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

[Eric Greisen]

Walter Brisken wrote:
> 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
> 
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The code for PARANG is below

C                                       Parallactic angle
             PANGLE(LOOPAN) = ATAN2 (COS (ARLAT) *
      *         SIN (HRANG), (SIN (ARLAT) * COS (DECAPP) -
      *         COS (ARLAT) * SIN (DECAPP) * COS(HRANG)))
C                                       Compute only for X-Y mount
C                                       (Hobart only?)
C                                       Co-parallacti
             IF (MNTYP(LOOPAN).EQ.3) THEN
                if (first) then
                   WRITE (MSGTXT,1000)
      *               'Using CO-PARALACTIC ANGLES for ', LOOPAN
                   CALL MSGWRT (2)
                   FIRST = .FALSE.
                   END IF
                PANGLE(LOOPAN) =  ATAN2 (-COS(HRANG),
      *            -SIN(HRANG) * SIN(DECAPP))
C                                       Nasmyth
             ELSE IF ((MNTYP(LOOPAN).EQ.4) .OR. (MNTYP(LOOPAN).EQ.5))
      *         THEN
                DPARM = TIME
                CALL SOUELV (LOOPAN, DPARM, HA, EL, AZ)
C                                       Right handed is FA=PA+EL
C                                       Left handed is FA=PA-EL
                IF (MNTYP(LOOPAN).EQ.5) EL = -EL
                PANGLE(LOOPAN) = PANGLE(LOOPAN) + EL
                IF (FIRST) THEN
                   WRITE (MSGTXT,1000) 'Using Nasmyth formula for ',
      *               LOOPAN
                   CALL MSGWRT (2)
                   FIRST = .FALSE.
                   END IF