Re: ATM Astroscan Houghton

["Bob May" <bobmay@nethere.com>]

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