Re: [ATM] Re: 25" mirror possibilities

["Jean-Guy Moreau" <jgmoreau@cablevision.qc.ca>]

Thanks again Vladimir for all the trouble  ;O)
I might as well keep on figuring the 26" f/3.7 mirror i am
now making. When used with a Televue Paracorr it should
give good images, as i was able to verify with the
smaller 10" f/3.3 i am now using.

Best regards,

Jean-Guy

p.s. i keep your design in file just in case...

----- Original Message ----- 
From: "vladimir sacek" <vladis.2@juno.com>
To: <atm@atmlist.net>
Sent: Thursday, November 18, 2004 3:39 PM
Subject: [ATM] Re: 25" mirror possibilities


> "...The primary is 610mm f/2 if that can make things easier..."
> 
> Will definitely be easier to make :) I thought it's 30". Also, will
> reduce linear astigmatism in proportion. Linear astigmatism (smallest
> blur diameter) in aplantic two-mirror systems is given by:
> 
> A=Da^2/{[m(2m+1)+B]/[2m(1+B)]}
> 
> where D is the aperture, "a" is the field angle in radians and the rest
> of parameters as before (secondary magnification "m" is negative for
> Gregorian,
> and so is the ray height "k" on the secondary, used to calculate back
> f.l. B).
> 
> The primary conic gets somewhat lower with lower secondary magnification,
> but
> not to the extent that it would make a real difference.
> Since median field curvature in aplantic two-mirror telescopes is zero
> when [m^2-B(m-1)] is zero, 
> the field can't be flat with practical secondary magnifications of about
> |2| and larger, and B of 
> about -0.5 and larger (towards zero). However, field curvature is
> moderate, and can be corrected 
> (together  with astigmatism) with a weak single positive lens near the
> image plane. So large secondary 
> is not really necessary.
> 
> One thing you might consider is to fold the design, by placing a small
> diagonal flat in the focal zone
> of the primary. It would reflect light out to the side, where would be
> located secondary mirror. 
> This diagonal flat would be between 1/2 and 1 inch in size (minor axis),
> depending on 
> secondary magnification and location, for ~40mm field. The tube would be
> shorter, and the spider
> much less massive. Effective c.obstruction (by the flat in the cone
> converging from the secondary) would be 
> approx. 15-20%. This setup would allow for lower secondary magnification
> (about |2| minimum) for 
> accessible image, and lower, more stable mount.
> 
> The only - but serious - drawback is that it would definitely make
> collimation more difficult.
> 
> Vlad
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