[fitsbits] Re: Q: Determining coordinates

[Doug Mink]

Marten Blixt wrote:
> 
> This should be an ordinary task for astronomers, that said, it
> implies that I'm not an astronomer.
> 
> However, in the past I've taken some CCD shots of the
> night sky (to capture the northern lights) and now I need to
> overlay a coordinate grid on them.
> In my FoV (~13 deg) I've identified a few stars (thanks xephem!) so that
> I know the *exact* ra and dec for them, what I need to do then is
> to fit a coordinate grid to them.
> I guess that this is a pretty ordinary task for an astronomer, but
> I'm getting stuck in nomenclature and .....
> 
> I've found this starast.pro program (IDL Astronomy User's Library) that seems
> to do exactly that, but I'm unsure on what it really does (don't know what
> the Coordinate Description matrix contains, and I'm a total newbie when
> it comes to FITS).
> 
> Is it common to assume rectangular coordinates inside the narrow FoV often
> used among astronomers?
> 
> Does anyone know of routines that calculates the coordinatne grid from a
> known set of stars inside the image? Or have tips/tricks that could
> point me in the right direction?

WCSTools at http://tdc-www.harvard.edu/software/wcstools/ is a set of tools for
fitting and manipulating image world coordinate systems (WCS) which define the
relationship between sky and image coordinates.  The IMWCS program fits
a WCS to an image after matching a list of image star coordinates (X and Y)
to a catalog.  If you know both sky and image coordinates for several objects
in the field (I like at least 5), the program can use that match to fit
a tangent plane projection, the one most commonly used for imaging detectors.
Once you have a WCS, you can use an image display program such as ds9 or
SAOimage
to display sky coordinates as well as image coordinates while you browse the
image.  Links to several display programs which track sky coordinates are
on the WCSTools home page.

-Doug Mink (WCSTools developer)