To obtain precise astrometry for the detected objects in our CCD frames, the regions of the SERC-J photographic plates containing our survey were digitized at the MAMA ("Machine Automatique à Mesurer pour l'Astronomie'' at Paris Observatory) which is developed and operated by CNRS/INSU (Institut National des Sciences de l'Univers).
The astrometry is performed in two steps.
(1) By alignment of the images of the digitized plates onto our CCD frames
(using existing MIDAS commands applied to objects in the field), we are able to derive
the coordinate transformation equations from CCD to photographic plate. There is one
set of transformation equation for each CCD frame.
(2) The transformation equations from photographic plate to equatorial
coordinates are provided by the MAMA facility as FORTRAN programs. The
astrometric reduction uses the PPM catalogue
(Roeser & Bastian 1991).
(3) Combination
of the two transformations yields for each CCD frame the transformation equations
from pixels to equatorial coordinates.
We use a set of MIDAS procedures which were written with the goal to perform the
transformation routinely for the numerous fields of the programme
(Revenu
& de Lapparent 1992).
The overlapping regions of neighbouring CCD frames allow us to estimate the
internal astrometric consistency.
In Fig. 4 (click here), we plot the median position difference defined as
in bin
sizes of 1 mag.
The NTT observations are represented by dots and those from the 3.60 m by crosses.
The error bars are the 1 
dispersion measured in each magnitude bin.
For the NTT, the differences of median position vary from
for 
to 
for 
,
and from 
for 
to 
for
with the 3.60 m.
The better accuracy for the NTT fields is due to the better sampling
(smaller pixel size) which
improves the measurement of the central position of the objects:
the seeing disk is spread over 
pixels for NTT frames, whereas it is
spread over only 
pixels for 3.60 m frames.
Figure 4: Mean difference in arcsec in the astrometric position of
common objects in CCD overlaps as a function of R magnitude in bin sizes of 1 mag.
The data for the NTT and 3.60 m telescopes are represented by dots and crosses respectively.
The error bars show the 1 
rms dispersion