[daip] X-Y mounts in FITS-IDI (and maybe AIPS too)

[Walter Brisken]

Hi DAIP,

Adam Deller has been digging up quite some issues with X-Y mounted VLBI 
antennas within the correlator.  We have noticed something in FITS-IDI 
that likely couples to AIPS as well.  In AIPS memo 114, the reviced 
FITS-IDI specification, the mount type can be Alt-Az, Equatorial, 
Orbiting, Nasmyth (R and L variants) and X-Y.  Unfortunatly X-Y mounts 
come in a continuum of flavors that depend on the orientation of the lower 
axis, with the East-West and North-South being the only actual variants I 
know about.  Since FITS-IDI cannot convey whether the X-Y mount is E-W or 
N-S (or something else) it has no hope of getting parallactic angle 
calculations correct in general.

A proposed solution might be to add two more mount types (XYNS and XYEW) 
to the FITS-IDI list with new identifying numbers (possibly 7 and 8), 
leaving the ambiguous XY as currently defined at a value of 4.

To be useful, AIPS will have to make use of this information.  Do any of 
you know which variant of X-Y is currently assumed in the parallactic 
angle calculations?

-Walter