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2 CCD photometry

2.1 Observations

CCD photometry was obtained using the JKT on La Palma on the nights of the 20th and 21st October 1995. A 1024 $\times$ 1024 pixel Tek CCD was used at the Cassegrain focus, which gives a pixel scale corresponding to 0.33 arcsec on the sky. Observations were made through Johnson BV and Harris RIfilters. Exposure times for the cluster fields were typically 700:100:60:60 seconds respectively. The coordinates of the 21 5$\times$5 arcmin fields are listed in Table 1. Repeat observations of each field were sacrificed in favour of total spatial coverage because of the extremely long readout time of the data acquisition system used on the telescope at that time ($\sim$ 5 min). A range of photometric standards from Landolt (1992) were observed during the course of each night.


Table 1: Coordinates of the field centres

$\alpha$ (J2000.0) $\delta$
$\alpha$ (J2000.0) $\delta$

59:08:282:14:40 59:23:28
2:15:19 59:08:282:15:19 59:23:28
2:14:01 59:13:282:15:58 59:23:28
2:14:40 59:13:282:14:00 59:28:28
2:15:19 59:13:282:15:58 59:28:28
2:15:58 59:13:282:13:60 59:33:28
2:14:00 59:18:282:14:40 59:33:28
2:14:40 59:18:282:15:19 59:33:28
2:15:19 59:18:282:15:58 59:33:28
2:15:58 59:18:282:14:39 59:38:28
2:14:00 59:23:28   


2.2 Data reduction and calibration

The basic CCD reduction was carried out using IRAF's CCDRED package (Tody [1986]). The raw images were trimmed and overscan subtracted. Multiple dome flatfields were averaged, and the data flatfielded using the resultant images. Dome flats were divided by sky flats to check that the dome was uniformly illuminated. No correction for such an effect was found necessary. The reduction of the crowded fields was carried out using the DAOPHOT (Stetson [1987]; Stetson et al. [1990]) package in IRAF. Cluster stars were analysed by means of point spread function (PSF) fitting. The stellar images were well sampled, and a gaussian PSF was applied. Standard star observations were reduced by means of aperture photometry, using an aperture of 14 arcsec in diameter, i.e. the same size as the one used by Landolt (1992).

We were not able to measure the extinction reliably, so we used mean values obtained from Derek Jones (private communication). Zero points and colour terms were derived in the standard way by applying linear transformation equations. For each star, errors in magnitude and colour were calculated. These include the statistical errors in flux, read-out noise of the CCD chip, and the errors in zero points and colour terms. The error calculations were performed within the IRAF tasks for photometry and fitting transformation equations (phot, fitparams, invertfit). Table 2 lists mean photometric errors, i.e. average values for several magnitude bins.


Table 2: The mean photometric errors present in the Stock 2 photometry as a function of magnitude


0.01 0.01 0.01 0.02
$15<V\le17$ 0.01 0.01 0.01 0.01
$17<V\le18$ 0.01 0.02 0.02 0.02
$18<V\le19$ 0.02 0.03 0.03 0.03
$19<V\le20$ 0.04 0.07 0.06 0.04
V>20 0.07 0.11 0.09 0.06

Using the STARLINK package ASTROM, about 10 to 20 stars on each CCD frame were manually identified on Super COSMOS scans of a POSSII photographic plate, and a transformation performed from pixel to equatorial coordinates. The resulting positions of all stars on the CCD frames are accurate to $\sim0.1$ arcsec.

A complete list of all stars with coordinates and CCD photometry is available by anonymous ftp from the Centre de Données Stellaire, Strasbourg, or from the Armagh Observatory WWW server[*].

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