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Subject: [M]: Re: Curious question on f-stop
From: Michael A. Covington
Reply To: mapug@shore.net
Date: Mon Aug 27 10:53:46 2001
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While you're here, how about checking out the
Astronomy Book
List ? |
> At the risk of showing how ignorant I may be.....
> =20
> I was reading and came across a specification of the LX200 F/10 10". It
> stated that the focal length was 2500mm and the aperture was 254mm. =
> Does
> this not calculate to F/9.84 not F/10?.
It does. But it's approximate. The reason is that the focal length of a
Schmidt-Cassegrain changes appreciably as you focus it, because you focus by
changing the separation of the two mirrors. Your 8-inch f/10 LX200 will
probably range from 1800 to 2300 mm focal length depending on where you put
the focal plane!
Michael Covington
Author, Astrophotography for the Amateur
www.covingtoninnovations.com <><
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