I need some help here from all you experienced mirror makers out there. The other day I was able to focus the suns light on the side of my shed to check the focal length. Well, I didn't see the sun today and there isn't any right now around my place. So, I am trying the old set it up in the house trick where it is nice and worm. I cut a hole in a 8 1/2 by 11 card stock and I sat my mirror up end on a bench so the face was pointing down the hall. My mirror diameter is 12.5 inches and my focal ratio is close to f7. So, if the radius of curvature is twice the focal length and that is supposed to be where a point source of light is supposed to return light to itself then we have 12.5 X 7 = 87.5 (focal length) X 2 = 175 inches (radius of curvature). I found the 175 inch location in the hall (heck, I was in the kitchen by then) I shot a laser pen light thru the hole onto the mirror face to see where the return light was going. I had to adjust the mirror face get the light to return to the card stock. Then I used an LED flashlight through the hole and I kind of got a focus + or - 6 or 8 inches. I didn't feel that it was very precise. Is there a way I can get better results with this method? Jim --------------------------------- Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, and more