Tuesday, February 10, 2009

CVEL and DBCON with ATCA Data

I'm reducing some archival, multi-configuration HI data from the ATCA and am having trouble getting the velocity axis quite right. I've put the correct velocity definition in the header using CVEL and then am combining the individual data sets using DBCON with DOPOS=1,1 so it checks the position and frequency information in the header. However, when I make the image and compare to the originally published image the velocities for various features are way off (~40 to 100 km/s). Any words of wisdom from the peanut gallery on using CVEL and DBCON together?

I've mostly been following The Analysis of ATCA data with AIPS, but it's a little light on detail on this issue.


Anonymous said...
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Adrienne said...

In my (limited!) experience with CVEL, it spits out a little warning about not being guaranteed to work if you have ever SPLIT your data.

I think this is because there can be additional frequency corrections in the CL table that might get lost with SPLIT. I haven't run into this problem myself, but we're not doppler tracking so there shouldn't be any offsets anyway. I think the ATCA doesn't doppler track either, so that's probably not it.

Have you checked to see that the features are in the correct place before DBCON?

My other suggestion is that the velocity reference could be different than the published image: optical vs. radio, barycentric vs. some other frame.

I'm also guessing you've thought of all of this before! My experience with non-doppler-tracked VLA data hasn't had any problems with CVEL/DBCON so far.

amanda said...

I ended up imaging the data in Miriad and double checking the line and it looked fine. The velocity structure was faint and intrinsicly weird, which is what was tripping me up.