[daip] Wrong (u,v) coordinates?

[Eric Greisen]

On 07/07/2014 04:23 AM, Arnaud Collioud wrote:
> Dear Eric,
>
> After the BLAPP problem, I am encountering a new un-expected problem.
> I successfully processed my multi-source S-band VLBI experiment (ending with SPLIT, then FITTP). But when loading the individual data in Difmap (or just looking at it in AIPS by UVPLT), the uv radius values are very low compared to the expected ones.
>
> For example, on a given baseline at a given time, I have these (u,v)-coordinates and uv radius:
> At X-band (8.646 GHz)
> 	u = -20.886 MLambda
> 	v = -342.66 MLambda
> 	uvrad = 343.29 MLambda
>
> At S-band (2.309 GHz)
> 	u = -1.482 MLambda
> 	v = -24.309 MLambda
> 	uvrad = 24.3547 MLambda
>
> The X-band values are typical for this kind of session (VLBI). But, the expected S-band values in this case are the X values divided by the frequency ratio (X/S = 8.646/2.309 ~ 3.744).
> The real ratio here is around 14.1 (which is almost (X/S)^2 ???).
>
> I suspect a frequency issue.
> But where should I look first to control my processing? Did someone already report such a problem?

Can you look at files from earlier in the processing to see when this 
issue arose?  I changed many tasks to shift the frequency reference to 
the center, but I see that you have not done that.  This shift does 
require a re-scaling of the uvw's but a very minor one.  Were the X and 
S data in the same file at one time?  There could be an issue then if 
the header had the X band frequency because the u,v,w should then be in 
wavelengths at X band even on the S band data.  Then, splitting out S 
band would lead to a really big correction and I wonder if it happened 
in some way twice???

ERic Greisen