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?

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Averaging Image Channels

Here is a good question from Sanch:

I am trying to average spectral channels in an image cube. Can anyone suggest what's the best way
to do it in AIPS?
I don't want to go back to the UV data and re-reduce it to get the image, as the data I am
working on is huge (VLBI data).

And some more details:

She wants to average together channels in a channel map. In other words, she has a channel map with, say, 100 channels and she wants to turn it into a map with 50 channels. Channels 1 and 2 would be averaged together and become the new "Channel 1," channels 3 and 4 would be averaged together and become the new "Channel 2," and so on.

Thoughts, AIPS gurus?

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SUBIM question

A question from reader Jenny:

I'm trying to combine two optical images using AIPS (don't ask why!) but the FITS images I have are not centred on the sources I want to combine. I want to use SUBIM to create images centred on my targets but is there a way to get the task to create the image using the ra and dec of the target rather than TRC and BLC?

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Total Flux in an Image Region?

A question from an anonymous reader...
But I have this issue: How to find the total flux in a given region? tvstat can make regions, but it shows only mean and rms. Is there any task that will show total flux under a selection?

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SMOTH and and regridding

Can anyone suggest an AIPS task that can help me with the following? I'm looking at emission lines in a diffuse source. I needed to try and increase my S/N levels, but could not smooth the line because it has some complicated structure (multiple velocity components, etc) that I want to fit. So, I smoothed spatially using SMOTH.

Since my beam is now almost twice the size that it used to be (6'' instead of 4'') I think the pixel size of the original image may be oversampling the beam. Can I regrid the smoothed image to have 1'' pixels instead of 0.5'' pixels? How?

Thanks,
Katie

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Gaussian Fitting

Here is a request from an anonymous reader; any advice, anyone?

Can you please help me with the following? I have an image with 5 substructures in it. The peaks inthe image decrease in size from left to right, the leftmost two are the brightest. All of them are close to each other. With AIPS jmfit I can fit not more than 4 gaussians. I played around a bit and noticed that dividing the whole image into two parts and fitting 2 gaussians and 3 gaussians respectively in the two parts does not work properly and gives a bad fit. So I came up with this idea. I can first fit four gaussians to first four peaks, and then from the whole image subtract the brightest two. To the residual image now I fit 3 gaussians. I checked whether this technique gives me a good fit to a slice of data (Wrote my own fortran programs for that). It seems to work but my problem is how to do it in aips.

So my problem is the following. I want to fit 4 gaussians to a selected range of data. Then subtract only the brightest two gaussian from the whole image. Then fit other three gaussians to another restricted part of the data. Could anyone please help me on this?

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