[Comm2011] astrometric solution

[John P. McFarland]

Hi Koen,

Excellent news!  I will get new values based on the new data and get them to 
you as soon as I can.

Cheers,


-=John


On Fri, 5 Aug 2011, Konrad Kuijken wrote:

> Now that the rotator issue has been traced to the software that combined 
> the information from the encoders of the rotator, we have a workaround 
> (only use one encoder instead of the four existing ones) that seems to 
> work fine. No more backlash, and the angular preset is now correct and 
> predictable.
>
> We're close to having an astrometric solution that is good enough for 
> automatic guide star acquisition - that will be a nice milestone. The WCS 
> that is in the image headers at the moment is good to about 25 pix all 
> over the field, including the aux.CCDs.
>
> If you overplot USNO stars over a full science array in skycat you still 
> see a radial pattern of residuals, up to 25 pix in the outskirts. This 
> implies that the plate scale is wrong by about 50/16000, ~ 0.3%. [NB there 
> was a bug in the header writing up to now - the CD values only had 3 
> significant figures. Not quite enough to explain this effect though].
>
> However, on individual chips the residuals change by about 5 pixels over 
> the length of the CCD, implying the plate scale is wrong by 5/4000, ~ 
> 0.12% . So apparently there is an issue both with the crpix values and 
> with the CD matrix.
>
> Could someone look into this and provide updated values? I think we can 
> assume here that the plate scale is constant over the full array, but it 
> needs to be determined together with the proper crpix's for each CCD. Once 
> this is set we are in business. (science frame and AG stars).
>
> I put a frame (OBS217_0011) and a simulataneous full-frame readout of the 
> AG CCDs in
>
> ftp://ftp.strw.leidenuniv.nl/pub/kuijken/ocmwcs/
>
> Thanks
>
> Koen
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