2. The data and the data reduction
An image of SL 538 and NGC 2006 is shown in Fig. 1. To the south-east of the cluster pair a third star cluster is located: KMHK 1019.
The data have been obtained on 15 December 1992 with EFOSC 2 at the ESO/MPI 2.2 m telescope at La Silla. A coated Thomson THX31156 chip (ESO #19) was used with a pixel scale of resulting in a field of view of . All data were obtained with the standard Bessell B, V, R, Gunn i and filters used at the 2.2 m telescope (see Table 1 for an observing log).
After standard image reduction with MIDAS, profile fitting photometry was carried out with DAOPHOT II (Stetson 1991) running under MIDAS.
The photometry was transformed using the Landolt standard fields around PG 0218+029, Rubin 149, and SA 98 (Landolt 1992) observed in the same night.
We applied the following transformation relations:
where X is the mean airmass during observation, capital letters represent standard magnitudes and colours, and lower-case letters denote instrumental magnitudes after normalizing to an exposure time of 1 sec. The resulting colour terms , zero points and atmospheric extinction coefficients are:
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998