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Astron. Astrophys. 352, 363-370 (1999)

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2. Observations and data reduction

SagDIG was observed on 1997 July 28 with the HiRAC CCD camera of the 2.56 m NOT at Roque de los Muchachos Observatory in La Palma island (Canary Islands, Spain). The seeing was [FORMULA]. A 2048[FORMULA]2048 CCD detector was used binned to [FORMULA], providing a total field of 3.7 [FORMULA] with a resolution of [FORMULA]/pixel after rebinning. Two frames were obtained in both the V and I bands with total exposures 2000 s in V and 1800 s in I. In order to estimate foreground galactic star number counts another field (8´ to the east) was imaged with exposures of 1200 s (V) and 1000 s (I). Standard stars from Landolt (1992) were used for calibration. Details of the calibration can be found in Aparicio et al. (2000b). A V band frame of SagDIG is shown in Fig. 1.

[FIGURE] Fig. 1. V image of SagDIG obtained with the 2.56 m NOT with a seeing of [FORMULA] (FWHM). Field size is [FORMULA]. North is up, east is to the left.

The derived images were processed with MIDAS implementation of the DAOPHOT II program (Stetson 1987; Stetson et al. 1990). A total of about 1550 stars were measured in the SagDIG frames down to a limiting magnitude [FORMULA]. Fig. 2 plots the PSF fitting errors as provided by ALLSTAR for V and I magnitudes.

[FIGURE] Fig. 2. PSF fitting errors as given by ALLSTAR as a function of V and I magnitudes.

Completeness has been analyzed using the usual procedure of artificial star trials (Stetson 1987). A total of 2000 artificial stars were added to the V and I frames of SagDIG in several steps of 100 stars each with magnitudes and colors in range [FORMULA] and [FORMULA]. Stars were considered as recovered if they were found both in V and I with magnitudes not exceeding 0.75 mag brighter than the initial, injected ones. The completeness curves for blue and red stars are shown in Fig. 3.

[FIGURE] Fig. 3. Completeness curves for blue [[FORMULA]; full line] and red [[FORMULA]; dashed line] stars in the SagDIG field.

Besides stellar photometry we also carried out aperture photometry of the galaxy in circular diaphragms that allows the total integrated magnitude and colour to be measured.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999