Mirror FL

Is there a quick and simple way to determine the FL of a mirror? I'm working on a small telescope and just want to rough in the approximate tub length.

I think I remember that you could use a drill bit and straight edge, but that seems not tobe the best idea to me. Steel against the surface could be a problem not to mention that I don't rember the formular!

Thanks for your time and efforts.

Scott

Depends on how accurate you need it.

The saggitta measurement (drill and straightedge) is a little fickle, since it requires very accurate measurement of small distances to get an accurate focal length.

For most accurate, set up a knife edge and slit light source like a Foucault Tester.

A much easier way would be to have you and an assistant go outside on a sunny day with the mirror and a tape measure. Reflect the sun onto a wall. Move the mirror in and out until the sun is as small as it can get. Measure the distance between the mirror and the wall. That is the focal length. Note that the images will be at a slight angle, which can be minimized by catching the sun from just outside the shadow of the wall. (Be careful you don't keep the sun focused long enough to start the house on fire. Incidentally, if you have a trail of ants going up the side of the wall, and a coated mirror you can be a telescope maker and a bug exterminator at the same time!) (You can also do this with the moon, but it doesn't kill bugs very well.)

A third way involves setting a lamp next to a wall. Use the mirror to reflect the light of the bulb back against the wall. When you see the lightbulb in focus, you are at the Radius of Curvature, which is twice the focal length.

On any of these tests, if the mirror is polished, you should see something. You can wet the mirror to improve reflection. If the mirror is coated, no such thing is necessary.

Alex