Re: C11 collimation woes

On Sun, 26 Nov 2006 10:17:24 GMT, "John Moore" <johnmoore@xxxxxxx>
wrote:

Thanks to all for their suggestions/comments so far (some rather critical). I
feel I should explain why I made these modifications. I already have an Intes
Micro MN78 which has taken some decent planetary pcitures (see
http://tinyurl.com/ckxk5 ), but I am striving for better. Having seen some
spectacular planetary images taken with a C11, I picked one up secondhand. I
immediatley compared it with the MN78 on Jupiter and its performance was much
worse, despite the much bigger aperture. Looking at the very glossy inside of
the C11 I surmised that at least some of the problem was caused by internal
reflections...

In my own experience, a telescope design such as an SCT with large
obstruction is not optimal for planetary observing. I have owned
three SCTs, at least six refractors, and probably eight Newts, I
believe; and the SCTs, despite other excellent virtues, were not
telescopes that ever gave me enjoyable planetary images.

My C-11 is collimated well so that in-and-out of focus diffraction
patterns look symmetrical and nearly identical. Yet, Jupiter is
"soft" and regularly disappoints me. I a direct "shoot off" between
the C-11 and a very well collimated, sharp figured 10" Newt with
smaller obstruction, the latter looked crisper but the diffraction
spikes from the spider were irritating...I preferred looking at
Jupiter in a lower power in my 120mm refractor. But, the best views
of Jupiter I've ever had, year in, year out, were always in
refractors, even achromats but preferably something like a 7" AP
StarFire, which--in perfect seeing--has such detail that one almost
imagines that it's a satellite image! Side by side SCTs -- Meades,
Celestrons, smaller Maks -- never look as good, bigger or smaller
aperture.

Now, I suppose it is possible for somebody to make a nice planetary
classical Cassegrain scope. But I do not think that commercial SCTs
are just the ticket for that type of observing.

Furthermore, with 11" aperture you will not get superbly sharp
planetary images except on rare nights with near-steady air. On those
occasions Jupiter might still *seem* almost perfectly steady and
detailed in a 4-6 inch refractor compared to your C-11 but not, of
course, if you run both scopes at "full bore", 50x per inch of
aperture (which, BTW, an 11" SCT can never do on a planet and seem to
retain sharp clarity.)

Furthermore, I find that Jupiter's detail tends to start breaking down
somewhere above 25x per inch of aperture though this will vary quite a
bit depending on the aperture diameter and any central obstruction in
the system.

So, I suspect that the problem here is that your original
determination that the C-11 was faulty or substandard is the thing you
might want to re-examine.

I notice that from night to night (observing at 3400 feet altitude in
the mountains, not far from Lick Observatory) that even though we have
a Meditteranean climate here in the SF bay area, and laminar airflow,
at least at high altitude sites, there is a HUGE variation in how
sharp things look in my C-11. I have to resist the impulse to "play"
with the scope or to feel let down. It's the air; and the large
aperture; and of course the slight extra image degradation of the
large central obstruction. When you improve each of these situations
the image gets better and better...but in the case of the C-11, you
can't change the optical design and size of the obstruction, so at
some point there is a limit to the way it will render the finest
detail at the highest magnification.

Yet I find that often I use mine at powers of 373x to 466x, to try to
see central stars of planetary nebulae, details in galaxies, etc. And
when doing so, on SOME nights the stars have nice, delicate, defined,
unbroken first diffraction rings; and on MOST nights they are fuzzy
blobs. So, I have determined that there is nothing wrong with my
telescope, nor its general collimation. Since the performance is good
but only in the confluence of weather and wind events that even here
rarely occur, I need not worry about endlessly modifying or tweaking
my scope: it's fine but simply will not show Jupiter the way a
friend's 7" StarFire will show it.

As to the internal reflections: well, for decades I struggled to try
to see IC 59/63 near brilliant Gamma Cass. I even once tried the 36"
refractor at Lick Observatory. Failed EVERY time; on Newts,
refractors, etc. At last I was able to see the faint nebulae using an
ancient 4" f/10 with thirty year old Edmund mirrors that had pitted
coatings: because the inside of the scope tube was nicely blackened,
and the light from Gamma Cass was relatively dim with this small
aperture, and did not affect my vision. I could see the nebulae,
finally. Then, I spent ten more years trying to see them with ANY
OTHER scope, and failed.

But, I *did* see them the first time I tried with my C-11. When Gamma
Cass was out of the field, by carefully choosing my eyepieces and
filters, I got an incredibly good view of both nebulae, much better
than before, and could actually recognize their shape. So, my
conclusion was that for visual use, my C-11's internal baffling is
quite satisfactory: again, it beats all the other scopes that I have
used to see particularly challenging, faint, pale objects that are
RIGHT near a blinding star. I have yet to try for Leo 1 but am
planning on it later this season. I have no doubt that it will be
accomplished: for the same reason. So I consider the baffling to be
more than adequate: better than my home made reflectors that I sweated
over for years, putting in flocking, using special paint, messing with
spider types, changing mirror coatings, etc.

In my opinion, for the kinds of observing I intend the C-11 to enable
me to do -- extremely faint extended objects of small angular diameter
-- it is almost perfectly optimized. It is also NOT perfectly
optimized for planetary viewing.

However, I have yet to find a telescope that does everything precisely
correctly. The solution I am favoring now is not to attempt to
redesign anything as complex as a commercial SCT, but merely to get a
couple of different telescopes to suit different needs.

I read the webpage you cite about modifying the C-11, shortly after I
acquired mine 18 mos. ago, and frankly was appalled by it.

Years ago I toured the Celestron factory and went into their sort of
low key clean room -- not exactly "NASA grade", but effective -- where
they laser-collimate scopes. I watched them adjust a small Mak using
900x, and saw that the Airy disk pattern was textbook perfect. I
realized then and there that no matter how much effort I'd put into
star testing at night, under any old sky conditions, I could NEVER
achieve that kind of accuracy and repeatability. It made me respect
the product, knowing that they were putting their scopes thru that
kind of test procedure after assembling the components.

If I were ever to undertake a program to modify my C-11, I would first
acquire optical bench testing components -- at least home made ones --
and a laser collimator. I would have a setup where I could
repeatably, carefully, analyze a perfect point source light so that I
could verify the condition of the scope BEFORE as well as during the
changes, and finally AFTERWARD when it was all finished and ready for
use.

But, I see no need. When my C-11 does produce those exquisite
textbook star images -- OCCASIONALLY -- then I can sigh with relief
and remember that it's an excellent optical system, and is working
correctly.

Now, this is not to say that any advanced observer can look through
one and not find details that fall short of perfection. But,
perfection is expensive. And, as you have seen, it is not achievable
by merely following some instructions given by any old web page,
without taking the precautions of having some kind of optical
reference test standards.

Your defocused image made me squirm with discomfort. At first, I was
thinking it might be due to axis alignment problems with your imager
but it is certainly far more likely that you have -- as several people
indicate -- pinched the optics.

The C-11's corrector plate is so sensitive to warping and temperature
change that I have noticed a very small aberration in star images when
I attach the heavy counterweight system that I purchased, and slide
the weight too close to the corrector. Furthermore, it is possible
that it can take MORE than just two hours to get the scope to total
thermal equilibrium. Often I find that stars just do not look near
perfect until my scope has been outside for five or six hours -- but
of course this could also be because my air seems to get steadier and
steadier as the night rolls on. Again: without a laser beam to test
the diffraction pattern, you cannot really tell by looking at stars,
because you CANNOT CONTROL THE AIR!

Your tone of despair was useful to me, at least. Reading of your
woes, and seeing your image of the terrible result, I could never be
tempted to take my C-11 apart and do anything to it...so, even though
it can't be of any consolation to you, at least I'd like to thank you
for telling us about the results.

I don't think "all is lost" but of course Celestron would be appalled.
Some further work will make this better and perhaps you can even end
up where you want to be (allowing of course for the fact that you
cannot reduce the size of the central obstruction.)

AstroApp




.

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