Re: [ATM] RC with multiple primaries

[Jim Burrows <burrjaw@earthlink.net>]

At 2004-11-02 23:14 -0500, matt tudor wrote:

> >Matt: why do the primaries need to be identical to sub-wavelength accuracy?
>
>for the same reason all zones of a single piece primary mirror needs to have
>a common focal length and not several. The multiple mirror is no different .

This is an interesting topic.  I think Matt is wrong here - the difference 
between a segmented mirror and an ordinary mirror is that the segments can 
be moved to correct for differing focal lengths (among other 
errors).  Consider a perfect parabolic mirror surrounded by an annulus of 
another perfect parabolic mirror with a different focal length.  Geometric 
optics says that if you move the center mirror until the foci coincide, 
you'll have a stigmatic mirror.  I won't hazard a guess what would happen 
off-axis in this case <g>.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84) 

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