2 Data

These data were obtained as part of the large HST GO cycle 9 parallel proposal aimed at the study of cosmic shear (see Pirzkal et al. 2001 and http://www.stecf.org/projects/shear for more details on the STIS data characteristics). The field considered here was, coincidentally, the first one observed as part of that program. The associated image is composed of 18 individual 400 s exposures with a field of view of $50'' \times 50''$, taken in the CLEAR filter mode. The exposures were individually cleaned for hot pixels and cosmic rays using the Eye algorithm in SExtractor (Bertin & Arnouts 1996), which creates masks of the pixels that are not used during the combination, eliminating 99% of the spurious pixels. The shifts between the different exposures were computed using the method described in Pirzkal et al. (2001), which can align these images to an accuracy of 1/10 of a pixel. Finally, the individual exposures were drizzled to a pixel size of 0.025'' (Fruchter & Hook 2002) and combined by median averaging with the IRAF Imcombine procedure, achieving a total exposure time of 7200 s. The use of this reduction procedure, basically similar to the one described in Pirzkal et al. (2001), is known to preserve the original PSF shape and size (Hämmerle et al. 2002) which is extremely important for an accurate analysis of the shear. The coordinates of the field are RA: $01^{\rm h} 35' 06.7''$and Dec: $-41^{\rm o} 21' 20.18''$ (J2000), about 7' north of the local Seyfert galaxy NGC 625, which was the primary target observed with the WFPC2. Throughout this paper we will refer to this field as the Slens1 field. The coadded image is available in fits format at http://www.stecf.org/projects/shear/slens/slens. fits. The SExtractor parameter file used is also available there in order to enable others to reproduce the SExtractor catalog.