National Radio
Astronomy Observatory

6.6.1 Hints for using (immoments)

In order to make an unbiased moment-0 image, do not put in any thresholding using includepix or excludepix. This is so that the (presumably) zero-mean noise fluctuations in off-line parts of the image cube will cancel out. If you image has large biases, like a pronounced clean bowl due to missing large-scale flux, then your moment-0 image will be biased also. It will be difficult to alleviate this with a threshold, but you can try.

To make a usable moment-1 (or higher) image, on the other hand, it is critical to set a reasonable threshold to exclude noise from being added to the moment maps. Something like a few times the rms noise level in the usable planes seems to work (put into includepix or excludepix as needed. Also use chans to ignore channels with bad data.

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