[fitsbits] Re: Q: Determining coordinates

[Stupendous Man]

  Marten Blixt <maarten at blues.phys.uit.no> wrote:

> 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.

  This isn't a very easy problem, even for astronomers.

> Is it common to assume rectangular coordinates inside the narrow FoV often
> used among astronomers?

  Often we do assume a projection of the spherical sky onto a flat
detector which yields very nearly rectangular coordinates, yes.
This is simple if the picture is taken near the celestial equator,
where RA and Dec are nearly rectangular, but it becomes complicated
as one moves towards the poles.   Your pictures are very probably
near the North Celestial Pole, _and_ they are 13 degrees wide --
a double whammy.  The overlay will differ greatly from rectangular.

> 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?

  This is going to be difficult.  I recommend you do this: 
First, print out your image onto paper.  Next, find
a good planetarium program, such as SkyMapPro, Voyager, TheSky, etc.
Use it to display on your computer's screen a region of the sky
which matches your picture, as nearly as you can.  Tell the program
to display a grid on the screen.  Print out the display onto paper,
then photocopy onto a sheet of clear plastic transparency (or print
directly to the transparency).  Fiddle with the size of the display
until the transparency matches the image on paper.

  Voila.  An analog solution.  I suspect that this will be much 
quicker and easier than a digital solution.

-- 
-----                                      
Michael Richmond                   "This is the heart that broke my finger."
mwrsps at rit.edu                     http://stupendous.rit.edu/richmond/