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Re: What Is F/10 Mean?

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Posted by John Glenn/">John Glenn on April 29, 1998 21:08:35 UTC

: : : : I'm about to enter the world of amature astronomy and : : : buy a new telescope. I'll probably start out in the : : : low end of things. I've been looking at Meade's 8" : : : LX10. I'm a little confused after looking at all of : : : the different telescope specs. What is focal ratio? : : : Does a larger focal ratio mean better?

: : : Thanks

: The f number or is the focal length of the mirror. For : example it you purchase a 10" diameter mirror with an F5 : focal length, the point at which the light collected by : the mirror would be focused would be 50" from the mirror. : Check out this web site: :

: Good seeing

The notation "f/10" is read "F Ten" and refers to the "F number" of the optical system. The definition of "F number" is the ratio of the focal length to the diameter and is directly proportional to magnification. A larger F number telescope has a larger magnification, given the same eye piece. For example, an f/10 telescope with a 6" mirror would have a 60" focal length.

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