Re: [ATM] RC with multiple primaries

["matt" <mariusrf@bellsouth.net>]

-----Original Message-----
From: Julio Sanchez <jsanchez@skipanon.com>
To: matt <mariusrf@bellsouth.net>; atm@atmlist.net <atm@atmlist.net>;
atm_ken_hunter@yahoo.com <atm_ken_hunter@yahoo.com>
Date: Tuesday, November 02, 2004 10:09 PM
Subject: RE: [ATM] RC with multiple primaries


>Hi Matt and Ken, thanks for the feedback. The MMTs that I have looked at
are
>different from what I described. For example, "the" MMT consists of several
>full telescopes with a common focus. This is different from having several
>primaries and a single secondary in a kind of Schiefspiegler array.
>

Keck is a multiple segment primary that uses a single secondary , the way
you described your project.
The only difference is yours is a subset of just one ring instead of several
rings . Keck has the segments warped and actively supported in place with an
accuracy of a few nm , and the position is corrected continuously .
For an amateur this is non trivial due to complexity. You'd need a Shack
Hartman wavefront sensor, a custom software to calculate Zernikes and
wavefront correction in real time and large but extremely precise
nanoactuators . Even very small nanoactuators cost hundreds of dollars each,
so how much would this project cost ? It appears beyond the budget of an
amateur, unless Bill Gates becomes interested in astronomy .

>Matt: why do the primaries need to be identical to sub-wavelength accuracy?
>If the focus point is not defined to this level then it seems that if there
>are small variations in the focal length of the primaries the system will
>still come to a common focus.
>


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 .
A good comparison would be the segmented primary is just like a large single
primary mirror with a large and oddly shaped spider , or like a  single
primary with a Hartman mask . A half wave error between the left and the
right side segments  would still produce the same effect as a half wave zone
in a regular scope . Light doesn't care whether it bounces off identical
parts of the same mirror and there's a spider in the way, or identical parts
of several segments and there's no spider. There'll still be diffraction due
to mirror edges and spacing, there will still be the same surface
requirements for the whole system as if it were a single mirror.

best regards,
matt tudor

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