The observations were obtained at the European Southern Observatory (ESO), using the 3.55 m New Technology Telescope (NTT) and the Danish 1.54 m telescope.
At the NTT we used the SUSI camera, which employs a thinned Tektronix CCD at the Nasmyth focus B. The pixel size is m (0.13'' on the sky), and the frame size is . The data were collected under excellent seeing conditions (0.6''), which is fundamental for the photometry of crowded bulge fields. We provide the log-book of observations in Table 1 (click here). We show in Fig. 1 (click here)a a NTT z image of Ter 6.
Figure 1: Image of Terzan 6: a) NTT-SUSI Gunn z image; dimensions are . b) Danish V image; dimensions are
Figure 2: Cluster V vs. (V-I) diagram for the NTT whole frame
Figure 3: Cluster V vs. (V-I) diagram for an extraction of radius r < 39''
Figure 4: Cluster I vs. (I-z) diagram: where the open circles correspond to stars inside a radius of r < 26'' (cluster) and crosses to stars of r > 39'' (mostly field)
Figure 5: V vs. (V-I) diagram corresponding to the Danish telescope whole frame , essentially dominated by the field stars
At the Danish we used the Tektronix CCD # 28 of pixels, with pixel size m, corresponding to 0.37 arcsec/pixel. The full field is . In Fig. 1 (click here)b the Danish V image is shown, where it can be seen that differential reddening effects are present across this larger field and that the cluster is located in an obscured area; the cluster nucleus looks almost stellar-like due to its post core collapse nature. The Danish observations were taken under photometric conditions, including Landolt (1983, 1992) stars, whereas the NTT data were not photometric. For this reason the zero-point calibrations were obtained based on the Danish observations, whereas the colour terms were obtained from NTT data of previous nights. The reductions were carried out in the standard way, and the calibrations equations, where Landolt stars were also used, are: and where the numbers are for 30 s exposures and airmass of 1.1.
We carried out the reductions with the DAOPHOT II package. The reduction procedure was described in detail in the Liller 1 and Ton 2 studies (Ortolani et al. 1996a; Bica et al. 1996), which were observed in the same run. The main sources of error in the photometry are the zero point accuracy ( mags) and the magnitude transfer from the cluster images to the standard stars due to the crowded field, which can amount to 0.05 mags. The photometric errors are approximately constant to I = 17.5, amounting to 0.02 mags; at I = 18.5 the error increases to 0.07 mags.
Table 1: Log-book of observations