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.

No comments: