Monday, April 2, 2007

Number of Channels = One Less Than I Expect

4/2/07: When I write my observe file, I ask for 8 channels. And when I get the data and FILLM, there are always 7 channels there.
Now, I have a new data set in which I asked for 4 channesl, and when I FILLM I am again missing a channel, there are only 3! This is a pretty significant loss of signal; I'd like that channel back.

I think I'm using pretty standard, default parameters in FILLM. Has anyone ever found a way around this problem? (I'm sure it's not a big deal if you're doing bona fide spectral line data).

UPDATE as of 4/14/07: After talking to my advisor Eric, it appears that Channel 0 actually counts as one of your channels!!! So when you ask for a correlator setup of 4 channels, you actually get 3 channels of original data, and then one of the channels is taken up by the average of these, CH 0. This is really one of the stupider things I've heard yet about radio interferometry. And let this be a warning to you, don't ever ever use the 2-channel correlator setup at the VLA!!!! I guess it's the same as doing continuum observations, but with less bandwidth!

4 comments:

katie d. said...

Are you using jobserve to write your observe files?

Laura said...

Yeeess.....

Laura said...

Maybe online flagging always flags the first channel?

amanda said...

Are there the same number of visibilities in all your channels? I'm wondering if fillm isn't separating out some of the channels or if it is splitting something off into a channel zero. (Although fillm with bchan 0 and echan 0 looks like it should do sensible things.)