[daip] IM2UV and UVMAP questions

[Eric Greisen]

AIPS en Tuktoyuk writes:
 > good afternoon,
 > 
 > 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??
 > I have done the same procedure for only a single uv-channel and the resulting 
 > amplitudes in the UVPLOT are the same 100x larger with less visabilities due to 
 > it only being 1 of the 128 uv-channels.
 > 
 > 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??
 > 
 > If I export the dirty maps to IDL and fourier transform to UV and then back to 
 > the image plane I get the same image back with the same brightness' as the 
 > original.  This is true for both the 6-channel cubes and a single channel image. 
 >  Therefore this only seems to be a problem with AIPs.
 > 
 > Is this due to a normalization factor of the FFT and inverse FFT of AIPs?? Or is 
 > it something else??
 > 

The IM2UV task does something quite wierd - it FFTs the image and then
pretends that each cell in that rectangle is a UV sample.  Your
comment suggests that there is a scaling error - something that FFTs
are prone to.  I should look for that as long as I am messing with
this code.

The IDL program you are describing would be like using the AIPS task
FFT.  That makes a complex image in the UV plane but does not then
pretend that these are samples for a whole new gridding and
weighting.  Thus the FFT and FFT back should produce something much
like the original output from UVMAP.

Eric Greisen