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

[jsgrmfg@gracemfg.com]

> >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

   But remember that this is intended as a planetary scope.  He needs every
last bit of contrast and resolution that can be squeezed from a 6"
aperture.  Accuracy criteria that may be more than adequate for deep sky
observers fall short of the mark for the dedicated moon and planet watcher.
Only the highest quality optics can take full advantage of those wonderful
split seconds when our busy atmosphere takes a break and gives us a glimpse
of an unblurred universe.

Jim Sturtevant

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