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Astron. Astrophys. 335, 929-942 (1998)

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2. Observations and data reduction

Observations were carried out during the nights July 15 and 16, 1993, at the ESO 3.58 NTT equipped with EMMI. A set of 17 CCD B-frames was secured with the Tektronix TK1024 (ESO #31) using filter ESO#603 at EMMI blue arm. 13 CCD V-frames were obtained with LORAL2048 detector (ESO #34) and filter ESO#606 at EMMI red arm. Six fields, partially overlapping, covering a total area of about [FORMULA] arcmin2, grossly centered on the cluster, were observed in good seeing conditions (FWHM 0.9-1.2 arcsec). An additional field centered at [FORMULA] arcmin from the cluster center ([FORMULA] tidal radii) was observed in order to estimate the field star contamination. The details of the exposures are given in Table 1. Flat-field and bias images were taken at the beginning and at the end of each night to correct for the detector response to uniform sensitivity.


Table 1. For each field and each filter the Table shows the number of frames (N), the exposure time (t) and the seeing for the best frame.

Data reduction was performed using the package ROMAFOT (Buonanno et al. 1979, 1983) for crowded field photometry. Standard procedures to determine the PSF, to group and to fit the objects were adopted (see Ferraro et al. 1990 for a description). For each field the master list was created from the individual frame photometry after interactively checking those objects for which the goodness of fit estimators assumed anomalous values. A total number of 10176 single stars was detected and measured in both B and V-bands, once the entire reduction procedure had been fully performed. The threshold values adopted in the search phase were fixed according to the crowding conditions and exposure times, on the basis of several tests, in order to limit the number of spurious detections. Information about the threshold values adopted and the number of objects detected for each field are shown in Table 2.


Table 2. For each field the Table shows the threshold adopted in the ROMAFOT/search procedure (Thr.) expressed in units of sky Poisson noise ([FORMULA]) and the number of stars detected.

Due to non photometric sky conditions, the observation of photoelectric primary standards outside the M 80 field did not allow a reliable calibration. In order to calibrate our data we used a set of 141 stars in common with Saviane & Piotto (1998) covering the ranges [FORMULA] and [FORMULA]. The following relations have been obtained:



where B and V are the magnitude listed by Saviane & Piotto, and b and v the instrumental ones. The former have been calibrated using a set of 12 standard stars from Landolt (1992); the uncertainty on the calibrated magnitudes quoted by Saviane & Piotto is 0.03 magnitudes in V and 0.04 magnitudes in (B-V). All the cluster fields are partially overlapping, so the common stars have been used to calibrate to the same instrumental magnitudes. Unfortunately, the background field do not overlap with any cluster fields. In this case the calibration has been performed by using, as secondary standards, the stars measured in the cluster frames obtained just before and after the exposures on the background field.

The constants present a dispersion of 0.06 and 0.05 mag in B and V, respectively.

The CMD for all the detected stars with a distance from the cluster center [FORMULA] px (pixel size 0.35 arcsec) is shown in Fig. 1. The data of the present investigation can be usefully compared with the photographic data presented by HR74. Lacking a finding chart for stars measured in 1974, we simply selected in our sample the stars in the same region covered by HR74 (100 arcsec [FORMULA] 250 arcsec), plotting the corresponding CMDs. Even if the HR74 data are affected by a larger internal photometric error, one finds that the CMD morphologies for the two dataset overlap quite satisfactorily.

[FIGURE] Fig. 1. CMD for all fields, only star with [FORMULA] (9822 objects).

The internal accuracy of our CCD measurements has been estimated on the basis of the rms frame-to-frame scatter of the instrumental magnitudes for each individual star, computed as in Ferraro et al. (1990). The mean internal errors are, approximately, [FORMULA] and [FORMULA] for [FORMULA]; [FORMULA] and [FORMULA] for [FORMULA].

The final photometric error includes also the error involved in the calibration procedure and the uncertainties related to the various transformations. We estimate that systematic total errors up to 0.05 mag in V and 0.08 mag in [FORMULA] cannot be ruled out in our measurements, at least for very blue and/or very red stars.

Nevertheless, our data and the photographic measurements by HR74 do not show significant zero point differences. In particular, there is a good agreement in the two color scales: they found [FORMULA] as mean red giant branch (RGB) color at the magnitude [FORMULA], while we obtain [FORMULA] at the same magnitude level from our CMD (see next section for a detailed discussion on the magnitude of the HB and the color of the base of the giant branch).

For what concerns the completeness of our sample, we made several tests following the procedure described in Ferraro et al. (1990): we randomly added to the original frame a group of stars of known magnitude ([FORMULA]), re-reduced the frame and calculated the ratio [FORMULA] which represents the degree of completeness, where [FORMULA] is the number of artificial stars retrieved. We performed the tests in four annuli at different distances from the cluster center, in order to account for the varying crowding conditions.

As shown by the artificial star experiments, the sample with [FORMULA] mag is fairly complete (i.e. [FORMULA]) down to 250 pix from the cluster center ([FORMULA] arcsec). Therefore, where in the following we make use of star counts (luminosity function, R parameter), we refer only to this subsample.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998