Re: [ATM] Looking for information on a telescope design

["vladimir sacek" <vla@toast.net>]

Scott,

> I don't see why you seem so negative towards this design.  You're
> like a black cloud here.

I merely state the facts. If you see that as a black cloud,
perhaps you should ask yourself why.

> And if someone is thinking of building one, we
> should offer whatever constructive help we can.

That is exactly what I am doing, much more so than by singing
Laudamus Te.

> I've looked though Clyde's scope a few years ago and he's not
> deluding himself; it is excellent.  It really does work.

I have never stated it doesn't. Have you read my posts more carefuly,
you would notice that I rate Mersenne visual field quality as generally 
good,
despite its strong off-axis aberrations. I don't see why it is better
to pretend there is no aberrations whatsoever, or play roulette with how
much of them one's particular arrangement will come up.

> To minimize the refractor-to-secondary distance and central
> obstruction, my idea is to mount the front refractor elements right next 
> to,
> or even in front of, the tertiary mirror(...)

That would reduce off-axis aberrations somewhat; not sure that a visible
benefit would be worth the added complications. Another way to
minimize stop displacement is to go with faster primary. Going from
f/5 to f/4 in the arrangement with the ED doublet apo reduces the 0.3
degree error from 3.7 wave p-v to 2.4 wave (comparable classical
Cassegrain would have it ~1 wave p-v).

Mersenne has easier to make secondary, no secondary tilt sensitivity due to
infinite magnification, lower sensitivity to decenter due to the lower 
secondary
conic, and similar mirror despace sensitivity. On the other
hand, classical Cassegrain has better field quality, and a distinctive
multiple advantage of no refractor needed.

Why build Mersenne? Because it's different is as good a reason as any.

Vlad 

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