Re: Off-Axis (Zero Obstruction) Reflector Telescopes

From: Dan Chaffee (dchaffee_at_blitz-it.net)
Date: 10/04/04 

Date: Mon, 04 Oct 2004 06:00:40 GMT

On Sun, 3 Oct 2004 20:01:19 -0400, "Dan McShane" <dgmoptics@erols.com>
wrote:

>
>Yes certainly the premium mirrors (CZ, Royce Optical, etc) will work, but
>for instance to make the OA-6.5 we start with an 18" f/3.6, not an f/4.5,
>which further raises the bar for required smoothness of figure. Try finding
>someone to make a 98-99% Strehl, 1/45th wave RMS 18" f/3.6.
>Most of the premium mirror makers have no interest in working at those low
>f/#`s.

The main the reason some large apertures give better images when
stopped down is that the RMS wavefront contained in the masked area
is far and away better than the whole surface. My first attempt at
mirror making resulted in a 6"f 7.3 hyperboloid with a pathetic .5
wave of overcorrection. Star testing it with a 2" mask showed a
nearly perfect star test---certainly a high .90's strehl. My point
is that a small section of a much larger mirror will almost always be
DRASTICALLY better than the whole surface, unless a rolled edge
or excessive roughness is included in the section.
A champion wavefront on a large, fast mirror is not necessary for
getting a very good idea of unobstructed performance from
30-40% of the original aperture, provided the mirror's edge zone is
not included in the mask and it's not too rough. A 16" f/4.5 with
a reasonably smooth surface and overall strehl of .7-.8, if even that
good, would easily do for a 6" unobstructed mask.

Dan Chaffee

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