[daip] UVMOD/UVFIX question

[Eric Greisen]

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