Astron. Astrophys. 348, 783-788 (1999)

2. Observations

NGC 6401 was observed in the night of July 4, 1998, with the 1.5m Danish telescope at ESO (La Silla). We employed an EFOSC camera equipped with a Loral/Lesser CCD detector C1W7 with 20522052 pixels. The pixel size is 15 µm, corresponding to 0.39" on the sky, which provides a full field of 13´13´.

In Fig. 1 we show a 15 sec V exposure of NGC 6401, where it is clear that we are dealing with a rich cluster.

Fig. 1. V image (15 sec.) of NGC 6401. Dimensions are 6.5´6.5´. North at the top and east to the left.

In the same night the template cluster 47 Tucanae was also observed, in order to have its bright sequences compared to those of NGC 6401. The frame is offset by 5´ south relative to the cluster center. This template V vs. (V-I) CMD diagram is shown in Fig. 2.

Fig. 2. V vs (V-I) CMD of the template 47 Tucanae, observed in the same night as NGC 6401. The sampling corresponds to an extraction of 1000 x 1000 pixels (6.5´6.5´).

The log of observations is provided in Table 1.

Table 1. Log of observations

Daophot II was used to extract the instrumental magnitudes. For calibrations we used stars from Landolt (1983) and Landolt (1992).

Reduction procedures in such reddened crowded fields were discussed in detail in a study of Liller 1 (Ortolani et al. 1996and references therein). The equations for the present clusters are:

reduced to 1 sec. exposure time and 1.1 airmass. 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 0.03 mag. The CCD shutter time uncertainty (0.3 sec) for a typical 10s 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 0.05. The atmospheric extinction was corrected with the La Silla coefficients (C_V = 0.16, C_I = 0.12 mag/airmass).

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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