Center Spot a Mirror?

Whats the most precise way to center spot a mirror?

Thanks,
David Pope

Physical center can easily be found using a compass and paper. Carefully draw a circle equal to the mirror's diameter, cut it out, enlarge the pinhole a little and mark the center with a Sharpie marker.

Place an adhesive/glue backed paper reinforcement (also known as a...ooops can't say that here) on the mirror to get a bigger collimation target. Some folks like to blacken the paper ring with a marker.

I use a router table to turn a plexiglass disk the same diameter as the mirror. A 1/4" hole in the center of the plexiglass serves as a pivot for turning the disk, and a guide for placing a black hard fiber washer in the center of the mirror. The washer is held with rubber cement. The plastic disk is a nice cover for the mirror when sending for recoating.

Here is a different way...with 2 elastics

Your mirror has 4 clips with 2 screws to each clip...

Think of the clips as at 1 o'clock (screw 1&2) 3 o'clock (screw 3&4) 6 o'clock ( screw 5&6)and 9 o'clock (screw 7&8)

put an elastic accross from screw 1 and 5 then put another elastic across from screw 3 and 7. Where they cross is your exact centre..Just put a dot with a permenant marker to mark the spot then put on a cirle paper washer.

I have used this method to check the factory center spot and someone elses center spot on a used scope..

Works great

Mark V