[daip] flux integral discrepancy

[Eric Greisen]

Lauren E. Davis writes:

 > I am currently working with data from a spiral galaxy.  I obtained some 
 > already-calibrated and published data from the author; when I image these 
 > data I produce a flux integral nearly half of the established value for 
 > the galaxy, including the value published using these same data.  Is there 
 > anything I could have done using only imaging tasks (IMAGR and APCLN) that 
 > would produce such a discrepancy?
 > 

If you started with the same calibrated visibility data as the
previous author, then there seem to be fewer possibilities.  Did that
author apply a primary beam correction (to the source off the pointing
center)?  Did that author apply some "known" scaling to the data -
e.g. based on his measured flux wrt a single-dish flux?  The
interferometer can easily miss total flux from large-scale features
that are too resolved to be properly accounted for.  Clean with its
bias to point sources can make this worse although not a factor of 2
all by itself.

One thing did happen to me when I cleaned a large HI data set with
point source Clean (100000 cc's per channel I think) - when I ran
ISPEC it gave a nice positive when integrated over half the galaxy in
the channels appropriate to that half, but a negative and non-trivial
flux in the channels appropriate to the other half.  The negative bowl
due to lack of zero spacing can integarte to a lot - in fact, with no
zero spacing and an integral to infinity, the flux should be zero.

I have re-imaged those data using muti-resolution in IMAGR and this
problem went away.

Can you ask the origial author what he/she did?

Eric Greisen