;$}lc... ... $0.09(240)$ &$~0.06$ & $0.08(296)$& \\ \hline \end{tabular}$$