2 The data

The data consist of two sets of observations, one from the HST archive and observed in 1994 (GO 5436) and the other our new data for the same field (GO 8696, PI Feltzing). We detail the number of exposures in each filter and epoch as well as integration times in Table 2. The first data set was taken on the 27 February 1994 and the new one (GO 8696) on the 1 April 2000. This gives a time span of 6.093 years between the two set of observations.

The first epoch of observations provided a long and a short set of exposures in F555W and F814W with the cluster centre on WF3. Our new observations were observed with the same WFPC2 orientation, but with four times as long total exposure time in F555W. The increased exposure times enable us to reliably detect all the possible stellar sources in the first epoch data.

A new pair of F814W images were also taken in order to improve the accuracy of the F814W magnitudes. Finally, one short exposure in F555W was also obtained in order to find the proper motions of the brightest stars in the field which are saturated in the longest exposures.

The images for each filter/epoch combination were combined using the DRIZZLE and CRREJ tasks in the STSDAS environment within IRAF. Drizzling removes the WFPC2 geometric distortion from the images.

For the first epoch data we combined the F555W and F814W images into one single image to obtain as deep an image as possible for object centering.