[daip] New Ticket - [!TXS-966244]: IMAGR and non-monotonic IF frequencies

[Michael Bietenholz]

New Ticket: IMAGR and non-monotonic IF frequencies

I think IMAGR is coming up with funny values for the centre frequency in cases where  gridding different IFs together but the frequency of the IFs is non-monotonic.  It probably grids the data correctly so everything is fine except the nominal frequency of your image is not the centre frequency of your visibility data.

I have a data set which has IF (SPW) freq (LISTR, OPTY='SCAN') like so:
First increases from ~32 to 33 GHz
FQID IF#      Freq(GHz)      BW(kHz)   Ch.Sep(kHz)  Sideband  Bandcode
       1      32.07500000  128000.0078   8000.0005      1
       2      32.20300000  128000.0078   8000.0005      1
       3      32.33100000  128000.0078   8000.0005      1
    ....
       8      32.97100000  128000.0078   8000.0005      1
Then freq jumps *down* to 31 GHz, but subsqeuently increases back up to 32 GHz
       9      31.07500000  128000.0078   8000.0005      1
     10      31.20300000  128000.0078   8000.0005      1
....
     16      31.97100000  128000.0078   8000.0005      1

Now if I run IMAGR (UVWTFN 'NO') on IF 1-8, I get a image freq. of 32.519 GHz, sensible enough. IFs 9-16 gets you 31.519 GHz, also sensible. But IFs 1-16 get you 32.545 GHz. That doesn't seem right, since IFs 9-16 are at a lower freq. than IFs 1-8, so the average frequency for all 16 IFs together should be *lower* than just the IF 1-8 freq.  By my calc., the IF1-16 image should be at something like 32.02 GHz

Admittedly this is an odd arrangement of IF frequencies, but I think I just used the Ka-band default at scheduling so I wasn't trying to do anything strange.

(above is not dependent on the unusual use of unity weights, I only tried that in case there was some weirdness in the IF-dependent visibility weights;  but it seems like IMAGR just chooses centre freq. of the gridded vis, rather than some weighted average).

31DEC13 AIPS, MNJ 26 June


Ticket Details
===================
Ticket ID: TXS-966244
Department: AIPS Data Processing
Priority: Default
Status: Open
Link:  https://help.nrao.edu/staff/index.php?_m=tickets&_a=viewticket&ticketid=3448