The observations of Palomar 13 were made using the 2 m RCC reflector of
the Bulgarian National Astronomical Observatory.
The images in Gunn system - (g, r) filters - were taken with the
Focal Reduser of the Max Plank Institute for Aeronomy (Jockers et al.
1992). An EEV 
type P8603/B was used as a
detector. The telescope/reducer configuration and the CCD's square
pixels provide an image scale of 0.8 arcsec/px for an
unvignetted field of about 
at an effective focal
ratio f/2.86.
The frames in B, V system were taken with the SBIG Model ST-6
camera. The detector is
, the scale is 
per pixel
and the image size is about 
. This camera was kindly
granted by the EAS/ESO support of astronomy in the Central/Eastern Europe
countries.
Table 1: The journal of observations
During the nights of observations the seeing was stable with measured stellar PSF on the raw frames about 0.8 - 1.2 FWHM arcsec.
A log of the observations is given in Table 1 (click here). The airmasses listed there are the mean airmasses over the duration of each exposure.
The preliminary reductions of the CCD frames including bias subtraction and flat fielding were carried out using the standard MIDAS (ver.92) data reduction package. No cosmic ray cleaning was done.
The photometry of the stars on the frames was performed by means of the point-spread function (PSF) fitting package DAOPHOT available in MIDAS (Stetson 1987). This package is widely used and there are numerous references describing the application of the package in details in addition to the description provided by Stetson. We followed closely the steps described in Smith et al. (1986) to produce a list of objects with PSF-derived magnitudes. The magnitudes were then corrected on the basis of a 10 pixels radius aperture. To determine the value of this correction we selected a range of 10 to 15 bright, relatively isolated and well-fitted stars per frame and carried out aperture photometry on the selected stars. The mean difference between the magnitudes derived by the PSF fitting and the magnitudes derived by the 10 pixels radius aperture for the selected stars was subtracted from the PSF derived magnitudes for all objects measured on the frame. We then rejected:
.
Table 2: Summary of calibration observations
The instrumental values were corrected for extinction and transformed
to the standard B, V and g, r systems using a technique similar to that
described by Christian & Heasley (1986) - the instrumental
magnitudes m were converted to the standard scale M by means of the
equation:
where a is a constant, X is the airmass and (B-V), res. (g - r) is the
color of the object. The coefficients (Table 2 (click here)) were determined
by reducing the frames of the standard fields in NGC 7006 (Christian et
al. 1985) and the three standard stars selected from the list of
Thuan & Gunn (1976), using an aperture photometry algorithm. The
rms. deviations of the reduced CCD photometry from the standard values are
0.016 in V, 0.02 in B, 0.023 in g and 0.017 in r over the range of
standards. A certain inaccuracy of the zero point in the g, r system is
possible due to the insufficient number of the used standards. This inaccuracy
was however estimated to be within the standard photometry error and thus could
not seriously affect the results of our study.
Tables 6 and 7 (available in electronic form) list our final photometric
results for 80 stars in the central field of Palomar 13 in the B, V system and
175 stars in the g, r system.
The star identifications in these tables are as follows:
identification number; x, y - the (
) positions give the locations of the
stars in pixel terms.
Columns g, g - r, V and B-V are the weight averaged
magnitudes and colors taken from all available data.
Table 3: The mean standard deviations
Table 3 (click here) lists the derived mean standard deviations for successive intervals of one magnitude in the g, r, V and B frames.