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"f" rating of a telescope

autoexer

Waukesha, US
1128 posts

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autoexer Silver Member Nikonian since 27th Jan 2011
Sat 20-Oct-12 04:46 AM | edited Sat 20-Oct-12 04:49 AM by autoexer

i have a 114mm (4.5 inch) with 900 focal length for an f/8 (7.89) - 900/114= 7.89

i hade a shorter tube, i.e. 630mm it would be f/5.52. question - when i think of f-stops in a lens, the smaller the number the greater ability to gather light, a f/1.4 gathers more light than an f/3.5. This does not seem to be the case in telescopes. I can see the difference if i had a 900mm focal length and increased the mirror size to say 6 inch (150mm) then the f-stop would be 900/150=6, f/6.0. The 6 inch will gather more light in the same amount of time as the f/8 scope.

If it works the same, f/6 is faster than f/8, how can the focal length of 630mm gather more light given the same size mirror?

what am i missing here?

i did find this: http://www.chuckhawks.com/telescope_focal_length.htm

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