Thursday, March 29, 2007

Smoothing in BPASS

What is up with all those warnings about smoothing in BPASS? It seems so fricking absurd that you can only smooth your bandpasses if you smooth your data.

(One of my data sets had very high spectral resolution, so each channel was quite narrow and typical integration times on bandpass calibrators led to very noisy bandpasses. Of course, I didn't want to add all this noise to my data, but I did want to calibrate the bandpasses. A good solution to this would be to simply smooth the bandpasses, so that you'd get the general shape of the bandpass without all the nasty channel-to-channel variations.) But is smoothing ok? I don't want to sacrifice the resolution of my data!

This advice in from Katie Devine at the VLA in NM:

Hey, I learned something today that you'll find interesting: the smooth warnings for smoothing the data itself can be ignored if you have an antenna with intrinsically nice bandpasses. The reasoning is such-- if you have a really crazy bandpass (like on the GMRT for example) that has large dips, and you smooth those out with out smoothing the data, then a line that may have fallen in a dip in the bandpass will not be calibrated properly, because that dip will have been smoothed out (make sense? I can clarify if you want).

On the other hand, the bandpass for the VLA just has a little bit of noise to it, but no other intrinsic shapes. So smoothing the bandpass without smoothing the data is ok in this case. Basically, your gut instinct was correct.

You'll want that bpass correction if you're citing line strengths, especially if you have more than one line in each band...

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