[daip] question about MOMNT

[Eric Greisen]

Doug Ratay writes:
 > Hi.  I have a question about the second moment calculation in the MOMNT 
 > task in aips.  I recently calculated the second moment of a HI cube, and 
 > found that the peaks in the velocity dispersion corresponded with the 
 > peaks in the HI distribution.  This could be physical and reflective of 
 > the real data, but I wanted to make sure that AIPS actually divides out 
 > the sum of the brightness temperatures.  Or is that something I have to do 
 > by hand later?  If there is a different equation that AIPS uses for the 
 > second moment from the standard statistics definition, is that listed 
 > somewhere.  Thanks alot.  If there is any other info that you need from 
 > me, please let me know.
 > 					Doug Ratay
 > 					University of Florida

You can read the code of MOMNT yourself to be sure.  It appears that
it computes:

M2 = scale * sqrt [ Sum{v*x*x)/Sum(v) - (Sum(vx)/Sum(v))**2 ]

Where v(i) are the data values and x(i) the spectral channel.  The
scaling corrects the units.  XMOM does a similar calculation.

Eric Greisen