An investigation into the selection effects of optical quasar surveys is
described. This is accomplished via the accumlation and comparison of
optical quasar surveys o f d ifferen t types, which are then used to
generate a su rvey free from selection effects.
A visual search and an automatic computerised search for quasars
using U.K. Schmidt plates were already available in a single high
galactic latitude field of 23 degrees.
An automatic search of a deep C.F.H.T. grens plate over 1 degree^ in
the centre of the field was undertaken. Slit spectroscopic
observations were made of 26 candidates from this survey. These data
allow conclusions to be drawn regardin g the grens as a survey
An automatic photometric su rvey was also undertaken, using direct
plates in the five wavebands U, J, V, R, and I from the U.K. Schmidt
telescope. CCD frames were obtained in each of the wavebands from
various sources, to allow calibration o f data from the Schmidt plates.
This has allowed the construction o f a conventional ultraviolet excess
sample, as well as a less conventional colour space density search
Locating the various low dispersion spectroscopic su rvey candidates
amongst the photometric su rvey data has brought to light various
aspects o f these surveys: in particular, the eye discovers a smaller
surface density of candidates at most magnitudes. Secondly, the
automatic prism plate search has significant progressive object losses
b righ ter than the plate limit, which are subject to complex selection
effects. Large numbers of galactic stars are mistaken for quasars
because of peculiarities in their spectra seen at low dispersion. Also
prism dispersion variations force a su rvey limit brighter than the
plate limit. Colour synthesis from the prism plate works well at
brigh ter magnitudes, unless spectra are saturated. The method is also
subject to limitations due to poor spectrum signal to noise at fainter
Spectroscopy indicates that the automatic grens survey is complete,
except for overlapped spectra. This is in part due to the linear
dispersion of the transmission grating. The present survey is however
subject to considerable contamination:- the reasons for this are
examined and discussed. Conclusions drawn from examination of the
grens sample colours are limited because the grens su rvey goes to a
much fainter magnitude, over a limited area.
The photometric survey readily identifies objects which have peculiar
colours, but a detailed examination of the properties of these objects
must await further work.
Finally, a synthetic survey has been constructed, using all of the
information available for each object. Redshift estimates were possible
for about 30% of the 130 candidates - the higher redshift objects.
Examination of the survey indicates that it is largely free from
selection effects below a redshift of z = 3.4.