How to measure focal length ?

[GrahamL]

Hi does anyone have a link or an expanation on how to measure the focal length of an unkown mirror? , the coatings are very worn .

I was thinking of turning it into a solar scope
8" and a 1.5 " secondary .I've seen quite a few how toos on this but do you think i can find a single one

cheers graham

[kinetic (Steve)]

One obvious way to get it within a few mm is to bodge up a mirror
cell on a long yardstick backbone.
Wet the mirror with water if the mirror coating is bad or
non existent...point to the sun carefully and slide a G-clamp with
some white card up and down the backbone board until the
5 cent piece sized focus is smallest.
But be careful...in full sun with a good coating, the paper
will burn with an 8" mirror or bigger.

Steve

[bojan]

Another way (and more accurate) is with point source (lamp in the cardboard box, with small hole on one side). For this you need a free horizontal surface (floor) more than twice the estimated FL.
Improvise the holder for the mirror in vertical position (but be careful not to let it fall on the floor, of course). Position the point course away from the mirror and try to focus the image on the same surface near where the pinhole is.
The distance between a hole (image) and the bottom of the mirror will be exactly twice the focal length .

[Ian Robinson]

If you have a straight edge and vernier caliper , carefully measure the sagitta (well the coating is stuffed anyway so wont matter if you mark the old coating when doing this).
Then go to this website http://stellafane.org/tm/atm/mirror-...g2fl-calc.html and you can get the focal length immediately.