) coordinates in arcsec, given relatively to a center
defined at 
- located
between both largest/brightest central galaxies, NGC 4874 and NGC 4889
(see Fig. 1 (click here)).
For spectroscopic purposes, the frames were taken with a large
superposition in Y (of the order of 40%, while almost negligible in
X), which caused double observation of many objects.
We carefully eliminated these double entries, both for stars and galaxies.
In order to estimate the final precision of our positions, we compared our
star catalogue with the Guide Star Catalogue (GSC) of the Hubble Space
Telescope limited to magnitude m<15.5, which has a 0.3 arcsec
accuracy. The positions of the same objects in both catalogues coincided,
after final tuning, within less than 3 arcsec (median result for 17 stars
identified in the field of our observations).