Wednesday, January 28, 2009

Clean Boxes from SETFC

Here is a question from an anonymous reader:

I'm new to AIPS, and noticed that using SETFC automatically sets clean boxes for every facet, the same size as every facet. When I've been wide field imaging, I've just been adding smaller clean boxes around sources on top of these. Is this ok, or is it better to delete these automatic boxes, and just manually add ones around obvious sources? Thanks.

I think I know the answer to this, if you click to read on...

You definitely want to delete the big clean box. By and large, you don't want clean boxes to overlap, and you do not want to clean your entire field (which is what you're doing if you leave that big clean box). Someone at NRAO once really stressed to me that you want your clean boxes placed as tightly around the real flux in your sources as possible. So get rid of those big boxes.

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Friday, January 16, 2009

Bad Gain Exponents

I'm trying to calibrate some archival VLA HI data from 1989 and FILLM keeps giving me scary messages about BAD GAIN EXPONENTS, e.g.,

MCCHED: 0/02:34:45.0 12 x 12 BAD GAIN EXPONENT 26

AIPS seems to really not like this. The source table of the resulting data file only contains the first source observed, which is not right. I'm was going to be more aggressive about the FILLM selection parameters to avoid the bad times, but was wondering if anyone else has encountered this?

EDIT: After playing with the TIMERANG keyword for FILLM, it seems like this is a problem with how AIPS is reading in the file. Everytime I change the TIMERANG keyword, the data with the BAD GAIN EXPONENTS changes to the earliest data in the file. Weird.

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