A photographic plate of our selected field located near NGC 450 was obtained in
1979 with the 48-inch Schmidt telescope of the Mount Palomar Observatory. The plate covers the
unvignetted part of the Schmidt telescope field and corresponds to a 
square. It was
first exposed through the B band during 7 minutes, then slightly offset and exposed through the U band
during 60 minutes. We thus got a U/B dual-exposure plate. The exposure times were calibrated in such
a way that a faint object with 
-0.4 gives rise to two images of similar brightness.
Bluer objects have a U image brighter than the B one and the selection of quasar candidates on the basis
of a U/B excess can easily be done by detecting these comparatively bright U images. The advantage
of using a dual U/B plate is that the selection does not suffer from the inhomogeneities in the response
of the emulsion since both the U and B images are very close together: the 
index threshold is
therefore not expected to strongly vary across the plate. Table 1 (click here) presents further information on
that plate on which
our survey is based.
Table 1: Characteristics of photographic plate PS26671
The plate has been visually inspected on an XY table equipped with a binocular (magnification factor of
30). In order to improve the objectivity of the survey, the plate has been systematically studied by two
different persons without any preliminary concertation. Each person slowly scanned the plate twice and
rescanned it a third time in order to attribute to each candidate a complex quality factor giving information
on the magnitude of its 
excess and on its brightness. Subsequently, the two lists were merged: in
general the agreement between both lists was rather good but certainly not perfect. The candidates from the
two lists have been arranged into three classes according to the associated quality factors and to the presence
of the object in one or two of the lists.
We came out with 140 candidates, 95 primary and 45 secondary ones. A third class exists (tertiary candidates) which contains objects that cannot be considered as good candidates on the sole basis of the present survey. Preliminary coordinates of the 140 candidates have been measured on a modified Zeiss blink comparator located in Liège and on the Optronics measuring machine of the European Southern Observatory (ESO) at Garching bei München. They have been converted to equatorial coordinates for subsequent observations.