[daip] frequency in SPLAT output

[Olaf Wucknitz]

Hello,

sorry to bother you again with a SPLAT-related problem.

I want to use SPLAT to average (in time and frequency) a data set with the 
following properties:

qh output:

AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: COMPLEX      3    0.000000E+00       1    1.0000E+00    0.00
AIPS 1: STOKES       1    LL                 1       -1.0000    0.00
AIPS 1: FREQ         8    3.207361E+08       1    5.0000E+05    0.00
AIPS 1: IF           1    1.000000E+00       1    1.0000E+00    0.00
AIPS 1: RA           1    02 24 36.260       1      3600.000    0.00
AIPS 1: DEC          1    34 22 00.286       1      3600.000    0.00
AIPS 1: --------------------------------------------------------------

FQ table:

   NO_IF    =            1
   Table can be written as a FITS ASCII table
 
COL. NO.        1             2              3             4             5
     ROW     FRQSEL     IF FREQ          CH WIDTH      TOTAL BAN     
SIDEBAND
  NUMBER                HZ               HZ            HZ
       1        1       0.000000D+00     5.000E+05     4.000E+06         1

When I average over a few channels (e.g. SPLAT with aparm(1)=3, channel=4, 
chinc=0), so that the output has more than one channel, the frequency 
information in the header is updated and the result seems to be correct.

Now I try the following:

task 'splat'
default
douvcomp -1
ind 2
getn 2
outd 2
aparm(1)=3
channel 8
chinc 0
go

This should average over all channels and produce one output channel.
If I understand everything correctly, the frequency of the output channel 
should be:
3.207361E+08 + 5e5 * (8-1)/2. = 3.224861e8

The output, however, has the following parameters:

AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: FREQ         1    3.207361E+08       1    4.0000E+06    0.00
AIPS 1: --------------------------------------------------------------

COL. NO.        1             2              3             4             5
     ROW     FRQSEL     IF FREQ          CH WIDTH      TOTAL BAN     SIDEBAND
  NUMBER                HZ               HZ            HZ
       1        1       0.000000D+00     4.000E+06     4.000E+06         1


In other words the frequency of the output is the frequency of the first input
channel even though I have averaged over all channels.

In SPLAT.FOR I find the following lines (starting at line 735):

C                                       Disable freq scaling under some
C                                       circumstances.
            IF ((OLDRP.EQ.1.0) .AND. (CNTCHN(1).EQ.ORIGCH))
     *         AVGCH(1) = OLDRP

This seems to be the piece of code that switches off the frequency scaling
"under some circumstances". There is probably a good reason for this, but in
my case it leads to incorrect results.

The result is the same with aparm(1)=1  (defaults for other parameters) in
SPLAT, which would also average over all channels in each IF.
It is different (but correct) with aparm(1)=2  (average over channels and IFs):

AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: FREQ         1    3.224861E+08       1    4.0000E+06    0.00
AIPS 1: --------------------------------------------------------------

COL. NO.        1             2              3             4             5
     ROW     FRQSEL     IF FREQ          CH WIDTH      TOTAL BAN     SIDEBAND
  NUMBER                HZ               HZ            HZ
       1        1       0.000000D+00     4.000E+06     4.000E+06         1


With SPLIT, there may also be problems.

aparm(1)=1 with nchav=8, chinc=8 changes the frequency by shifting the
reference pixel.
qh output:

AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: COMPLEX      3    0.000000E+00       1    1.0000E+00    0.00
AIPS 1: STOKES       1    LL                 1       -1.0000    0.00
AIPS 1: FREQ         1    3.224861E+08       1*   4.0000E+06    0.00
AIPS 1: IF           1    1.000000E+00       1    1.0000E+00    0.00
AIPS 1: RA           1    02 24 36.260       1      3600.000    0.00
AIPS 1: DEC          1    34 22 00.286       1      3600.000    0.00
AIPS 1: --------------------------------------------------------------

imh output:

AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: COMPLEX      3   0.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: STOKES       1  -2.0000000E+00       1.00 -1.0000000E+00    0.00
AIPS 1: FREQ         1   3.2073609E+08       0.56  4.0000000E+06    0.00
AIPS 1: IF           1   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA           1    02 24 36.260       1.00       3600.000    0.00
AIPS 1: DEC          1    34 22 00.286       1.00       3600.000    0.00
AIPS 1: ----------------------------------------------------------------

This seems to be correct.
But if I use aparm(1)=2  (all other default), I get the same incorrect result
as with SPLAT. With aparm(1)=3 it again works fine.



Cheers,
Olaf



-- 
Olaf Wucknitz
Argelander-Institute for Astronomy               wucknitz at astro.uni-bonn.de
University of Bonn, Germany          http://www.astro.uni-bonn.de/~wucknitz