From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


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{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.00 MP