: 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?

Hi Mike:

Yours is f:6. That means when focused on infinity or anything else a relatively long distance away, the distance (EFL) between mirror and focus will be about 48". As the object is moved toward the mirror, the focus will move away. Object and focus pass one another at the radius of curvature which is twice the Effective Focal Length so 2x48"=96"

Have Fun -Rick

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