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


Rich, I really don't quite understand the comment on the shadow entering
from the left and going right rather than from the outside going in.  With
any zone, the shadow does indeed enter from one side and go to the other
side of the half moon zone aperture if that is the way that the KE is
going. There should be a point at which both halves of the zone do the same
thing at the same time and this is the balance point for the zone
indicating the ROC of that zone.
If your mask has more than about 45 deg. of angle on the cutout part, you
will be seeing parts of the zone which are not of any consequence in the
measurement as the real part of the test is on the axis of the mirror and
not over the full area of any zone.  Thus, any mask opening which is more
than 30 deg. of angle is really seeing more than is needed. FWIW, I don't
use the Couder ratio of constant area zones which TEX and others use as
this leaves a huge central hole which is of not much use as it has some
definite shape to it.  Use a much smaller center and add a zone if needed
to make the zones not so wide.  I normally use a 5 zone mask and the radii
for the mask on a 8" are 1", 2", 2.7", 3.3" and
4".  This gives me decent sized zones which can be measured without
problems and a decent sized central hole that can be easily set. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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