Re: ATM Astroscan Houghton

["Bill Kelley" <flex@commspeed.net>]

Bob,
As you should have already read on the list, the Astroscan does not have a
corrector plate, just a window. The mirrors are parabolic. Your argument is
moot.

Furthermore, the center of rotation of a sphere is it's COC. Quite "unique"
I dare say . Tilting the mirror ( read "collimating" ) means the same thing
as maintaining an optical axis defined by the secondary in the Astroscan,
not repositioning the primary with respect to it's COC.
Off axis you are rotating it around it's focal point, and spherical or not,
it is out of collimation. My beginners classes make a lot of Newtonian's
with 5" F10 spheres - you'd have a hard time persuading them that
collimation is not important!

My 2 cents worth now spent. I'm retiring from the fray.

Bill Kelley




>
> Actually, Bill, the difference in surfaces is indeed important.  A
parabolic
> mirror has a center of rotation and that is only at one point and angle.
> That center of rotation must go through the center of the eyepiece for
best
> operation of the surface in the scope.  A spherical surface has no unique
> center of rotation and thus it can be merely put into the tube and crudely
> alligned to somewhere near the appropiate direction.  The corrector plate
> will reform the wavefront which will then be reflected back to the center
of
> that center of rotation of the surface of the corrector and that will be
> where the light is picked off and sent to the side for the eyepiece.  The
> full symetric image may not be present if the spherical mirror is really
off
> of collimation but the image presented to the eyepiece will still be
on-axis
> as the important part is where the focus is relative to the corrector
plate,
> not the primary mirror.
> Bob May
> http://nav.to/bobmay
> bobmay@nethere.com
> NEW! http://bobmay.astronomy.net
>
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