[daip] flux integral discrepancy

[Lauren E. Davis]

Hi Eric,

Thanks very much for all your suggestions.  Unfortunately I don't seem to 
have found the answer yet. The author states specifically in his paper 
that they didn't seem to lose any flux do to zero spacing, and having 
played around with it myself it doesn't seem to correct the problem.  The 
global profile also looks symmetric -- the shape of the profile 
coorresponds closely to previous results, it's just scaled down from the 
expected flux density.  I'm going to contact the author now, I've really 
tried everything I can think of.  Thanks again for all your help.

Lauren



On Wed, 1 Mar 2006, Eric Greisen wrote:

> 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
>