[daip] visibility weight problem in output of DIFUV

[Huib Intema]

Dear DAIP,

I use DIFUV to subtract a model UV database from a real UV database. The 
model UV database is created with UVSUB with OPCODE = 'MODL', using the 
real UV database as input, which results in model visibilities having 
the same weights as in the input. Before (using a mid-2008 copy of 
AIPS), I needed to run WTMOD on the output of DIFUV with APARM(1)=0.5 to 
compensate for the unwanted summation of (quadratic?) weights by DIFUV.

I found that DIFUV in the current 2009 AIPS causes the output visibility 
(quadratic?) weights to be halved rather than doubled. This I could fix 
by using WTMOD with APARM(1)=2.0. However, when the input visibility 
weights are both zero the output weight is NaN. This looks like a zero 
division error while adding the reciprokes of the input visibility 
weights rather than the weights themselves.

Regards,

Huib

(running the binary MNJ AIPS on a 64-bit Linux PC with Redhat 5)



-- 
Huib Intema (Jansky Fellow)

NRAO CV room 353             National Radio Astronomy Observatory
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hintema at nrao.edu             Charlottesville, VA 22903-2475
www.cv.nrao.edu/~hintema     United States
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