2 Observations and reductions

The observations reported on here were acquired on December 2 to 7, 1992, at the 2.2 m Max-Planck-Institute telescope of ESO, La Silla, with the EFOSC-2 camera. The detector was a Thompson THX 31156 CCD ($1024\times1024$ pixels, $19\times19$ microns each, reduced to a $900\times900$ pixels window) with a readout noise of 9 e^- rms. The field covered was $5\times5$ arcmin, its center was positioned approximately in $1\hbox{$.\mkern-4mu^\prime$}5$ north-north-east from the cluster center. A reproduction of the field can be found in Alvarado et al. (1993), Fig. 2 (Plate 96). A total of 12 frames in B light (exposure times -- from 30 to 900 s), 13 frames in V (6 to 240 s), 12 frames in r (Gunn) (6 to 120 s), and 10 frames in i (Gunn) (10 to 180 s) were acquired. The seeing for most frames was from 1.0 to 1.5 arcsec.

The reductions of CCD photometry were performed at the Institute of Astronomy (Moscow) using the software described by us earlier (Samus et al. 1995) and based upon the DAOPHOT II ALLSTAR application (Stetson 1991). We reduced the photometric data to the standard $BV(RI)\rm _C$ system. The formulas taking into account the color terms of photometric reductions were derived by us using observations of the standard sequence in NGC 1978 itself (Alvarado et al. 1993).

The formulas used in this study are:

$$V=v+0.083(\pm0.014)(b-v)-0.008(\pm0.009),$$

$$V=v+0.048(\pm0.021)(v-i)-0.014(\pm0.021),$$

$$B-V=1.349(\pm0.014)(b-v)+0.032(\pm0.010),$$

$$V-R=1.170(\pm0.032)(v-r)-0.051(\pm0.015),$$

$$V-I=1.031(\pm0.010)(v-i)-0.012(\pm0.010).$$

These formulas are derived from observations of 7 stars covering a wide range of colors (-0.16 to 1.71 in B-V; -0.13 to 0.81 in V-R; -0.05 to 1.51 in V-I). The photoelectric magnitudes of the standards are well reproduced by our CCD data; this can be seen from Table 1 presenting photoelectric magnitudes and colors of the standard sequence along with residuals for our CCD values, expressed in the sense (Phe-CCD). Our first formula agrees well with the corresponding formula derived, for the same photometer, by Buonanno et al. (1993) (their (b-v) coefficient is 0.084). However, the formula for (B-V) following from Buonanno et al. would have a different (b-v) coefficient (1.197). The reason for the discrepancy is not clear, taking into account the small error associated with our formula.

  

Table 1: Data for photoelectric standards

The internal accuracy of our magnitudes, for most stars, is: for $V\le18$,$\sigma_B<0.03$, $\sigma_V\approx\sigma_R\approx\sigma_I<0.01$;for V>18, $\sigma_B<0.06$, $\sigma_V\approx\sigma_R\approx\sigma_I <0.04$.

We have identified 332 stars of our sample with stars measured photographically by Olszewski (1984). Figure 1 shows the comparison of photometries. The agreement of V magnitudes is reasonably good, taking into account typical accuracy of photographic photometry. However, one can see systematic deviations in B-V color, especially for blue stars. We also find some systematic trends of $\Delta V$ values with B-V, despite the above-mentioned excellent agreement of our reduction formula for V with that from Buonanno et al. (1993).

 

Figure 1: Comparison of our photometry with that from Olszewski 1984. a) $\Delta V$ (present paper minus Olszewski) vs. V (present paper); b) $\Delta(B-V)$ (present paper minus Olszewski) vs. B-V (present paper)

Our investigation is the second BVRI CCD study of NGC 1978, and a detailed comparison with photometry from Will et al. (1995) is of considerable interest. To compare photometric results, we used the data deposited in the Star Data Center in Strasbourg. Unfortunately, only V magnitudes and B-V color indices are available for Will et al. (1995) photometry in NGC 1978 accessible electronically, whereas BVRI photometry is discussed in Bomans et al. (1995). We have identified 1899 stars in common with Will et al. The comparison of photometries is illustrated by Fig. 2. Its striking feature is a large difference of zero point in V magnitude, at least by $0\hbox{$.\!\!^{\rm m}$}4$,despite the photometry of Will et al. being based upon the photoelectric standards of Alcaíno & Alvarado (1988), created with the same equipment as the standards used by us (Alvarado et al. 1993). The standard "J'' from Alvarado et al. (1993), with photoelectric V=15.33, has V=16.140 in the CCD data by Will et al. Taking into account our overall agreement with Olszewski (1984) in the magnitude zero point, we tend to attribute the revealed disagreement to a calibration error in Will et al. (1995). A systematic error could be introduced into our data because we use photoelectric, and thus aperture, magnitudes for the standard stars, located in a crowded field, along with our PSF CCD data. But this error is not expected to exceed several hundredths of a magnitude. The distribution of stars in Fig. 2a is rather asymmetric, with more stars scattering below the average line than above it. This may be due to errors in both studies. In B-V color, our data show systematic effects resembling those revealed in comparison with Olszewski (1984).

 

Figure 2: Comparison of our photometry with that from Will et al. 1995. a) $\Delta V$ (present paper minus Will et al.) vs. V (present paper); b) $\Delta(B-V)$ (present paper minus Will et al.) vs. B-V (present paper)

Our photometry in NGC 1978 is available upon request from the authors (N. Samus).