[Comm2011] astrometry information in raw frames / platescale

[Konrad Kuijken]

Hi all,
greetings from Paranal again.
A nice postcard for your enjoyment (I think it looks better with the  
chip boundaries than it would without, actually)



On the astrometry, I looked into the raytracing again, to check what  
the plate scale should be, and what the expected locations of the GS  
CCDs are.

Raytracing the 2-lens corrector, I get a plate scale of 0.21343, very  
constant from 0 to 0.7 degrees off-axis. We've been using 0.214x  - I  
cannot now reproduce where this came from (ADC?) but that is what  
Andrea has been writing to the headers. John's astrometry also gets  
this plate scale of close to 0.2134.

Implication is that all over the array, also on the aux ccds, the  
mapping is very close to linear between pixels and angles.

Also, the mystery of why the plate scale suddenly changed a couple of  
nights ago is solved: some of the data were with ADC and some without!  
I hould have thought of that earlier. That is what you get when you  
leave a Belgian in charge.

So it seems as if the raw frame's astrometry header information  
situation is now as follows:

* plate scale is stable (but different between 2LC and ADC, as  
expected). What is written to the header of the raw frames at the  
moment does NOT take into account whether ADC is in or out. It gives a  
radial discrepancy of 10" in the corner of the field (2LC), -5" (ADC).
*****Will need to be changed: write difft solution depending on ADC in  
or out.
* coordinates have an offset which can even be 50" in the center of  
the field - which I find hard to understand since the telescope DOES  
find its guide star, and pointing at a bright star DOES put that star  
in the middle of the array between the 4 central chips, within a  
couple of arcsec. The offset we see in the header astrometry is  
variable. We suspect that the coordinate for the center of the field  
that is written to the header is wrong. *****Needs to be investigated  
where this comes from.

Koen


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