Monday, August 24, 2009

Measuring Spectral Lines

From reader Fred:

When you've got spectral lines in emission, what's the best way of actually reliably measuring them in AIPS? Half the time, when JMFIT is run on a line, the peak and integrated fluxes come out identical (which makes me suspicious!).

my (small amount of) insight below the fold...

I don't usually measure spectral lines in AIPS, and am much more likely to do it in say, IDL, so I don't have any direct insight for you. I'm guessing that JMFIT is not a good way to measure lines, though. I have used JMFIT a lot for measuring continuum point sources, and I can tell you that when the peak and the integrated flux come out to be about equal, this usually means that the source is unresolved. I'm guessing that your spectral lines aren't all actually unresolved (<1 channel in width?), so perhaps JMFIT just doesn't really know what to do with spectral lines. The problem might be that it expects everything to be in terms of mJy/beam, and it usually knows what the beam is in RA/Dec coordinates, and so it is all set up to fit a gaussian to a source and figure out how many beams it covers and how many mJy it emits. This whole set-up doesn't makes much sense when you are talking about one of the coordinates being mJy/channel...

So, I'm skeptical of using JMFIT, but it might be possible. Unfortunately I don't have other suggestions in AIPS, but I bet other people do?

1 comment:

Chandra said...

Hi, I'm pretty much a novice when it comes to working on AIPS. My work entails me to extract the spectral flux from data cubes.I use ISPEC to do the same.To extract the spectra I wrote a procedure to call ISPEC and provide it with the BLC and TRC values(the pixel values corresponding to the sources in the image)for it to extract the spectra.But the output of ISPEC seems to give out incorrect RA and DEC values corresponding those pixels that was inputed.There seems to be a problem transforming the pixel values to their corresponding RA and DEC values.
Hoe does ISPEC do the pixel to co-ordinates tranformation?
I would be grateful if somebody could help sort this problem.Thanks in advance!