[daip] UVMOD/UVFIX question

[Hanno Spreeuw]

Thank you,

It seems that inserted sources with FPOS<>0 do not appear in the images 
exactly where they should. The errors increase away from the tangent 
point. This confirms earlier results by James Miller-Jones (CC). 
However, with FPOS=0, the source always appears at the tangent point.
We are testing new source extraction code. In order to check our 
deblending algorithm, we are inserting many sources in a small patch of 
the sky. With these artificial sources one always knows the true flux 
and position, so that is good for testing.
I am considering the workaround with UVFIX for these tests.

Hanno.

Eric Greisen wrote:
> Hanno Spreeuw wrote:
> 
>> Hello, I have a question about UVMOD. Suppose you are inserting a 
>> source in a dataset with an offset from the tangent point (set by FPOS).
>> Would that be equivalent to shifting the dataset using UVFIX (with 
>> SHIFT<>0), inserting a source using UVMOD (with FPOS=0) and then do 
>> the inverse shift using UVFIX?
>> If yes, would SHIFT=FPOS?
>>
> The one difference here is that UVFIX recomputes u,v,w which may make a 
> difference even without any position shift.  Reading the help files 
> certainly suggests that SHIFT=FPOS would produce the same results.  But 
> you can test this - although I do not see why you don't just use UVMOD.
> View the code as experimental and try things.  In the old days, 
> computers had no disk and were verrrrrry slow.  Now, these things take 
> little time to test.  Add a big point source with UVMOD and then make an 
> image.  If that is as it should be, add the source you really want to add.
> 
> Oh - one other detail.  The shift parameters are defined with a cos(dec) 
> in them and so the inverse shift in UVFIX - if dec changes a lot - may 
> not have the same values exactly.
> 
> Eric Greisen
>