Re: (ATM) Focal ratio and atmosphere

[OPTIXWIZ@aol.com]

Dear Dan,

There is a lot of confusion about this design. Basically, the all-reflecting
part is an afocal telescope.  The disadvantage to the design is that the FOV
vignettes. Significantly. How significantly, I couldn't say without careful
analysis. 

That a 20-inch has been made with a 5X afocal beam probably proves that the
image quality demanded of the system is only a fraction of what a perfect
20-inch could theoretically do.

A Mersenne has two conic mirrors whose figures are both paraboloids. The
primary is concave, matched to a convex secondary, looking superficially just
like a standard Cassegrain, except the optics produce a collimated beam,
rather than a focus.

Your 41" could be designed to feed the same 4" Nagler lens. But why not just
design it "right," in the first place?  A properly designed three mirror
Johnsonian could give you a telescope tube less than 10-feet long, with the
focus coming out the trunion of you alt-az or fork mount, or off the short
tube of your Newt looking tube, yet with a very small sec. obstruction
(effectively 5:1 obstruction or LESS, IF YOU WANT IT, for an f/6 - f/7
focus). [If you know someone eho can do an f/1.2 paraboloid (I do, but not
cheaply!!!), you could have a 60-74" long tube and an f/3 system, which, for
a one meter class telescope is amazing!]

Bye for now,

Gene Cross