Tuesday, October 14, 2008

Imaging GMRT Data

A plea for advice from an anonymous reader:

Off topic, & quite possibly a stupid question from an AIPS newbie, but I'd really appreciate any help. I'm currently processing some GMRT data (30mins, 610MHz, 38 facets, each imsize 512, cellsize 1.5), & IMAGR seems to take a very long time to run, ~2hrs for 1000 iterations. I'm wondering if this is normal, or if something has gone wrong during the SPLAT process, where I split off the source from the multi source dataset, and averaged every 7 channels between channels 1 & 105.

IMAGR, with NCHAV=15,when running, appears to process channels 1-15, then 2-16,3-17 etc, is this normal for data that has been averaged as above?


emily said...

imagr can take a while, especially with many facets and averaging (if nchav is set). 2 hours does not seem abnormal for that combination of parameters.

regarding the second question, i think you want nchav=15 AND chinc=15. then imagr will average together channels 1-15, 16-30, etc.

Anonymous said...

Thank you for your reassurance, and especially for your quick reply, much appreciated.

I am slightly confused on one point, in that a couple of manuals which I have been following average the data as described in SPLAT, then only have nchav=(eg.)15 in IMAGR, with chinc=1. My initial reaction (I may have misunderstood something here) was why bother averaging in SPLAT at all, if IMAGR is just going to cycle through all the channels anyway? Is there an advantage to setting the chinc parameter to 1?

emily said...

you want to average in IMAGR so that you aren't bandwidth smeared over most of your image. you can do a small amount of averaging in SPLAT beforehand to make IMAGR run faster.

i'm assuming that you care about 610 MHz continuum emission in which case you want to smash all the channels together into one map in the end anyway. so you'll want nchav=chinc=total number of channels in your uv data file.

Anonymous said...

Lightbulb has now switched on! Thanks very much :-)

Tony Rushton said...
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