On Tue, 07 Sep 2004 13:53:13 GMT, Chris L Peterson
wrote:

The standard formula for pixel scale (in arcseconds) is 206265 * d / F , where d
is the pixel size and F is the focal length, both in mm. In the case of Venus,
taking a width of 60 pixels gives a pixel scale of 19.6 / 60 = 0.33
arcsec/pixel. Plugging that into the rearranged formula you get F = 206265 *
0.0074 / 0.33 = 4625 mm. (4676 mm for the Saturn image). Your focal ratio, of
course, is just 4625 / 80 = f/58.

Thanks Chris. That's what I needed! :-)