3C 273 (calibration): again, as a calibration target, there is
no requirement for a particular roll angle.  However, since there
is a jet, it would be useful to keep the dispersion direction
from aligning with the jet, which would smear features.  The
position angle of the jet is 135deg W of N, or 225 deg E of
N.  Thus, roll angles of 135 deg W of N and 45 deg E of N
are desired so that the dispersion axis is perpendicular to
the jet axis.  The desired range is +/- 60 deg.  The visibility
plots show that it can be observed with rolls from 50 deg
to 270 deg (W of N), so the first range spans the allowed
range nicely.