[daip] forwarded message from Eric Greisen

Wed Mar 29 12:09:35 EST 2006

Here is what I sent.  Note that IM2UV attempts to make a UV data set
not simply an fft of the image.  If you want to compare what IDL does
you must use the AIPS task FFT.

Eric Greisen

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From: Eric Greisen <egreisen at nrao.edu>
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To: AIPS en Tuktoyuk <kyle at inaoep.mx>
Cc: daip at nrao.edu
Subject: Re: [daip] UV amplitudes X100??
Date: Tue, 21 Mar 2006 17:29:36 -0700

AIPS en Tuktoyuk writes:
 > 
 > from the original interferometer UV data I created a dirty map with IMAGR 
 > (niter=0), and the map looks like it should dirty.  Then using IM2UV I fourier 
 > transformed the dirty map back to the UV plan.  The original UV data had a max 
 > amplitude around 12Jy while the IM2UV version has a max amplitude around 1200Jy, 
 > wow. Do you know why this happened?? Why 100X rather than another multiplication 
 > factor??

           IM2UV is not used much and may well have a scaling error of
one sort or another.

 > 
 > As per the image brightness: the original dirty map has a max of ~0.5899Jy/bm 
 > and a total flux of ~0.069Jy.  When I use UVMAP on the IM2UV set to transform it 
 > back to the image plane the image has a max of 0.969Jy/bm and a total flux of 
 > 2.00Jy.  this equals to the total flux being ~29X larger in the new map from 
 > UVMAP.  I assume this is directly related to the increase in the different UV 
 > data sets, true??

     The total flux in a dirty map is zero unless there is a zero
spacing value.  The total flux over a limited area may be > 0 but is
still affected by the beam pattern.  Clean is really needed before
flux measurements mean much.  Imaging IM2UV output will provide that
but with no real data to define a correct value.  It will also mix the
initial dirty beam with a very grid-like beam due to the rectangular
"samples" of IM2UV.  It certainly is not a correct method of imaging.

 > 
 > Is this due to a normalization factor of the FFT and inverse FFT of AIPs?? Or is 
 > it something else??

I do not know what you are trying to get at here - I will take a look
at the scaling from IM2UV, but would have no faith that it is right or
that I will figure out how to put it right.  Tasks like IM2UV are
intended to assist you in examining your data for bad samples.  FFT is
probably more effective.

Eric Greisen

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