[daip] Wrong (u,v) coordinates?

[Arnaud Collioud]

> Can you look at files from earlier in the processing to see when this issue arose?  I changed many tasks to shift the frequency reference to the center, but I see that you have not done that.  This shift does require a re-scaling of the uvw's but a very minor one.  Were the X and S data in the same file at one time?  There could be an issue then if the header had the X band frequency because the u,v,w should then be in wavelengths at X band even on the S band data.  Then, splitting out S band would lead to a really big correction and I wonder if it happened in some way twice???

Yes, originally X and S frequencies were together (even if there were no X data). I split the 2 using UVCOP with the following parameters:

AIPS 1: UVCOP     Task to copy selected times and channels of uv data.
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     'IYA09_S.3'             Input UV file name (name)
AIPS 1: INCLASS    'UV'                    Input UV file name (class)
AIPS 1: INSEQ         1                    Input UV file name (seq. #)
AIPS 1: INDISK        1                    Input UV file disk unit #
AIPS 1: OUTNAME    'IYA09_S.CLN'           Output UV file name (name)
AIPS 1: OUTCLASS   'UV'                    Output UV file name (class)
AIPS 1: OUTSEQ        1                    Output UV file name (seq. #)
AIPS 1: OUTDISK       1                    Output UV file disk unit #.
AIPS 1: SELBAND      -1                    Bandwidth to select (kHz)
AIPS 1: SELFREQ      -1                    Frequency to select (MHz)
AIPS 1: FREQID        2                    Freq. ID to select. 0=> all
AIPS 1: BCHAN         1                    First channel selected.
AIPS 1: ECHAN         0                    Last channel selected.
AIPS 1: BIF           0                    Lowest IF number 0=>all
AIPS 1: EIF           0                    Highest IF number 0=>all
AIPS 1: SOURCES    *all ' '                Source list
AIPS 1: QUAL         -1                    Source qualifier -1=>all
AIPS 1: UVRANGE       0           0        Annulus in uv plane selected
AIPS 1:                                     (thousands of wavelengths)
AIPS 1: TIMERANG   *all 0                  Timerange:
AIPS 1:                                     1-4 = start day,hr,min,sec
AIPS 1:                                     5-8 = end   day,hr,min,sec
AIPS 1: ANTENNAS   *all 0                  Antennas to copy 0=>all
AIPS 1: BASELINE   *all 0                  Baselines with ANTENNAS
AIPS 1: SUBARRAY      0                    Limit copy to one subarray
AIPS 1:                                    0 => all.
AIPS 1: FLAGVER      -1                    FG table to apply to data
AIPS 1:                                    <= 0 => do not apply
AIPS 1: UVCOPPRM      1           0        Flags:
AIPS 1:               0           1         1 > 0 => keep flagged data
AIPS 1:               0           0         2 > 0 => drop subarray info
AIPS 1:              -1        *rest 0      3 = 0 => copy xc and ac data
AIPS 1:                                       = 1 => copy only xc data
AIPS 1:                                       = 2 => copy only ac data
AIPS 1:                                          of antennas, named in
AIPS 1:                                          antenna list. Baseline
AIPS 1:                                          list must be zero.
AIPS 1:                                     4 > 0 => report progress
AIPS 1:                                     5 => flag data with weights
AIPS 1:                                          below this limit
AIPS 1:                                     6 = 1,3 -> do NOT flag TY/SY
AIPS 1:                                       = 2,3 -> do NOT flag SN
AIPS 1:                                     7 = extend copy of tables
AIPS 1:                                         by UVCO(7) min, 0 -> 15
AIPS 1: NCOUNT        0                    If UVCOPPRM(3) = 2 then
AIPS 1:                                    NCOUNT is the approximate
AIPS 1:                                    ratio of AC to XC in file.
AIPS 1: FQCENTER      0                    >= 0 -> center frequency axis

I did not change FQCENTER because I was not sure about what it will do (I was also not aware of the importance of it).

As you said, the initial file has the X-band frequency as reference in the header:
>imh
AIPS 1: Image=MULTI     (UV)         Filename=IYA09_S.3   .UV    .   1
AIPS 1: Telescope=VLBI               Receiver=MKIV
AIPS 1: Observer=Bonn Cor            User #=    4
AIPS 1: Observ. date=18-NOV-2009     Map date=03-JUL-2014
AIPS 1: # visibilities   4848965     Sort order  TB
AIPS 1: Rand axes: UU-L-SIN  VV-L-SIN  WW-L-SIN  TIME1  BASELINE
AIPS 1:            SOURCE  FREQSEL  INTTIM  WEIGHT  SCALE
AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: COMPLEX      1   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: FREQ        32   8.4059900E+09       1.00  2.5000000E+05    0.00
AIPS 1: STOKES       1  -1.0000000E+00       1.00 -1.0000000E+00    0.00
AIPS 1: IF           4   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA           1    00 00 00.000       1.00       3600.000    0.00
AIPS 1: DEC          1    00 00 00.000       1.00       3600.000    0.00
AIPS 1: ----------------------------------------------------------------
AIPS 1: Coordinate equinox 2000.00
AIPS 1: Maximum version number of extension files of type HI is   1
AIPS 1: Maximum version number of extension files of type BS is   1
AIPS 1: Maximum version number of extension files of type CL is   1
AIPS 1: Maximum version number of extension files of type SU is   1
AIPS 1: Maximum version number of extension files of type AN is   1
AIPS 1: Maximum version number of extension files of type FQ is   1

What should I do? 
Set FQCENTER to '1' in UVCOP?  Could you explain to me what this parameter is for?

Thanks!
Cheers,
Arnaud