Nice mirror, might be interested myself.
Forgive my dodgy memory of this stuff, been a while since I made my mirror.
For a true focal length measurement you need an 'infinite' distant light source, the Sun is perfect but
very dangerous with a coated mirror this size! so
don't use the sun!
I used to use the Sun but only when the mirror was uncoated polished glass.
I think(again, dodgy memory) with a torch or some other light source, the focal point shifts depending on how distant the light source is.
If you adjust the light source to get it and the focal point the same distance from the mirror(it's centre of curvature), you then halve the measured distance to get the true focal point distance(the right distance for parallel-ray light source like stars) and then divide that by the mirrors diameter to get the f-ratio.
Another way is to measure the sagita. To do this you need to measure the depth to the centre of the mirror in relation to the edge.
This is done with a flat bar placed across the mirror and then measure using a micrometer depth gauge, then plugging the numbers into a formula, which is F=r^2/(4D) where F=focal length, D=depth and r=mirror radius.
Ofcourse there is the danger of damaging the mirror edge/coating etc with the flat bar and depth gauge.
Someone please correct me if I've messed that up!
but yeah it's 355mm accross x 30mm thick.
