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

["Scott Ewart" <scotte@frontiernet.net>]

> 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).


Vlad,
	You and others keep mentioning doublets and even triplets, but
that's not what Clyde uses.  A 4-element Petzval does make a difference.
I'm talking about a refractor with an extremely well corrected 4-degree true
field (on its own).  And yes, I agree, cutting the refractor in half does
present quite an engineering challenge.  Also, instead of a mask around the
secondary, why not just use more mirror for a larger fully-illuminated
field?
	Just thinking through this design keeps me going around in circles.
It's full of trade-offs and compromises.  Want to keep the EFL short?  Got
to make a bigger secondary ....unless you don't care about a wide true
field.  But why keep the EFL short if...  Keeps coming back to a central
obstruction pushing 30%.
	And if I were to go to the trouble of making a 22" at f/4 or faster,
I might just prefer to climb a couple of steps and use a Paracorr.  But then
again, if it were a 28" (or larger), I might just prefer to sit.



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 


	Obviously, there's the comfort of sitting versus climbing a ladder
of a Dob.  But if you're comparing it to a Cass, what's the EFL and FOV of a
30" Cass?  Yikes!
	Pros and cons, trade-offs everywhere.  There is no perfect scope.

Scott Ewart


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