Carlos Moreno wrote:

Given that I don't see any replies, I'll go with a possibly uninformed
and irresponsible suggestion :-)

I'm assuming that you have the convex mirror in your hands (as opposed
to inside the telescope and without access to it).

If you have a visible laser, you could point it to the mirror, parallel
to the optical axis, and try to measure the [maximum] angle of
reflexion. With the angle of reflexion and the radius of the mirror,
you obtain the focal length: f = radius / sin(angle)

Of course, the tricky part is that the laser has to hit the mirror
perfectly parallel to the optical axis -- that might not be too
hard to do. To obtain the maximum angle of reflection, you could
maybe do such that the reflected beam hits a plane surface (e.g.,
a white wall) that is parallel to the mirror (perpendicular to
the mirror's optical axis). Then, move the vertical (or horizontal)
position of the laser, and measure the distance between the two
points at which the reflected beam disappears. Something like this:

_ -| ---
_ - | ^
_ - | |
_ - | |
____________________ | |
\ ___________________ | |
|------------------- -- laser | Dmax
/_------------------- | |
- _ | |
- _ | |
- _ | v
-| ---
|


Actually, that way you don't even need to calculate the angle;
you obtain the focal directly because Dmax divided by the distance
between the mirror and the wall is equal to the diameter of the
mirror divided by the focal length.

Don't know if it is easy to implement or useful given any
constraints you might have, but it's something that comes to
mind.

HTH,

Carlos
--

I will think about your suggestion

I foud a method in this link, on the bottom of the page

http://www.astronomydaily.com/atm/focal.asp

But i'm not shure that this method is OK. May be my english is not ok.
What do you think about that ?