From: Jan Bentz 
To: Bob May 
CC: atmlist 
Reply-To: Jan Bentz 


There seems to be some confusion about aplanat.  An aplanat is any optical
system corrected for both spherical abberation and coma.  An anastigmat is
an aplanat corrected for astigmatism.  They may or may not
  have "flat fields".  Any cassegrain type may give a flat field if the
secondary and primary are approximately equall in radius.  They cannot be
exactly equal if there is much astigmatism.

Jan Bentz

Bob May wrote:

> I don't know what really can be considered professionally as an aplanet but
> I would suspect that a optical system where the focal plane is flat enough
> to keep the aberration within the Airy Disc would be considered good enough.
> When you are discussing focal plane curvatures of miles, the Sag. of the
> curve of that focal plane is on the order of extremely small!   My
> calculator just refused to give a value of the sag for a 1" radius spot for
> a 1 mile ROC curve.  It just said 0.
> Bob May
> http://nav.to/bobmay
> bobmay{at}nethere.com
> NEW! http://bobmay.astronomy.net
>
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