Tuesday, November 10, 2009

Heads Up! Inverted Axes for EVLA-VLA data..

I recently encountered an issue with one spectral line data set observed during the EVLA-VLA transition (like all my other data sets). So far, only this data set in particular is affected, but it could possibly apply to others if you notice this behavior. If you notice rotation that should not be there compared to other images, this may affect you!

Here are the weird things about this data set:

  1. The velocity axis appears to be backwards, but the header seems normal (is, the cdelt3 value in the header is not of the opposite sign).
  2. Compared to other observations of the same galaxy taken in different configurations, the map is rotated by 180 degrees on the plane of the sky, but again, the header seems normal (cdelt1/2 are both normal).
I spent a lot of time trying to figure out the cause of this and originally chalked it up to user error - i.e., something went wrong in the calibration, which gave the phases the opposite sign. Or so I thought.

Well! Turns out it was a bona fide problem with the EVLA transition, which inverted the velocity axis and gave the phase an incorrect sign. This has mostly been corrected in the archive, but is apparently still present in a few, lingering data sets. The fix for this is to use the task "FLOPM" on the data (after SPLIT) with opcode "VLAE".

Voila! Problem solved. Now if I could only get back the three weeks I spent banging my head against the proverbial AIPS wall.

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IMERG is a program that combines single dish and interferometer data (see Stanimirovic (2002) for details on the algorithm). In short, this algorithm (also referred to as "feathering") fourier transforms both the single dish and the interferometer data, uses the overlap region in uv space between the interferometer and single dish data to determine a scaling factor for the single dish data, adds the interferometer image and the scaled single dish image in fourier space, and then fourier transforms it back.

IMERG has a couple of extra caveats that the user should be aware of before running:

  • Both images need to be square and the dimensions a power of two.
  • Make very sure there are no blanks in either image. Use REMAG to replace any blanks with zero. Note that OHGEO will blank any region in the transformed image that is outside the boundaries of the original image. One of the signs that you may have some blanks in your image is really small scaling factors (~1e-8).
  • Make sure that the single dish image is big enough, i.e., make sure the galaxy is much smaller than the entire image.

Edit (11/13/09): As far as I can tell, IMERG does absolutely no checking that the two images have the same axes, so beware and double check that both images are the same. Also the beam that gets put in the resulting image is the single dish beam, not the interferometer beam. Use ADDBEAM to change.

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Friday, November 6, 2009

Multi-frequency synthesis in AIPS?

(Edited based on re-reading the documentation this morning.)

Does any one here have experience with multi-frequency synthesis in AIPS? I've got a WSRT data set with 8 continuous IFs each with 64 channels. I'm currently setting

bif=1; eif=8
bchan=2; echan=50; nchav=49


I was going to run it through self-calibration as usual.(See the following post for details.) Is there anything else I need to do?

[Edited 11/11/09: See Michael Rupen's comment on this post for the answer.]

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