[daip] AIPS question

[Eric Greisen]

Bill Cotton wrote:
> Eric,
> 
>    One of my users was having a problem using the combination
> Imager+CALIB+CLCAL (Imager in Obit, CALIB,CLCAL) in AIPS.  When he did
> this on a calibrator and reimaged it, it moved one pixel to the right.
> I've confirmed this behavior and that Imager+Calib+CLCal (all in Obit)
> does the right thing.  Also, the position in the CC table for the
> calibrator was (0,0) and the reference position in the header correct.
> I claim this should always give the desired results.
>    I suspected that the difference in which pixel is the reference was
> causing trouble, (nx/2,ny/2+1) in AIPS, (nx/2+1,ny/2+1) in Obit.
> I've traced the problem to the following segment in GRDAT.FOR:
> C                                       Get reference pixel offsets from
> C                                       tangent point
>       IF ((KLNBLK(KIITY).EQ.1) .OR. (KLNBLK(KIITY).EQ.2)) THEN
>          XPOFF(JFIELD) = CLNR(KRXPO)
>          YPOFF(JFIELD) = CLNR(KRYPO)
>       ELSE
>          XPOFF(JFIELD) = (CLNR(KRCRP+IRAOFF) - KLNBLK(KINAX+IRAOFF)/2)
>      *      * CLNR(KRCIC+IRAOFF)
>          YPOFF(JFIELD) = (CLNR(KRCRP+IDECOF) - KLNBLK(KINAX+IDECOF)/2 -
>      *      1) * CLNR(KRCIC+IDECOF)
>          END IF
> What, exactly, is this doing?

My memory is a bit hazy - but one needs the phase shift parameters in 
order to subtract the components from the correct location.  All images 
made bu AIPS will have KIITY set to 1 (do3d false) or 2 (do3d true) so 
that the shift - XPOFF and YPOFF used all over the code that you 
initially wrote - will be correctly known.  It is "hidden" in an aips 
history card which FITTP writes and IMLOD reads.  The "center" of the 
image comes in to some of the handling of the CC files for reasons I 
could probably figure out if I took it to be important enough to spend a 
couple of days.  The image creation routines set up

       IF (DO3DIM) THEN
          CATBLK(KIITY) = 2
          CATR(KRXPO) = 0.0
          CATR(KRYPO) = 0.0
       ELSE
          CATBLK(KIITY) = 1
          IF (IFIELD.LT.1) THEN
             CATR(KRXPO) = 0.0
             CATR(KRXPO) = 0.0
          ELSE
             CATR(KRXPO) = (CATR(KRCRP) - ICNTRX(IFIELD)) * CATR(KRCIC)
             CATR(KRYPO) = (CATR(KRCRP+1) - ICNTRY(IFIELD)) *
      *         CATR(KRCIC+1)
             END IF
          END IF

I can see where confusion will come when the Clean image has an odd (in 
AIPS) ref pixel but no shift and no evidence in the header to cover this 
information.

Looks like another place where the difficulty of popping back and forth 
between packages is further revealed.  XPOFF is all over QCLEAN from the 
early 1990's.  I think I hit trouble trying to use subimages for models 
and that may be when I started to record these data for recovery later.

Eric