To: John Swenson 
From: Douglas Braun 
Cc: atm{at}shore.net
Reply-To: Douglas Braun 


Hello,

Perhaps this is sort of a naive question..

By "astigmatism", do we really mean "off-axis
astigmatism"?  I do not see how any properly built and collimated
scope could have astigmatism in the center of the field (except for
off-axis designs).

Doug Braun

At 09:55 PM 12/17/02 -0800, John Swenson wrote:

>The Lurie Houghton design has essentially zero coma, zero spherical aberation
and an almost perfectly flat field, it does have a little astigmatism. The
basic geometry is a newtonian with spherical primary and a corrector right
before the secondary (the secondary is usually attached to the corrector).
The corrector consists of two weak lenses that are NOT achromats, the same
glass type can be used for both lenses.

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