Re: [ATM] two mirror folded Newtonian

["Everett Cairns" <ecairns@mcmaster.ca>]

----- Original Message ----- 
From: "Mike Lockwood" <melockwo@uiuc.edu>
To: "atm" <atm@atmlist.net>
Sent: Monday, March 06, 2006 12:16 PM
Subject: Re: [ATM] two mirror folded Newtonian


> Hi,
>
> Peter wrote:
>> Here is a scope you should check out. This is an 18" f/8 two mirror 
>> folded
>> design which is well baffled.
>> http://www.johanneswilm.org/mike/telescope/

    One thing of interest on this page is the folded Schmidt (first proposed 
circa 1931).If you make the fold 90 degrees and mount the scope as an 
altazimuth with horizontal axis along the eyepiece (or film holder) axis  of 
the tube you have a "geriatric" mount where the eyepiece remains horizontal 
.(for those of us who no longer "fold" in the back) For a photographic 
instrument, the whole would need to be set on an equatorial platform. The 
Schmidt would retain the distinct advantage of offering access to the film 
position, the original intent of the 1931 suggestion, where incidently a 
much larger fold angle was suggested, to the limit allowed by good baffling.
     A 12 inch f8 FLATFIELD Wright constructed in this format would be 
OPTICALLY, but not mechanically,better than Meade's new $6000 "Ritchey 
Chretien" partially from better geometric correction and more so from the 
much smaller obstruction of the optical path, caused in the Wright by the 
central hole in the fold mirror and in the Meade by the secondary. The Meade 
instrument has a curved focal surface. The entrance section of the Wright 
can be made long or very short depending on how aspheric you wish to make 
the primary. The corrector in the Wright is very forgiving of axial 
positioning, and less sensitive to angular misalignment than a Schmidt 
Cassegrain. This would seem a possible amateur instrument if one is up to 
figuring the humungus diagonal! Failing the courage, you could make it as a 
Wright Newtonian with only a minor increase in obstruction.The short tube 
version could be constructed to support the small diagonal on the corrector 
plate.
    It is a shame that the commercial scope maker is largely constrained by 
the perceived requirement that the average buyer wants the scope to be not 
only be portable, but also fit on a coffee table. Oddly Meade has decided 
that their customers will tolerate lifting a 96 pound instrument (feb 2006 
Sky and Telescope).(maybe it is to spend most of its life on the coffee 
table)
    This is why it is so much fun to be an amateur telescope maker.You can 
make anything you want without worrying about what the average user needs or 
wants!

    Everett Cairns 

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