How do you calculate effective FL/FR based on object pixel size?

On Tue, 07 Sep 2004 09:50:48 GMT, Drew wrote:

Can someone tell me how I can reverse engineer the following to figure
out effective focal length and f/ ratio?

I did some eyepiece projection astrophotography yesterday morning of
Venus and Saturn on a variable adapter.

The eyepiece is a 9mm 45 degree Ortho 58-70mm (variable adapter) from
the Canon Digital Rebel focal plane (CCD is 0.625 factor of a standard
35mm frame, the pixels are 7.4 microns I think, CCD size is 3072x2048
pixels).

Telescope is the Orion ED80 80mm f/7.5 telescope.

Comparing several images, the pixel sizes came out to this:

Venus pixel size: 45-60 pixels
Venus angular size: 19.6"

Saturn pixel size: 40-53 pixels
Saturn angular size: 17.3"

Well, as you can see from this planets are not very good objects to use for
calibration since you can't easily determine their edges. If you shoot a star
field you can do this much more accurately.

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.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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! :-)