[daip] Oddity in FLATN

[rlaing at eso.org]

On Thu, 1 Jun 2006, Eric Greisen wrote:

Dear Eric

> What version of AIPS are you running?

31DEC06

>
> UVFIX corrects the observed RA/Dec to J2000 beginning with the 31DEC03
> version.

Indeed, that's what I wanted and got.

> There should then be no problem in FLATN.  Note that FLATN
> uses the coordinate it chooses to grid the coordinates from the images
> and so if it chose a B1950 coordinate and then used J2000 values from
> the images, bad things will result.

That's what I think happened. The header of one of the input images is:

AIPS 1: Image=1553+24   (MA)         Filename=1385 10     .ICL001.   1
AIPS 1: Telescope=VLA                Receiver=VLA
AIPS 1: Observer=AR402               User #=   90
AIPS 1: Observ. date=20-SEP-1999     Map date=23-MAY-2006
AIPS 1: Minimum=-2.42138514E-04      Maximum= 7.47922510E-02 JY/BEAM
AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: RA---SIN   256    15 56 03.910     128.00         -3.000    0.00
AIPS 1: DEC--SIN   256    24 26 52.900     129.00          3.000    0.00
AIPS 1: FREQ         1   1.3851000E+09       1.00  5.0000000E+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=     678
AIPS 1: Conv size=  10.00 X  10.00   Position angle=   0.00
AIPS 1: Observed RA   15 53 56.200    DEC  24 35 33.00
AIPS 1: Phase shifted in X     -0.002  in Y     -0.046
AIPS 1: Rest freq      0.000         Vel type: OPTICAL wrt YOU
AIPS 1: Alt ref. value  0.00000E+00  wrt pixel    1.00
AIPS 1: Maximum version number of extension files of type CC is   1
AIPS 1: Maximum version number of extension files of type HI is   1
AIPS 1: Keyword = 'WTNOISE '  value =  1.227314E+00
AIPS 1: Keyword = 'CCFLUX  '  value =  2.043409E-01
AIPS 1: Keyword = 'CCTOTAL '  value =  4.143397E-01
AIPS 1: Keyword = 'PARANGLE'  value = -2.755295E+01
AIPS 1: Keyword = 'ZENANGLE'  value =  1.149982E+02

Observed RA, DEC is in B1950. So FLATN picked this up, interpreted it as J2000
and used it as the central pixel for a set of images which are actually in
J2000. Is this the "bad things" you expect? Has the equinox of the observed RA
and Dec been lost? [It would be quite useful to have this displayed to avoid
confusion]. Not a big problem once one realises what is going on, but it threw
me for a bit.

Regards

Robert