On Tue, 05 Aug 2003 15:48:43 -0400, "André P."
wrote:

I have a cassegrainian miror kit but I don't know the negative focal lenght of
the secondary convex mirror. Someone have a method to find it?

If you know someone who has a spherometer he can measure the radius of
curvature for you. The focal length is 1/2 the radius of curvature.