Re: [ATM] Re:(ATM) F/10 or F/8?

[Michael Peck <mpeck1@ix.netcom.com>]

At 08:44 6/14/05, jsgrmfg@gracemfg.com wrote:




> >Can anyone tell me if there would be any advantage in making a mirror
> >in an F/10 vs. a F/8?  Seems like everything is a an F8.  This will be
> >for a 6" mirror for primary use in a planetary scope.
>
>   F/8 is common and often recommended because it is the longest focal ratio
>that can be tested easily by a beginner.  An F/10 is too short to leave
>spherical but so close to a sphere that the Foucault shadows are very

A 6" f/10 sphere is diffraction limited by any reasonable criterion. A 6" 
f/8 sphere falls just short of satisfying the Marechal criterion, and also 
fails by varying amounts on other, older criteria.

Mike Peck


------
Michael Peck
mpeck1@ix.netcom.com

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