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Re: Focal Length

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Posted by John Suback/">John Suback on December 4, 1999 16:00:19 UTC

: This is my first attempt at grinding a mirror. I used "Amateur Telescope Making 1" by Albert G.Ingalls as a guide. On page 112 he describes the "spit test" as a rough guide to focal length. I used this method but at the end of fine grinding I noticed, that with a wet mirror I could focus sunlight, and that this focus is much less than with the "spit test". Am I correct in assuming that this (the sun) is the true focal length and if so will my mirror still work? It looks like it will have a focal length of about 4.6. If this is not acceptable what can I do to correct the problem?

There is a Formula in Jean Texura's (

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