Mirror f ratio

Hi I have a 12.5" mirror. Is there a quick and simple way to determine if it is an f6 or f5?
Thanks
Ivan

you need to try and focus on something to measure it. I would bet its f5.
jim

Many ways, this one works fine: set it e.g. against a wall. Take any flashlight (little MagLite works good) and a piece of paper and aim it at the mirror from about 11 feet away. There will be a bright image of the light on the paper. Move in and out until you get a sharp image at same distance of the light. Measure that with a tape. Half that is the focal length. 62.5 in would be f/5; 75 would be f/6. This is a Very Good measurement and can be used to build the tube etc. with no unpleasant surprises. The same technique can be refined to become the "Foucault Test." I always built this "finder" into my testers. Beauty is you never even have to go dark, so can use it in a lighted room (kitchen, office, lab) without bothering anybody. Tom Dey