: Pretty close. Man, that's a small mirror. Most mirrors I have worked with have a diagonal of 20% of diameter or less. In your case you'll want less. 3 5/16 mirror at 20% is .66" How did you find the radius? I have taken mirrors with unknown focals and use the "flash the light" on a wall method. I use a flashlight, shine it in the mirror and the reflection on a wall. When the element of the flashlight focuses on the wall (by moving the mirror back and forth, use two people)Then I measure the distance from the mirror to the image on the wall and divide by two. I've been pretty lucky that way. It works. As for fast focus. I believe your mirror qualifies. 3.3" mirror at f/8 is fast. I'm sure Robert May will correct me if I'm wrong.

Hey Kip, yes it is a small mirror. Trying to get a couple of grandsons interested in ATM. If it takes hold we will build a 6", maybe a 8" I used the card and light method to find the ROC. Checkout http://www.atmpage.com/focal.html I have another mirror, smaller than this one. I will try your method when we check it out. Thanks for the come back! YARB

• Re: Are my figures correct? - bobmay - September 19, 1999 - 16:28 UTC