Re: [ATM] secondary size dillemma

[Hugues.Laroche@ses-astra.com]

Hi Tim and all,

>If I was to go up to a 3.1'' secondary the figures
> would be 85.68% edge of field illumination, 0.44 degree
> or 15.37mm 100%illuminated field, and a 19% central
> obstruction. Should I bite the bullet <SNIP>

I would follow the advice of Bob May. Just try it
an see by yourself. As other have said, the tight
size might even be an advantage if your primary is 
less than perfect, and the eye will not see much 
difference.

Now if you intend to also make pictures with the
scope, you will likely end up by buying a larger
secondary because there the vignetting effect will
be obvious (though nuerical treatment of CCD pics
allows to marvalously cancel many nasty effects).

Re the 16 vs 19% obstruction effect on diffraction,
I guess it would take a very seasoned observer to
see the difference. Not me at least...

Hugues
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