The optical, infrared, and narrow band images were re-sampled and registered by
common stars in the fields. The relative offsets between these images
is less than 0.".
To register these images with the radio images requires absolute as well
as the relative
astrometry. Astrometric measurements were made with the NRAO Charlottesville
measuring machine. The positions of 10 or more faint stars on the
Palomar Sky Survey
plates were measured relative to 10 or more nearby SAO stars. The
positions of the
POSS faint stars were then measured on the CCD images, and the
absolute coordinates,
plate scale, and rotation were solved for assuming a tangential
projection. The random
astrometric errors were typically less than 0.3" and the absolute
errors should be less
than 0.5". Given a radio positional accuracy of 0.1 the overall
relative positioning of
the optical images to the radio images is 0.5"(). However,
some of the objects had
special astrometric problems or issues and these are discussed below
in the sections on the individual objects.