To set the position angle of a prime focus receiver box and to track the parallactic angle change, the following options exists under obsinp (FSwitch, PSwitch):

The default is None and StartPosAng = 0.0.

Please note, that in some rare cases (e.g., multi-feed OTF mapping) it is not necessarily the correct approach to simply correct for the parallactic angle. In example if one measures an OTF using the "descriptive" projection system, one would rather need to correct the feed angle in a way such that the beam pattern is always fixed with respect to the motion of the telescope position. This is currently not implemented, hence, for these rare cases we recommend to use the "absolute" projection.