Astron. Astrophys. 326, 528-536 (1997)

2. Observations and data reduction

NGC 5102 was observed on 1995 May 26, using the 512 512 pixel imaging mode (pixel size 0:0001435 0:0001435, field of view 7:003 7:003) of the f/96 camera of the COSTAR-corrected FOC (Jedrzejewski et al. 1994). Several exposures were taken using the broad-band filters F175W and F342W, centered at wavelengths of 1730 and 3410 Å and of FWHM 716 and 702 Å, respectively. The ultraviolet filter suffers from non-negligible red leak (transmission of a few 10^-4 in the visible). In anticipation of saturation effects in the F342W image, one exposure (769 s) was taken with 4 mag of neutral density filter (F342W+F4ND). All images were taken in FINE LOCK mode. The data were automatically processed and calibrated by the Routine Science Data Processing system. The images obtained with the same filter were found to be aligned to within a fraction of pixel and therefore were co-added. This left us with total exposure times of 4075 s for the F175W image and 1789 s for the F342W image.

Fig. 1. Greyscale representation of the F342W (left) and F175W (right) summed images of the central region of NGC 5102. The images are displayed on a logarithmic scale. The field of view in each image is 7:003 7:003, and North is inclined at 45o to the vertical, with East pointing to the left. This makes the (optical) major axis (position angle of 48o) almost vertical. The regular pattern of dots is due to the reseau marks etched onto the FOC photocathode, which are used for geometric registration of the images. The central regions of the F342W image are heavily saturated, so the intensity distribution appears to have a "hole" in the center.

The final co-added F175W and F342W images are displayed in Fig. 1 and show a sizable number of resolved point-like sources over the diffuse bulge light. The FWHM of these sources ( 0:0003) as well as the photometry discussed in the next section allow their identification as stars in the galaxy NGC 5102. These stars contrast better in the outer parts where the underlying galaxy light is fainter. In the corners of the F175W image the count rate is of the order of 0.9 10^-3 count/pixel/s and may still include a small contribution from the galaxy light as shown by comparison with the typical value of 0.7 10^-3 count/pixel/s reported in the instrument handbook (Nota et al. 1995) and actually found at the edge of the exposure with the neutral density filter (where the galaxy contribution is negligible). In the F342W image the contribution from the unresolved galaxy light remains dominant over the dark count rate at the edge of the field. Both images reveal several dust patches.

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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