[daip] Help with AIPS CC files

[Eric Greisen]

Jose Afonso wrote:
> Dear Eric,
> I need to modify my AIPS CC files in a peculiar way, and I wonder if there is an easy route to do it.
> 
> The data is from GMRT, taken during a period when there was a timestamp issue (timestamp at the beginning of the integration instead of the middle, or the other way around, can't remember exactly but I think that's ok). The result is that the sources in the resulting image are shifted by some amount in RA and DEC (which I can either calculate, if I go and precisely get what the timestamp problem was, or I can just compare with a more recent map with accurate positions and get a observed deltaRA and deltaDEC). 
> 
> I need to use the CC files resulting from the old data (with the RA,DEC shift) as input model for the self-cal of the new data, so I need to change the CC  components adding a given deltaRA and deltaDEC. I would think there is an easy way to do this, as long as you know the structure of the CC files (which I don't, I'm afraid). Could you help me with this? Is there an easy way to just add this deltaRA, deltaDEC to all CCs? Even exporting to an ascii table, modifying it, and re-importing, or something? I'm not sure if this matters, but although the old and new images are from the same region, the pointing is slightly different - which would result in an extra headache in case the CCs are stored in pixels relative to the tangent point I would guess...
> 
> Many thanks for whatever help you can provide.
> Jose

You can look at the structure of the CC files with PRTAB which exposes 
it pretty simply.  The x and y are in arc sec wrt the reference pixel.
TABED will let you change all values.  Be sure to make a copy of the 
data and work on the copy.

I suspect that there are other things you could do such as change the
reference pixel location or change the RA and Dec in the header.  I 
think self-cal will manage all those too.

If it is a timestamp, then I suspect what you need to do is a shift 
dependent on the distance of the CC point from the reference pixel and 
so tabed might want to be tried.  I am not sure if it is up to that 
however.  A POPS proc with TABGET and TABPUT could always work.

Cheers,

Eric Greisen