ATM Forum Message Forums: Atm · Astrophotography · Blackholes · Blackholes2 · CCD · Celestron · Domes · Education Eyepieces · Meade · Misc. · God and Science · SETI · Software · UFO · XEphem
 Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...The Space and Astronomy Agora RE: Help,ceramic Tiles Focal Length Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response ToPosted by Rick Crockett on August 3, 2000 18:24:12 UTC

Hi:

You can use drill bits to determin the depth of curve know as Saggita. Shimms would be better. Do not use bits once into polishing because it is too easy to scratch the surface.

My version of the formula is this:

Fl=D^2/16S or S= D^2/16Fl

D=8", Fl=40" so S=8"*8"/16*40"=64""/640"=1/10th inch or 0.100" for Saggita.

I have a wooden box that I an set the primary in and near one corner it has a 2x2" pole with a sliding cross arm made of cardboard. If neccessary I soap up the mirror (in the rough grinding stage) and reflect an image of the sun onto the cardboard. Measuring the distance from mirror to focus gives me the focal length. When you have a good polish the Foucault method will give you an extremly accurate measurement for Radius of Curvature which is just twice the focal lenght.

Have Fun,
-Rick