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

[Don R Surles <Don.R.Surles-1@usa.dupont.com>]

My experience with 6" f-8 and f-10 scopes:  the image scale of the f10 is 
much better for planets/moon/bright objects and is much more pleasing than 
the f-6 &, f-8.   Neither  of them is suitable for widefield viewing. 

Spherical mirrors are ok for most nights and low  to medium power.  But on 
nights when the seeing is good and more power is needed there is nothing 
better than a 1/8 or 1/10 wave system (that includes the secondary).

I prefer long focal length over barlows every night.

Don...





"Jonathan Lavoie" <atm@lavoielactee.net> 
Sent by: atm-bounces@atmlist.net
06/14/2005 11:00 AM

To
<atm@atmlist.net>
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Subject
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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