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Astron. Astrophys. 339, 61-69 (1998) 2. Observations and data reductions
The data were collected on October 20, 1997 at the ESO
Danish 1.54m telescope equipped with DFOSC. The camera employed a
Table 1 lists the complete log of the observations. The
weather conditions were good during the night, which was stable and
photometric, and the seeing was Table 1. Journal of the Pal 12 observations for Oct 20, 1995 The image processing was carried out within the IRAF environment. First, a map of the bad features of the chip was created and they were removed from the raw images. Then, the bias stability was checked by comparing frames taken at different times during the entire run, and no significant discrepancies were found. A 0.4 % spatial gradient was found along the x direction, thus a master bias image was created by taking the median of all the bias images. This master bias image was subtracted from all the remaining frames. Sky flats were used to create master flat fields as medians of the single frames. In order to avoid the fall of quantum efficiency (QE) all around
the border of the Loral CCD, we cut our images outside the limit where
the QE was 90% of the central value. From an inspection of the flats
this limit imposed an effective area of
Stellar photometry was performed using DAOPHOT , ALLSTAR (Stetson, 1987), and ALLFRAME , according to a standard procedure (see Paper I). Observations of Landolt's (1992) standard stars were used to
calibrate the photometry. In addition, the shutter delay time was
measured with a sequence of images taken with increasing exposure
times. A value of where The normalized instrumental magnitudes were then compared to the Landolt's (1992) values, and the following relations were found: where the uncertainties represent the 90% confidence ranges of the fit for one interesting parameter. The standard deviations of the residuals are 0.013 mag in V and 0.022 mag in I, respectively. In order to transform the PSF magnitudes into aperture magnitudes
we assumed that Our photometric catalogs are compared with those of Harris &
Canterna 1980 (HC80), GO88, S89 and DA90 in Fig. 2. Noticeable
differences are found for the V band, where a
We tried to sort out the possible reason for the observed
discrepancies in the V band, while no significant differences
are found for the I band. From a comparison with existing
photometry of the Fornax dwarf galaxy, SH98 conclude that their
V band calibration is consistent with the previous works. A
possible source of uncertainty could be a problem with the V
exposure times: however, the (small) shutter delay has been included
in Eq. 1. Moreover, when the individual zero points of the 7
available V frames are compared, no differences larger than
0.01 mag are found, which furtherly confirms that there is no
shutter delay problem. In principle, thin cirrus could have been
present at the beginning of the night, although it should have blocked
a remarkably constant percentage of light during the
In any case, the global zero-point difference in V between our Pal 12 photometry and that of the previous works will not affect our conclusions on the relative age of this cluster. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: September 30, 1998 ![]() |