Friday, January 4, 2008

Double Sources-- and not the AGN kind

I'm having this problem with some archival data from 1990. I have two nights of data on a galaxy, spaced a few nights apart from one another. I calibrate the nights separately, and image them separately. It rapidly becomes obvious that the images from the two nights are shifted relative to one another, but not in any simple way. The galaxy center, at the image center, is at the same position in both, but as you go out radially, the first night's sources are further out, radially. It's as if the image is stretched one night, relative to the other.

AIPS thinks that both images have the same pixel scale, so that means a source will have different R.A. and Dec (by about an arcminute, pretty bad!) between the two nights' images. If I DBCON the two nights, I get lots of double sources.

The observational setup claims to be exactly the same between the two nights-- same pointing center, same correlator set-up.

Anyone have any idea what might be causing this? I think I've had a similar problem with GMRT data before, and gave up temporarily and went on to a different project. I'm guessing it's a calibration issue?

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