[daip] fshift

[Eric Greisen]

Katherine Blundell wrote:
> Hi Eric,
> 
> But somehow fshift is definitely deriving us different numbers…  there is no other shift in these particular data that should be added onto the RASHIFT and DECSHIFT other than what is required to move the peak to the nearest pixel centre (as per my email yesterday) - here is the imheader:
> 
>> getn 155
> AIPS 1: Got(1)   disk= 1  user= 301   type=MA   SS433_C0.ICL001.20
>> imh
> AIPS 1: Image=SS433_C0  (MA)         Filename=SS433_C0    .ICL001.  20
> AIPS 1: Telescope=VLBA               Receiver=VLBA
> AIPS 1: Observer=BB301A              User #=  301
> AIPS 1: Observ. date=05-NOV-2011     Map date=08-FEB-2013
> AIPS 1: Minimum=-2.34071817E-03      Maximum= 2.99269054E-02 JY/BEAM
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 1: RA---SIN  2048    19 11 49.567    1024.01      -0.001500    0.00
> AIPS 1: DEC--SIN  2048    04 58 57.873    1024.85       0.001500    0.00
> AIPS 1: FREQ         1   1.6664275E+09       1.00  1.6000000E+07    0.00
> AIPS 1: STOKES       1   1.0000000E+00       1.00  1.0000000E+00    0.00
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Coordinate equinox 2000.00
> AIPS 1: Map type=NORMAL              Number of iterations=     420
> AIPS 1: Conv size= 0.00973 X 0.00497   Position angle=  -2.40
> AIPS 1: Phase shifted in X      0.000  in Y      0.000
> AIPS 1: Rest freq      0.000         Vel type: OPTICAL wrt YOU
> AIPS 1: Alt ref. value  0.00000E+00  wrt pixel    0.00
> AIPS 1: Maximum version number of extension files of type CC is   1
> AIPS 1: Maximum version number of extension files of type CG is   1
> AIPS 1: Maximum version number of extension files of type HI is   1
> AIPS 1: Maximum version number of extension files of type MF is  19
> AIPS 1: Maximum version number of extension files of type PL is   1
> AIPS 1: Keyword = 'WTNOISE '  value =  1.696171E+33
> AIPS 1: Keyword = 'SUMWTIN '  value = -6.839778E-32
> AIPS 1: Keyword = 'CCFLUX  '  value =  1.771047E-01
> AIPS 1: Keyword = 'CCTOTAL '  value =  1.771047E-01
> AIPS 1: Keyword = 'OLDRFQ  '  value =  1.59449000D+09
> AIPS 1: Keyword = 'SUBARRAY'  value =            1
> AIPS 1: Keyword = 'MAXABSU '  value = -1.000000E+00
> 
> I'm really quite flummoxed by all this…  I just don't understand why the numbers don't converge  :(

I do at least in part:

1. Your computation for FSHIFT(1) included erroneously a cos(dec) term.

2. The shifts in your header are not 0, just not big enough to print 
with this inadequate format (the code prints nothing when they are both 
0).  Do a GETHEAD with KEYW = 'XSHIFT' and 'YSHIFT' to see what is in 
the header already.  I predict ~0 for X and not zero for Y.

Eric