[daip] Coordinate system of antenna table in FITS file produced by FITTP

[Eric Greisen]

Bojan Nikolic writes:

 > As far as I understand the FITS-IDI document, the coordinate system
 > should be that such that the x-axis runs from centre-of-mass of the
 > Earth to the intersection of Equator and the Greenwich meridian. This
 > coordinate system should apply both to ARRAYX, ARRAYY, ARRAYZ keywords
 > and the STABXYZ column values.
 > 
 > Looking at output of AIPS, it seems to me that (ARRAYX, ARRAYY,
 > ARRAYZ) are indeed in this coordinate system, but that somehow for the
 > STABXYZ values the coordinate system x-axis is redefined to run from
 > centre of Earth to the intersection of *local* meridian and the
 > equator. 

Actually no.  The actual X, Y, Z coordinate of an antenna is the sum
of the ARRAYX and the BX and they are in the same coordinate system.
Note that the IDI document is for VLBI and in that case normally the
ARRAYC = 0 (center of earth) and the antenna locations carry the full
location information.  In the VLA case and perhaps others, the array
center is the physical center of the array and the BX, BY, BZ are wrt
to that.  For computation of projected baselines, the center does not
matter since two antenna locations are differenced.

I notice that FITLD, when reading the VLBA correlator data, reverses
the sign of By, i.e the handedness.  AIPS everywhere tests for VLB vs
VLA and assumes opposite handedness.  It is curious that the
correlator uses the handedness of the VLA.

Eric Greisen