2 Observations and reductions

NGC 6256 and NGC 6717 were observed in 1994 May and 1985 June respectively with the 1.5 m Danish telescope at ESO (La Silla).

In the 1994 May run the Tektronix CCD ESO # 28 of $1024 \times 1024$ pixels was used; the pixel size is 24 $\mu$m, corresponding to 0.37'' on the sky, providing a full field of $6.3'\times 6.3'$.

In Fig. 1 is shown a V 10 min exposure of NGC 6256. Notice the post core-collapse structure.

  

Figure 1: V image of NGC 6256. Dimensions are $6.3'\times 6.3'$. North is to the top and East is to the left

In the 1985 June run the $320\times 512$ pixels RCA CCD ESO # 1 was used. The pixel size is 30 $\mu$m, corresponding to 0.47'' on the sky and the array gives a full field of $4' \times 2.5'$. In Fig. 2 the V 15 min image of NGC 6717 is shown.

  

Figure 2: V image of NGC 6717. Dimensions are $4.0'\times 2.5'$. North is to the top and East is to the right

The log of observations is provided in Table 1.

  

Table 1: Log of observations

Note to the table: The long B and V observations of 24.06.85
were taken offcenter, west of the cluster.

Daophot II was used to extract the instrumental magnitudes. These magnitudes have been calibrated using Landolt (1983) stars for NGC 6717 and Landolt (1992) for NGC 6256.

The reductions in the crowded reddened fields of NGC 6256 are described in detail in a study of Liller 1 (Ortolani et al. 1996 and references therein). The equations in the case of NGC 6256 are:

V = 26.91 + 0.04(V-I) + v

I = 26.09 + i

reduced to 30 s. exposure time and airmass of 1.1. Due to crowding effects arising in the transfer of the aperture magnitudes from standards to the field stars, the zero point calibration errors are dominant, estimated to be about $\pm 0.03$ mag The CCD shutter time uncertainty (0.7 s) for a typical 20 s exposure time for the standard stars, produces an additional 3% uncertainty, which is propagated to the calibrations of the long exposure cluster frames. The final magnitude zero point uncertainty amounts to $\pm$ 0.05. The atmospheric extinction was corrected with the La Silla coefficients (C_V = 0.16, C_I = 0.12 mag/airmass). The zero point of the V filter was checked relative to the Alcaino (1983) photometric stars and a good agreement within 0.02 mag was found.

NGC 6717 was observed in the same run as Palomar 12, and characteristics of the instruments and calibration procedures used are described in Gratton & Ortolani (1988). The equations in the case of NGC 6717 are:

B = 24.88 + 0.16(B-V) + b

V = 24.25 + 0.08(B-V) + v

for exposures of 15 s. in V and 30 s. in B (airmass = 1.15). The atmospheric extinction was corrected with the standard La Silla coefficients (C_V = 0.14, C_B = 0.22 mag/airmass).

NGC 6717 has the bright field star $\nu_2$ Sgr 3' to the north, which was placed outside the CCD frame; the cluster is offcenter but the star glare is avoided, and checks showed that in fact no diffuse light disturbed our frames.