Re: [ATM] two mirror folded newtonian

[Mike Lockwood <melockwo@uiuc.edu>]

Shawn,

McHatten, Shawn wrote:
> 	Our Centre has recently acquired a 2in thick 12.5in f8 mirror.
> We are looking at building a truss tube scope out of it but a normal
> Newtonian design will push the Zenith observing height to around 8ft. A
> little high for our liking. I suggested that we look at the folded
> design that I have seen a couple of you working on. 
> 	Did a quick wander through google, our archives and the ATM ring
> and only found one hit with two mirror folded Newt. Most involve a three
> mirror (primary, secondary, and tertiary) design. If anyone has worked
> on the two mirror folded design can you drop me a line with URL if you
> have a website up. 
> 	Not even sure if this is considered a "folded" design since all
> we are doing is positioning the secondary at something less than 45
> degrees to push the eyepiece down the OTA but that's what I've seen you
> guys refer to it as. 

OK, I might get chastised for suggesting this, but I will anyway.....

The main reason for using a 12.5" F/8 mirror in the first place is to 
have a long-focus system with small central obstruction.  To markedly 
reduce the eyepiece height, it is unavoidable that you must increase 
the central obstruction.  (A 1.5" secondary would probably work in 
that scope, and thatis a NICE small mirror.)  As the builder of a 10" 
F/8.8, I greatly enjoy the benefits of the small obstruction, and it 
is kind of a shame to increase it.  Also, if you fold it, you can have 
a funny angle when viewing near the zenity, causing neck pain, and 
possibly requiring the use of another diagonal.

If you want lower eyepiece height, it might make sense to try to trade 
the mirror for a shorter focal length mirror, and if possible, one 
that is thinner.  The trusses will be shorter, the scope will be 
lighter and easier to handle and build, and assembly will be easier. 
Also, a thinner mirror will equilibrate more quickly and provide 
quality images throughout more of the night.  A thin 12.5" F/6 would 
be ideal.  One or two steps on a small stool would be all that is 
required to reach the eyepiece at the zenith.

Just want you to consider all options.

	Mike Lockwood


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