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Astron. Astrophys. 360, 120-132 (2000)

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2. Observations

We selected our targets from the photographic photometry of Buonanno et al. (1986, see Table 1 and Fig. 1). For our observations we used the ESO 1.52m telescope with the Boller & Chivens spectrograph and CCD #39 (2048[FORMULA]2048 pixels, (15 µm)2 pixel size, read-out noise 5.4 e-, conversion factor 1.2 e-/count). We used grating # 33 (65 Å mm-1) to cover a wavelength of 3300 Å - 5300 Å. Combined with a slit width of 2" we thus achieved a spectral resolution of 2.6 Å. The spectra were obtained on July 22-25, 1998. For calibration purposes we observed each night ten bias frames and ten dome flat-fields with a mean exposure level of about 10,000 counts each. Before and after each science observation we took HeAr spectra for wavelength calibration purposes. We observed dark frames of 3600 and 1800 sec duration to measure the dark current of the CCD. As flux standard stars we used LTT 7987 and EG 274.

[FIGURE] Fig. 1. The colour-magnitude diagram of NGC 6752 (Buonanno et al. 1986). Stars analysed in this paper (including 10 stars discussed by Moehler et al. 1997b) are marked by open squares (some of which overlap due to almost identical photometric data).


Table 1. Coordinates, photometry, and heliocentric radial velocities for the target stars in NGC 6752. The numbers refer to Buonanno et al. (1986) and "acc." refers to targets that happened to be in the slit by accident. We also give the newer UBV photometry of Thompson et al. (1999). For the NTT spectra obtained in 1998 no radial velocities could be determined due to problems with the wavelength calibration (see Moehler et al. 1999b).

We also analyse data that were obtained as backup targets at the NTT during observing runs dedicated to other programs (60.E-0145, 61.E-0361). The observational set-up and the data reduction are described in Moehler et al. (2000, 1999b). These data have a much lower resolution of 5.4 Å.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000