The observations were performed with the Danish 1.5 m telescope at
ESO La Silla during 2 nights on November 2 and 3, 1994 (the third night was
cloudy, and this accounts for the missing fields in Fig. 4 (click here)).
A sketch of the observed fields is displayed in Fig. 4 (click here).
Field 1
was centered on the coordinates: 
,
(2000.0).
There was almost no overlap between the
various fields (only a few arcseconds). The Johnson
V and R filters were used. Exposure times were 10 mn for all fields; 1 mn
exposures were also taken for a number of fields with bright objects in
order to avoid saturation. The detector was CCD #28 with 10242 pixels of
, giving a sampling on the sky of 0.377''/pixel, and a size of
for each field.
The seeing was poor the first night: 
for fields 1
and 2, 
for
field 3 (in which consequently the number of galaxies detected is much
smaller), and good the second night: 
. On the
other hand, the photometric quality of the first night was better than that
of the second one. However, the observation of many standard stars per night
made a correct photometric calibration possible even for the second night as
indicated by a comparison with an external magnitude list: the photometric
catalogues from the six fields have the same behaviour for both nights
(see e.g. Fig. 8 (click here)).
Figure 4: Distribution of the fields observed with CCD imaging. The size
of each field is . Positions are drawn relatively to
the centre with equatorial coordinates
and
Corrections for bias and flat-field were performed in the usual way with the IRAF software. Only flat fields obtained on the sky at twilight and dawn were used; dome flat fields were discarded because they showed too much structure.
Each field was reduced separately. The photometric calibration took into account the exposure time, the time at which the exposure had been made, the color index (V-R), the airmass, and a second order term including both the color index and airmass. The photometric characteristics of both nights were estimated separately.
Objects were automatically detected using the task
DAOPHOT/DAOFIND. This task performs a convolution with a gaussian
having previously chosen characteristics, taking into account the
seeing in each frame (FWHM of the star-like profiles in the image) as
well as the CCD readout noise and gain. Objects are identified as the
peaks of the convolved image which are higher than a given threshold
above the local sky background (chosen as approximately equal to
of the image mean sky level). A list of detected objects
is thus produced and interactively corrected on the displayed image so
as to discard spurious objects, add undetected ones (usually close to
the CCD edges) and dispose of false detections caused by the events
flagged in the previous section. Since exposure times were the same in
V and R, the number of objects detected in the R band is of course much
larger.
We used the package developed by O. Le Fèvre (Le Fèvre et al.
1986) to obtain for each field a catalogue with the 
galaxy positions,
isophotal radii, ellipticities, major axis, position angles, and V and R
magnitudes within the 26.5 isophote.
Star-galaxy separation was performed based on a compactness parameter q
determined by Le Fèvre et al. (1986, see also Slezak et al. 1988), as
described in detail e.g. by Lobo et al. (1997). We chose q=1.45 as the best
separation limit between galaxies and stars; very bright stars were
classified as galaxies with this criterion and had to be eliminated manually.
After eliminating repeated detections of a few objects, we obtained a total
number of 805 galaxies detected in R, among which 381 are detected in V.
The errors on these CCD magnitudes are in all cases smaller than 0.2 magnitude,
and their rms accuracy is about 0.1 magnitude; these rather large values are
due to the bad seeing during the first night and to pretty poor photometric
conditions during the second night.
Positions of the galaxies detected in the R band relative to the
centre defined above are displayed in Fig. 5 (click here). Notice the smaller
number of galaxies detected in field 3 due to a sudden worsening of the seeing
during the exposure on this field.
The astrometry of this CCD catalogue is accurate to 
arcseconds as
verified from the average mutual angular distance between CCD and MAMA
equatorial coordinates for 174 galaxies included in both catalogues.
Figure 5: Positions of the galaxies detected in the R band relative to the
centre defined as the centre of the diffuse X-ray emission (see text)
Figure 6: Histogram of all the R magnitudes of the galaxies in the
CCD catalogue
The histogram of the R magnitudes in the CCD catalogue is displayed in Fig. 6 (click here). It will be discussed in detail in Paper III (Durret et al., in preparation). The turnover value of this histogram is located between R=22 and R=23, suggesting that our catalogue is roughly complete up to R=22.
The (V-R) colours are plotted as a function of R for the 381 galaxies detected in the V band in our CCD catalogue (Fig. 7 (click here)). Unfortunately, since the observed CCD field is small, there are only 50 of these galaxies with measured redshifts, and therefore it is not possible to derive a colour-magnitude relation from which to establish a membership criterion for the cluster.
Figure 7: (V-R) colour as a function of R for the 381 galaxies detected
in the V band in our CCD catalogue. The 50 galaxies indicated with a square are
those with redshifts in the interval 
assumed to belong to
85
576 stars were also measured on the CCD images and used to calculate
calibration relations between our photographic plate 
magnitudes and
our V and R CCD magnitudes.
For stars:
For galaxies where only R is detected:
For galaxies where both V and R are detected:
The observed R band CCD magnitude 
as a function of the R
magnitude calculated from the photographic magnitude is plotted
in Fig. 8 (click here) for galaxies, showing the quality of the correlation
for the six different CCD fields,
especially for objects brighter than R=19.
All the CCD fields appear to behave identically.
Figure 8: Observed R band CCD magnitude as a function of the
R magnitude calculated from the photographic magnitude. The six different
symbols correspond to the six CCD fields described above
The CCD photometric data for the galaxies in the field of 85 are given
in Table 2. This Table includes for each object the following information:
running number; equatorial coordinates (equinox 2000.0); isophotal radius;
ellipticity; position angle of the major axis; V and R magnitudes;
X and
Y positions in arcsecond relative to the centre assumed to have coordinates
and
(equinox 2000.0) (this centre was chosen to
coincide with that of the diffuse X-ray gas component as defined by
Pislar
et al. (1997)).