Crab Pulsar (calibration): as an EA calibration target, the PA
doesn't really matter.  The continuous clocking mode observation
would be best done if the bright portion of the nebula were
kept away from the column containing the pulsar, so roll
angles of -95 to -60 (E of N or 265-310 W of N) are preferred,
as well as the values with 180 deg added: 85-120 deg (W of N).