Monday, May 21, 2007

Continuum Subtraction & POSSM

I'm reducing line (OH 1665/1667 MHz) VLA data for the first time, and I don't know if I should trust my continuum subtraction. When I make an image, it looks like most of my continuum structure is gone, but....when I look at the continuum-subtracted baseline-based spectra in POSSM, the spectra look pretty much exactly the same as they did before I subtracted the continuum.

I would expect that if the spectra had an average value of the continuum of say 2 originally, after the continuum subtraction the specta would have an average value of zero, with emission being positive and absorption negative. But it appears that they still have a value of 2.

What does this mean? Any ideas? Oh yeah, I'm using UVLIN.


KED said...

I've never used UVLIN, but after a quick look at the help file, a few ideas: have you set docont < 0? Otherwise it looks like it doesn't remove the continuum that it fits. A problem I've encountered w/ continuum subtraction using UVLSF is the order of the fit... if you've already applied your bandpass calibration, and have a flat baseline, so use 0th order. Also, make sure you're using as many good line free channels as possible in ICHANSEL. ISPEC can help you select these channels.

A similar task to try is UVLSF, which is like UVLIN but has different flagging options and the ability to write out an output "continuum" uv data set to show you what has been removed.

If your problem persists I suggest trying again with UVLSF and imaging the continuum output. Does it look like what you're seeing in the line-free channels?

KED said...

Also, it may be that POSSM is the culprit; try using ISPEC to examine the spectra and see what that tells you.