2.2. Data analysis
All the images were corrected for bias and
flat-field using standard IRAF procedures. The relative photometry was
carried out with the PSF-fitting code DoPhot
(Schechter et
al. (1993)).
Each frame was independently reduced. A 3
threshold above the
background noise was adopted for the search for sources on the frames
and the spatial variations of
the PSF were modeled with a quadratic polynomial function. Only the sources
classified as stars by the code were retained. All the V and I
catalogues of a
given field were reported to the (instrumental) photometric system of the
best-seeing image acquired under photometric conditions.
The catalogues were then cross-correlated, the magnitudes were
averaged and the standard deviation adopted as the photometric
uncertainty of the individual stars. In the final catalogue of instrumental
magnitudes and positions we have retained only the sources that have at
least three valid measures of the magnitude for each passband (e.g. at
least 3 V and
3 I). Moreover, all the stars with an associated error (either in V or I
magnitude) larger than three times the average uncertainty at their
magnitude level were excluded from the sample. The final F1 and F2
catalogues contains 26399 and 1937 sources, respectively. Accurate
aperture corrections have been
obtained for each field on a few tens of bright and isolated stars.
The stellar crowding is quite low in the considered fields that sample external low-surface brightness regions of the galaxy. Even in F1 the average stellar density is as low as 0.08 stars/arcsec2 (considering all the detected stars). For the applications presented in this paper the effects of incompleteness are not an issue since we always compare subsets of stars that are homogeneous in magnitude and color (see Sect. 3.3 and 3.4 below). No attempt is made to determine the true Luminosity Functions of the observed sequences. The TRGB level, the main target of the present study, occurs more than 3 magnitudes above the limiting magnitude, a range in which the considered sample is likely ~ 100% complete. Since the photometric uncertainties of individual stars (in each passband) are empirically estimated as the standard deviations of 3 to 8 independent measures of the magnitudes there is no need for artificial star experiments to characterize photometric errors.