2 Observations and reductions

ESO 93-SC08 and ESO 452-SC11 were observed at ESO (La Silla) with the 3.55 m NTT and 1.54 m Danish telescopes.

The SUSI camera was employed at the NTT; it consists of a $1024\times 1024$ thinned Tektronix CCD ESO # 17 at the Nasmyth focus B. The pixel size is 24 $\mu$m (0.13'' on the sky), and the frame size is $2.2'\times 2.2'$. We show in Fig. 1 a NTT V image of ESO 93-SC08 and in Fig. 2 a NTT V image of ESO 452-SC11. They are both loose clusters, such as Palomar globular clusters or old open clusters.

  

Figure 1: NTT V image ESO 93-SC08. Dimensions are $2.2'\times 2.2'$

The Danish observations of 1986 June were carried out with the $512\times 320$ RCA ESO CCD # 1, with pixel size $30\times 30$ $\mu$m (0.47''), giving a field size 4.0' vs. 2.5'.

The Danish 1994 May observations used the $1024\times 1024$ Tektronix CCD # 28, with pixel size 24 $\mu$m, corresponding to $0.37''\!$/pixel. The full field is $6.3'\times 6.3'$.

The Danish 1998 observations have been carried out using the DFOSC with the tinned $2048\times 2048$ pixel CCD Loral/Lesser C1W7. The pixel size is 15 $\mu$m, equivalent to 0.39'' on the sky, with a full field of $13'\times 13'$.

The log of observations is given in Table 1.

The stellar photometry was made with the DAOPHOT II code in MIDAS environment. Landolt (1983) standard stars were used for the early observations and also Landolt (1992) for the recent ones.

For the 1986 June observations the transformation equations are

${V\; =\; 25.0\;+0.09(B-V)\;+v}$;

${B-V\; =\; 0.66\;+1.075(b-v)}$

where coefficients are for 15 (V) and 30 (B) s exposures and airmass of 1.15.

 

Table 1: Log of observations

The zero-point calibrations for the 1994 May 17 NTT observations, which had excellent seeing, but poor photometric quality, were derived from the Danish observations, whereas the colour terms were derived from NTT data of previous nights. For ESO 452-SC11 the Danish 1994 May data were used. The reductions were carried out in the standard way, and the calibration equations are:

${V\; =\; 26.91\;+0.04(V-I)\;+v}$;

${I\; =\; 26.09\;+i}$

where coefficients are for 30 s exposures and airmass of 1.1.

For the NTT data of ESO 93-SC08 the Danish 1986 June and Danish 98 July calibrations were used.

The calibration equations for the Danish 1998 observations are:

$$\begin{eqnarray} V &=& 24.25 + v \pm 0.02\nonumber\\ I &=& 23.04 + i \pm 0.02.\nonumber\end{eqnarray}$$

The coefficients are for 1 s integration. The results are similar to Danish 1986 V calibration within 0.04 mag. A check of the V-I colour could be done following Munari & Carraro (1996) - see their Fig. 2 and Eq. (23), which allows a transformation from (B-V) to (V-I) for MS stars. For such purpose, we selected 10 bright disk MS stars with 16 < V < 19 (Fig. 3a). The results confirm the (V-I) Danish calibration within 0.04 mag.

  

Figure 2: NTT V image ESO 452-SC11. Dimensions are $2.2'\times 2.2'$

The main source of error in the zero point accuracy ($\pm$0.03 mags) is the magnitude transfer from the cluster images to the standard stars which can amount to 0.05 mags. An additional error source affecting observations obtained with the early CCD ESO # 1 was due to the charge transfer inefficiency, causing a slight elongation of the stars in the east-west direction.

The photometric errors at the Danish 1986 run are approximately constant to V $\approx$ 19.5 and B $\approx$ 21, whereas at the Danish 1994 run they are approximately constant to V $\approx$ 19 and I $\approx$ 18. They amount to 0.02 mag, and increase for fainter magnitudes.

At the NTT the errors of $\approx$ 0.02 - 0.03 mag are constant to V $\approx$22.5 and I $\approx$ 22.5, increasing for fainter magnitudes.

We also present a high quality CMD of NGC 6752 that we use for comparison purposes. The diagram is a combination of that previously published in Rosino et al. (1997), to a deeper one obtained under excellent seeing conditions, 5' offcenter. The calibration was carried out using standard stars observed in the same night, which was photometric.