The initial CCD reductions were performed using the CCDRED package
within IRAF (Tody 1986). This procedure involved the
replacement of bad pixels, subtraction of the mean overscan level,
correction for any 2-dimensional structure in the bias level, trimming
of the data section, subtraction of the preflash level and flat-fielding
the CCD frames. Further details of this process can be found in Massey
(1992).
The stars were subsequently photometered using tasks within the IRAF
package DAOPHOT (Davis 1994). Instrumental magnitudes for
the standard stars were determined using digital aperture photometry, where
for each night we have adopted an aperture radius to which all photometric
measurements for that night are referred. This is essentially the same
technique as used in standard photoelectric photometry. The choice of radius
is governed by two opposing effects. Using as large an aperture as possible
will include more starlight, however this will be accompanied by an
increased sky contribution, as well as bad pixels and cosmic ray events.
Although adopting a small aperture will provide the best signal-to-noise
ratio, the results will not be consistent for different CCD frames as the
effect of seeing and telescope focus will dominate the stellar profiles.
Throughout the run, 2-3 arcsec seeing was typically encountered which
corresponds to a stellar profile with a full-width-at-half-maximum (FWHM)
of 5-8 pixels. As some 200 standard star observations were obtained,
we investigated when the increase in starlight for larger apertures was
masked by the photometric errors. We found that this occurred for a radius
that was 6-7 times the FWHM of the stellar profile, and so a mean aperture
was determined for each night's data.
Transformation equations of the form
were adopted, where bvri are the instrumental magnitudes, BVRI the
standard magnitudes, X the airmass and b1 to i3 are the
transformation coefficients. The adopted zero points, colour terms and
mean extinctions for the entire run were as given in Table 2 (click here).
| Filter | Coefficient | |||
| 1 | 2 | 3 | 4 | |
| b | 4.513 | -0.046 | 0.28 | -0.032 |
| v | 4.449 | 0.018 | 0.15 | |
| r | 4.123 | 0.006 | 0.13 | |
| i | 4.908 | -0.049 | 0.11 | |
| Magnitude range | V | B-V | V-R | V-I |
 | 0.001 | 0.003 | 0.003 | 0.003 |
| 13.0-15.0 | 0.003 | 0.005 | 0.005 | 0.005 |
| 15.0-17.0 | 0.012 | 0.024 | 0.017 | 0.017 |
| 17.0-19.0 | 0.056 | 0.112 | 0.077 | 0.080 |
| V=19.0 | 0.104 | 0.197 | 0.140 | 0.145 |
Point-spread function photometry (PSF) was undertaken for the cluster
fields, with independent PSF's being calculated for each CCD frame which
were then used to derive instrumental magnitudes. An aperture correction
was then applied to account for the smaller fitting radius adopted in the
PSF photometry compared to the large digital apertures used for the
standard stars. IRAF estimates an internal error for these magnitudes
which is based on the fitting procedure and in Table 3 (click here) we give the mean of
these errors as a function of magnitude within each band-pass. Additionally,
independent measurements of stars that were observed in the overlap region
between adjacent fields were generally found to be consistent within
the formal errors quoted in Table 3 (click here).
Photometry of all stars observed in this region of sky can be obtained
electronically by ftp from the Centre de Données Stellaire, Strasbourg
(130.79.128.5) or from the Armagh Observatory World Wide Web server
(http://star.arm.ac.uk/) or by anonymous ftp
upon request.