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Astron. Astrophys. 342, L9-L12 (1999)

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2. Observations and reductions

The Calar Alto 2.2m telescope and its focal reducer Cafos was used in August 1998 to obtain images in B, V, and I. We obtained 3 frames in every band, slightly shifted from one exposure to the next. Total exposure times were 1800, 1500, and 1500 sec, respectively. The observations were accompanied by the usual CCD calibration measurements for flat-fielding and de-biasing. The flux calibration was established by observing two Landolt (1992) faint standard fields several times at various airmass. The seeing was 1.5 (I), 1.5 (V), and 1.7 (B) arc sec (FWHM). An I fringe pattern frame was derived from all 24 science frames of this run with various locations on the sky. This fringe pattern frame was substracted after debiasing and flatfielding, while the B and V frames were only debiased and flatfielded. The field of view of the frames is 12 by 12 arcmin, far larger than the extent of the And VI dwarf galaxy, estimated as 2 by 4 arc min by Karachentsev & Karachentseva.

The co-added And VI frames were searched for stellar objects in each band independently by the MIDAS 97NOV version of Daophot (Stetson, 1987). The PSF was constructed from some 30 to 40 brighter, non-saturated, and well isolated stars. The PSF-photometry was done with the ALLSTAR routine. Several thousand objects were detected in each of the bands. Aperture photometry of the standard stars allowed us to calibrate the PSF magnitudes. We are using here only those stars which are detected in at least two colors, either V and I, or B and V. This largely reduces false detections. 794 stars were simultaneously identified in B and V while 979 were found in V and I. The PSF photometric errors become larger than 0.15 at 23.75 (B), 23.25 (V), and 21.75 (I). The faintest objects detected are typically a magnitude fainter than this, but we will use in the following only those stars with errors less than 0.15 mag.

As we here intend only to describe the dwarf galaxy and to establish its distance from the location of the tip of the red giant branch (TRGB), we postpone artificial star tests to evaluate the incompleteness to a later paper.

To get a quantitative description of the structure of And VI, we cleaned the V frame from the brightest stars (about 18, they are probably MWG foreground) and smoothed the images with a large spatial filter (11 by 11 arc sec). On this smoothed frame, we derived the structural parameters by applying the ellipticity fit of Bender & Möllenhoff (1987).

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998