Dob vs Cass

Thank you Roland and Vahe for the pointers to sources of premium SCTs.
Clear skies,
Bill Meyers

Vahe Sahakian wrote:

My problem with SCT's is that if I want a premium scope, I don't know where to
obtain one even if I am willing to pay more. Are they available anywhere, or has
the economic logic you explain above made only production line SCTs available?

If I am not mistaken there is one premium SCT available, it is a 10"
f15, made in Europe and I think that it is procuced by Opticon. This
is not a mass produced instrument and expect to pay a big $$$ for it.
The other option is to find a used Tak SCT, it is a 9" f/12, they do
show up once in a while on Astromart, both of these instruments are a
light year ahead of the best sample of Meade/Celestron SCT's.

Thanks,
Vahe

Alan French wrote:

For a full thickness 10" mirror on a 3 point support, about 1/36 wave, which
would translate into 1/18 wave at the wavefront. With a 9 point support,
about 1/285 wave, or 1/142 at the wavefront.

Based on Based on "Mirror Support: 3 or 9 Points?," Sky & Telescope, Sep 94,
pp. 84-87.

Yes, mine is a full-thickness mirror in a 9-point cell. The cell is a Novak,
although I really *hate* that cell because I don't like the way the mirror
clips intruded over the optical surface before I "altered" things a bit. I
also didn't like the way the edge supports could be moved to take into account
slightly different mirror sizes, as getting the mirror properly centered is a
real pain in the butt. The heads for the locking screws of the edge supports
unfortunately sit *under* the mirror, so you have to take the mirror out,
reset the screws and move the support to what you hope is the correct position
to hold the mirror, tighten the screws down, put the mirror back in, and then
repeat the process over and over until you either get the mirror centered and
properly held at the edges, or throw the cell out the window in a fit of rage
:-). Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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While they are probably superbly made optically, they both have large
obstructions. Seems that the manufacturers just cannot bring themselves to make
something a bit more exotic.

Do not know about Opticon specs, but the Tak SCT has a 27% c.o. which
is not bad at all for a f/12 cat.

Thanks,
Vahe

Do not know about Opticon specs, but the Tak SCT has a 27% c.o. which
is not bad at all for a f/12 cat.

You sure about that? I thought it was a lot more - closer to 33%. I think the
mirror size is 27% but the housing and baffle are bigger.

Roland Christen

"Alan French" wrote in message .. .
"andrea tasselli" wrote in message
om...

Perhaps one should and could wonder how much a mirror deflects under
its own weight, maybe to realize that any such claim, even if true
(which I don't think it is for 99.99% of those claiming out there) in
some ideal situation, isn't going to happen in practice.

Andrea,

For a full thickness 10" mirror on a 3 point support, about 1/36 wave, which
would translate into 1/18 wave at the wavefront. With a 9 point support,
about 1/285 wave, or 1/142 at the wavefront.

Based on Based on "Mirror Support: 3 or 9 Points?," Sky & Telescope, Sep 94,
pp. 84-87.


Hello Alan,

I assume that the article refers to azimuth pointing deflections (I
haven't got that article so I can't comment on the accuracy of the
numbers). As you move toward the horizon things change quite a bit and
of course depend on the actual lateral support system you have.
Besides, full thickness mirrors have more then few cons on their side,
even with a smallish size like 10".

Regards

Andrea T.

David Knisely wrote:

Alan French wrote:

For a full thickness 10" mirror on a 3 point support, about 1/36 wave, which
would translate into 1/18 wave at the wavefront. With a 9 point support,
about 1/285 wave, or 1/142 at the wavefront.

Based on Based on "Mirror Support: 3 or 9 Points?," Sky & Telescope, Sep 94,
pp. 84-87.

Yes, mine is a full-thickness mirror in a 9-point cell. The cell is a Novak,
although I really *hate* that cell because I don't like the way the mirror
clips intruded over the optical surface before I "altered" things a bit. I
also didn't like the way the edge supports could be moved to take into account
slightly different mirror sizes, as getting the mirror properly centered is a
real pain in the butt. The heads for the locking screws of the edge supports
unfortunately sit *under* the mirror, so you have to take the mirror out,
reset the screws and move the support to what you hope is the correct position
to hold the mirror, tighten the screws down, put the mirror back in, and then
repeat the process over and over until you either get the mirror centered and
properly held at the edges, or throw the cell out the window in a fit of rage
:-). Clear skies to you.

How would you suggest modifying the 'University Optics' 10" cell into a
true 9-point system?


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You sure about that? I thought it was a lot more - closer to 33%. I think the
mirror size is 27% but the housing and baffle are bigger.

A friend of mine used to own one of these, I am pretty certain that
the c.o. is about 27%, everything included. Based on my observations
this size c.o. is right at the limit for a reasonably acceptable
planetary contrast and overall performance, increase the size another
2 or 3%, you will see things go downhill big time.
My little TEC6 has a c.o. of about 29.5%, compared to your
unobstructed 155EDT the contrast loss is fairly evident on Jupiter,
although it is acceptable on Saturn.

Thanks,
Vahe

The performance of some is *close* to that of a decent Newtonian, so close
that, other than the slight edge in planetary contrast,

Hi Guys:

Well, I resolved this for all and good for myself this weekend.

All telescopes are good. But some we love more than others.

I didn't worry about obstruction sizes or Strehl ratios, or much else this past
weekend...I just got out under the dark skies of Chiefland and enjoyed seeing
what a 1984 Celestron C8 could bring me. And I enjoyed thoroughly what this
simple old scope delivered. No, it wasn't perfect optically...apparently it was
an intimation of what was to come with Halley, but more than good enough for my
hardly discriminating tastes and eyes.

It showed me wonders aplenty.

I ALSO enjoyed seeing what a beautiful 10 inch dob next to me could deliver,
what an exquisite 6 inch TMB refractor could show, and what I could see in Tom
Clark's Beast, his new 42 inch dob (you can imagine). Believe me, it was ALL
good, guys!

But I _still_ liked that silly, simple, imperfect little C8 (on a
crackerjack-cheap EQ4) best. I couldn't help it, I just did. I'm not completely
crazy, am I? ;-)

It's nice to get back to your roots once in a while. ;-)


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Rod Mollise wrote:

I ALSO enjoyed seeing what a beautiful 10 inch dob next to me could deliver,
what an exquisite 6 inch TMB refractor could show, and what I could see in Tom
Clark's Beast, his new 42 inch dob (you can imagine). Believe me, it was ALL
good, guys!

A 42-inch scope is a serious instrument. I would feel a little guilty
about owning such a large scope if it wasn't used, at least
occasionally, by college level students for research and teaching. I
hope Tom has considered this.


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