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Re: [ATM] Nasmyth secondary mirror cell ?

Don Anderson wrote:

>>Do what you can to keep your central obstruction close to or below 25%.
>>You're putting in a tremendous effort, you shouldn't give up contrast ...
>>    
>>
>I have been figuring obstruction percent on area not radius. By area it is
>13.22%, by radius or diameter it is 36.36%.  Which is correct?
>
For contrast considerations it is usually figured by radius (or 
diameter).  Less than 20% has minimal effect.  Some people consider 25% 
an upper bound.  S/C scopes are often >30%.

Jeff

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