[ATM] Re: Off-centered Mirrors?

["Cross, Timothy" <TCross@NRCan.gc.ca>]

Don,
These off-axis Newts are currently made by DGM Optics
(http://users.erols.com/dgmoptics/), and Orion has just joined the fray with
a 3.6" (or 3.8"?) off-axis reflector
(http://www.telescope.com/jump.jsp?itemID=77066&itemType=PRODUCT&path=1%2C2%
2C4%2C8&KickerID=429&KICKER); these are all presumably made by cutting
primaries from large, parent parabolic mirrors (i.e. you might cut 4 4"
mirrors from a 10-12" parent mirror.  Thus they are rotationally asymetrical
(like a concave wedge); I expect that it would be very difficult to make one
on its own.  This is a very different design than is used in
Schiefspiegler-variant telescopes (see
http://www.seds.org/~spider/scopes/schief.html for further details), which
use traditional concave primary mirrors and tilted secondaries; some may
also use corrective lenses and/or flexed mirrors to correct for aberations,
such as coma.  This is how I understand the situation in any case; please
feel free to correct this anyone, if incorrect.
Regards,
Tim Cross


------------------------------

Message: 21
Date: Sat, 23 Oct 2004 15:54:59 EDT
From: DHA352@aol.com
Subject: [ATM] Off-centered Mirrors?
To: atm@atmlist.net
Message-ID: <9.353451d1.2eac1113@aol.com>
Content-Type: text/plain; charset="US-ASCII"

Has anyone tried an off-centered mirror?  I will try to explain:   Imagine a

22 inch f/6 Newtonian, mask it down to 10 inches.  Now move the  mask near
to 
an edge such that the shadow from the secondary and its support is  not on
the 
mirror.  Now the hard part (thus my question), make only the  unmasked 
portion of the mirror.  Voila, an unobstructed scope without  astigmatism.
The 
"Center" for COC grinding would be 2 inches off  the  glass. etc.

Don

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