[daip] moment maps

[Eric Greisen]

Yoshiaki Hagiwara writes:

 > I made a 1st moment map of the VLA-A data using the MOMNT, in which
 > one can see weak velocity gradient (10 kms/0.1arcsec) along PA=-20deg.
 > The detected velocity field on the map extends within 0.1 arcsec, 
 > and the beam size is 0.1 arcsec.

    You seem to be saying that the object is only 1 beam width across
and yet shows some velocity gradient (10 km/s / beam width).  You do not
say what the spectral resolution is in km/s.  Is 10 km/s a fraction of
a channel or many channels?

 > 
 > However, there is no such a velocity gradient, looking
 > at the P-V map, sliced at PA=-20deg! 
 > 
 > I guess that the velocity gradient seen in the moment map 
 > is false. How can I clarify, when the production of moment 
 > maps goes wrong?

A moment computation is affected by all pixels that get included.  If
one bad pixel at a high velocity is included then the first moment
will appear to be quite different from a computation that did not
include it.  Thus blanking is very important.  Your eye in looking at
the P-V image does something far less mechanical in ignoring that
which does not appear to be signal and so your eye can get a different
result.  There is another moment task XMOM and it would be good to
compare the two outputs.  Almost certainly, the images should be
blanked by eye (AIPS TV) with task BLANK before either XMOM or MOMNT
are run.

Eric Greisen