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4. Data reduction

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
Table 2: Transformation coefficients



Magnitude range V B-V V-R V-I
tex2html_wrap_inline1397 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
Table 3: Internal photometric errors as a function of


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 ( or from the Armagh Observatory World Wide Web server ( or by anonymous ftp upon request.

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