around the positions of NGC 1750 and NGC 1758.
CCD stellar positions from different CCD frames were assembled into
a pseudo-plate through a method that ensured astrometric uniformity.
This allowed us to use the whole set of CCD positions as one more plate for
astrometric purposes. The interaction of CCD
and photographic information, combined with a careful treatment of the
data, allowed the complete removal of magnitude effects from the
astrometric positions and proper motions. A new implementation of the
iterative central overlap algorithm was devised for this study.
One film was included among our plate material. The tests performed indicated that this detector has a good photometric quality, but it is not useful for astrometry. This casts doubt on the reliability of film-based detectors for wide-field astrometry, although further tests on other film plates are needed in order to clarify this point.
In spite of the inhomogeneity of the plate material, a detailed
consideration of the plate-to-plate transformation models allowed us to
achieve a good proper motion precision. The resulting relative proper
motions have a typical standard error of 
1 mas yr-1 for
well-measured stars in the magnitude interval R=9-14 mag.
Positions and proper motions were transformed into the FK5 and ICRS reference systems through comparison with PPM, Tycho and Hipparcos catalogues.
Photographic Johnson-Cousins BVR photometry was obtained for
39762 stars by calibrating four filtered modern epoch Schmidt plates,
using CCD photometry as reference. The resulting photometry has a mean
precision of 0.08 mag in B and V, and 0.04 mag in R. The
catalogue limiting magnitudes are 
and
mag.
The analysis of the data from a spatial point of view allowed us to distinguish without ambiguity two real open clusters in the field. The objects are sparsely populated and show a strong spatial overlap. For their spatial positions and distributions, and for their apparent main sequences, the clusters can be identified with NGC 1750 and NGC 1758. The entry number 1746 of Dreyer's NGC catalogue most probably does not correspond to any real object in this field.
Further analysis of the data presented in this paper will allow to
assign membership probabilities and to determine the main astrophysical
parameters of both clusters.
We express our thanks to W. Van Altena and to all members of
the Astrometry Group of the Department of Astronomy at Yale University
for their help for the treatment of the data. Special thanks to I. Platais and
T. Girard for their sharp comments and advises for dealing
with magnitude equations.
The authors are very thankful to C. Alard for
providing us with his original C codes, to N. Robichon for his
comments and suggestions, and to R. Chesnel for his patience and
efficiency at plate scannings.
We are also very grateful to E. Schilbach and S. Röser for their
loan of Tautenburg Schmidt and Bruce Astrograph
plates for this work. Thanks also to S.E. Urban for providing us with
the original individual measurements of Astrographic Catalog plates
from Paris zone. Thanks a lot to K. Birkle, J. Zamorano and Ó. Alonso,
who kindly exposed and developed for us the CAHA Schmidt plates
for this work.
D.G.-E. is indebted to the staffs of
the Centre d'Analyse des Images and the
Astronomy Department of Yale University
for their hospitality and unvaluable help,
without which this work would not have been possible.
This work has been supported by CYCIT under contract ESP95-0180 and
DGPC. D.G.-E. acknowledges partial support by the Spanish Ministerio de
Educación y Ciencia (Ref. AP92 30526274) and by the Comissió
Interdepartamental per a la Recerca i la Tecnologia (Generalitat de
Catalunya, Ref. 1996BEAI200346).
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