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Astron. Astrophys. 332, 93-101 (1998)

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2. Observations and data reduction

The central region of M 3 was observed with the WFPC2 on 25 April 1995 (GO-proposal #5496; P.I: FFP). The cluster core was imaged on the [FORMULA] field of view of the PC1 which has a linear scale of [FORMULA] per pixel. Four sets of exposures of 300, 800, 100 and 140 sec were sequentially taken in U (F336W), V (F555W), I (F814W) and UV (F255W) respectively. Exposures were performed between the date 1995.115:10:17:39 and 1995.115:16:50:39. Here we discuss essentially the PC1 images which properly sampled the cluster core.

The raw data images were first processed by the standard HST/WFPC2 procedure as far as the data handling, flat-fielding and pixel corrections are concerned. For every filter, the images were median filtered to remove the cosmic rays, and the resulting image has been used to search for all the objects present in each field. The PSF fitting photometry was then performed on each individual frame separately using the HST-updated version of ROMAFOT (Buonanno et al. 1979, 1983). The complete description of the available material, the reductions, and other results will be presented elsewhere (Ferraro et al. 1997b and in preparation). The instrumental magnitudes have been transformed into the UVI-Johnson system following the prescription given by Holtzman et al. (1995); the F255W magnitudes are in the STMAG system. To avoid low count statistics we have set a limiting magnitude cut-off at U = 20.5 beyond which the photometry becomes less reliable and we only consider hereafter objects which fulfilled the following requirements:

i)- the star image in the F255W filter consists of, at least, two pixels at [FORMULA] DN) above the sky background;

ii)- the star image is present on all four images taken with the same frame. The individual images of the stars of interest were manually checked for the occurrence of multi-pixel cosmic rays events;

iii)- the candidate star has detectable counterparts at all wavelengths (2550, 3360, 5550 and 8140Å);

Stars located within 2 arcsec from the frame edges were rejected as well as those heavily affected by known hot pixels or bad columns. Fig. 1 displays the image of the cluster core through the F255W filter.

[FIGURE] Fig. 1. The ultraviolet (F255W) image of the core of M 3 obtained with the HST /WFPC2 PC1 camera is displayed. The image is the median of four 300 sec exposures. For the sake of visualization, a filtering by a gaussian (3x3 pix., [FORMULA] pix.) has been applied. The intensity scale is linear. Reference stars are labelled as AC (from Aurière & Cordoni, 1983). AC29 (#7521) is a bright horizontal branch star, #7785 is a blue straggler. Small open squares labelled "a" or "b" refer to the expected positions of the radio source discussed below. The cluster centre as defined by von Zeipel (1908) (labelled C_vz; filled square) is at [FORMULA] East and [FORMULA] north of star AC999. The orientation and the approximate plate scale are indicated.

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998