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

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

Vlad,
	I don't see why you seem so negative towards this design.  You're
like a black cloud here.  There are all kinds of designs that, for whatever
reason, don't look so good on paper, but in the real world, work very well
to fill a particular need.  Yes, the Mersenne does have its drawbacks, but
for some, it's just right.  And if someone is thinking of building one, we
should offer whatever constructive help we can.
	I've looked though Clyde's scope a few years ago and he's not
deluding himself; it is excellent.  It really does work.  Sure there are
design issues that need to be addressed, like the dual field, which I
actually liked and could easily get used to.  Clyde said he waited to build
one until a suitable refractor came along.  He uses Tele Vue Petzval scopes
because they offer the needed speed, and color and off-axis corrections.
	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, then mount the rear elements and
focuser into the altitude bearing.  The two sets would need a rigid
connection to maintain refractor collimation and scope size would be limited
by R4-to-R5 refractor spacing, but I think it could be done.
	Maybe I'll build one someday.  But in the mean time, this was the
year of the Schupmann at Stellafane.  Will next year be the year for the
Mersenne?

Scott Ewart


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
vladimir sacek
Sent: Wednesday, September 05, 2007 1:33 AM
To: atm@atmlist.net
Subject: Re: [ATM] Looking for information on a telescope design

Scott,

>(...) From this standpoint, the sole impact of the
> secondary magnification is to reduce the angular field of view in the sky
> that is projected onto a given linear field size, with field curvature 
> being
> the only aberration that is aggravated in combination.

Yes, if we follow the logic of the increased incident angle being
cancelled by as much smaller angular field in the Mersenne
arrangement. But things don't seem to be that simple. The widely displaced
stop is a wild card in that it can significantly affect off-axis 
aberrations.
I've put two 4" f/10 apos from ATMOS, one ED doublet with low residual
third-order coma and astigmatism, the other triplet with corrected 
third-order coma
and low astigmatism into a 400mm aperture Mersenne with 0.25D
minimum secondary size,  at half the f.l. of the f/5 primary from the 
secondary
(in Oslo).

At 20mm off-axis (1.2 degree), refractors alone have about
1 wave  and 1.3 wave p-v, mostly astigmatism on best curved field,
-340mm and -380mm, respectively. In the Mersenne, at the same 20mm off-axis
(0.3 degree), the doublet has 3.7 wave p-v of the combined third- and 
fifth-order
coma and astigmatism, on the best surface radius of -120mm. The triplet has
4.7 wave p-v of mainly third- and fifth-order coma plus some fifth-order 
astigmatism,
on somewhat more strongly curved field. Average blur size in 20mm eyepiece 
is
roughly 20 arc minutes, which probably doesn't appear too offensive, but 
leaves
something to be desired.

Not only than the displaced Mersenne stop can cause significant off-axis 
aberrations
in refractors, it seems that their level cannot be even approximately 
predicted.
It would require raytracing to find out what the field quality is.

Vlad 

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