Re: Updating RA/DEC fits headers with true location?

[Arne Henden <aah@nofs.navy.mil>]

If you have the transformations to calculate RA,DEC from a given x,y
then you just use those equations with the center pixel as input.
It sounds like you are one step earlier, though; having matched
a set of stars with known RA,DEC and with CCD x,y how do you
determine the transformation?  I'd suggest looking at the
astrometry chapter in Berry/Burnell, The Handbook of Astrnomical
Image Processing, Willmann-Bell.  Good book to own in any case.
There is a nice set of routines in slalib to do the math.
Arne

Robert Creager wrote:
> Hey folks,
> 
> I need to start updating my fits headers with true RA/DEC location so manually
> viewing the images is actually useful.
> 
> So, I have a bunch of stars with known astrometric positions (RA/DEC) and known
> frame positions (x,y).  I'm sure there is a 'proper' way to calculate the
> center of the image, but I'm not much on spherical geometry yet.
> 
> I know using a simple interpolation solution would get me close, as would
> finding the closes two stars too the center of the x,y positions, provided the
> image wasn't rotated much (which could be accounted for).  But, I figure there
> is a better way.
> 
> Anyone?
> 
> Thanks,
> Rob
>