[daip] IMAGR question

[Eilat Glikman]

Hello,

I am conducting some tests on FIRST images and have run into a peculiar problem.  
My project involves stacking images of radio-quiet quasars from the FIRST survey 
to see if we can detect these objects and study their average properties.  This 
is an effective technique and we are able to detect a source when stacking 
thousands of cutouts.  
To calibrate these results we put 100 40uJy sources using UVMOD into FIRST UV 
files and rand IMAGR in the same way as the FIRST survey does to make its maps.  
As a control group we also input 100 4mJy sources with UVMOD and process the 
data in the smae way.
Our initial tests put the points in a 10X10 grid.  This resulted in a 60% bias 
in the stacked flux (inputting 40uJy sources resulted in 25uJy sources).  We 
repeated the tests on 80uJy, and 200uJy sources.  Since these sources were below 
the cleaning level (FLUX parameter in IMAGR is set to 5.0e-4 Jy) the stacked 
cutouts showed the dirty beam.  
We then hypthesized that perhaps the regular shape of the grid caused sidelobes 
to overlap and may have introduced this bias.  To correct this I made 100 
randomly positioned sources and repeated the test.  
The results of this test are even more bizarre: the 40uJy sources produced a 
similar bias of 60% reduction in fitted peak flux (using jmfit)
we also made a test with 4mJy sources and they too exhibited a bias of the same 
amount!  so cutting out the sources and stacking them yielded a 2.5mJy source.

We have conjectured several possibilities for this, but in an effort to save 
time I thought to ask an expert.

Does any of this make sense?  Could it be (with the 4mJy sources) that we are 
adding 400mJy to the map and that could be causing the bias?
Could this be a result of the snapshot nature of the FIRST UV images whose 
integration time is only 165sec?

Any help would be appreciated.

Thanks,
Eilat Glikman

Columbia University Department of Astronomy