Re: [ATM] Re: Aplantic Gregorian

["matt" <mariusrf@bellsouth.net>]

-----Original Message-----
From: vladimir sacek <vladis.2@juno.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Sunday, November 28, 2004 10:32 AM
Subject: [ATM] Re: Aplantic Gregorian


>Tom Krajci wrote:
>
>Jarvis probably has in mind relations for primary conic needed for
>Gregorian aplant and the distances to its foci. Primary conic for
>Gregorian aplant is given by
>
>K= -1 - 2(1+B)/(m-B)m^2
>
>with "B" being the back focal length (primary-to-final-focus separation)
>in units of primary's f.l. and "m" the secondary magnification. Back
>focal length is positive for the final focus behind the primary, negative
>for
>final focus in front of primary, and zero for the final focus at the
>primary.
>So if you go for an arrangement with B=0 and a diagonal in front of the
>primary,
>the relation reduces to:
>
>K= -1 - 2/m^3
>
>Important thing to remember is that secondary magnification "m" is
>negative
>for the Gregorian.
>
>Relations determining foci positions for an ellipsoidal mirror in terms
>of its
>conic K and r.o.c. R are:
>
>near focus distance = R/(1+sq.rt.|K|)
>far focus distance = R/(1-sq.rt.|K|)
>
>|K| stands for the absolute value of K; you get it simply by neglecting
>the minus sign.
>
>In order to make testing at the foci practical, the far focus needs to be
>close enough
>to enable a practical/workable setup. Plugging in the numbers, you'll
>find out that
>for any workable telescope arrangement K falls between -0.9 and -1. Even
>with K=-0.9, the far focus for, say, 12" f/3 primary would be nearly 39
>times the
>radius (some 78 yards) away from the mirror. For K=-1 (parabola) the far
>focus is
>at infinity, and the close focus is at R/2.
>
>Obviously, testing of Gregorian - classical or aplantic - primaries at
>their foci is not practical.
>
>Btw, there was a question what are the terms for two-mirror systems in
>regard to
>coma correction. It may have been answered but, just in case, coma
>corrected systems are
>"aplantic" and those that aren't (with parabolic primary) are usually
>called "classical".
>
>Vlad
>_______________________________________________
>ATM mailing list http://www.atmlist.net/

I have not seen yet any reference to a Gregorian Mersenne arrangement . Is
there any book, article, publication that treats it in detail ? I found
historical references, and short of simulating it myself in Oslo is there
any way to avoid reinventing the wheel ? There are large professional
Gregorian instruments that are used in Mersenne configuration, but besides
pretty pictures very little in the way of optical prescriptions or clear
details is available on their websites. I have done a complete ATM list
archives search and only came up with a few relatively well known and little
talked about scopes, like Clyde Bone's plus a couple of older ones . Clyde's
scope was apparently featured in the September 1999 S&T issue, but other
than that, no other web references with images, data etc.

best regards,
matt tudor

_______________________________________________
ATM mailing list http://www.atmlist.net/