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Re: [ATM] Re:(ATM) F/10 or F/8?
----- Original Message -----
From: "Michael Peck" <mpeck1@ix.netcom.com>
To: <atm@atmlist.net>
Sent: Tuesday, June 14, 2005 10:48 AM
Subject: Re: [ATM] Re:(ATM) F/10 or F/8?
> 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.
>
Why don't we make ellipses enstead of spheres for slow F/ratio ? It is as
easy to do by just moving the light source near the mirror then work the
same way as making a sphere. Correcting half way to the parabola would make
it well inside the diffraction limit.
Jonathan
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