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Focal Length Of Primary Mirror

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Posted by Mike on January 7, 2000 00:35:04 UTC

I am building my first telescope and purchased an 8-inch mirror with a reported focal ratio of F6. I have tried to determine the focal length of the mirror using a flashlight and a candle. With the flashlight or candle near a wall, I have focused the image of the element or flame on the wall next to the Light source. The distance between the mirror and wall has varied between 96 and 97 inches (I tried three times and found it easier to tell when the image was sharepest using the candle). I know the posts here indicate that the distance between the mirror and the focused image is the focal length of the mirror. I have read elsewhere, I think, that the focal length of the mirror is one-half this distance. Which is right, and do I have an 8-inch F6 or an F12?

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