From: "William Cook" 
To: atm{at}shore.net
Reply-To: "William Cook" 


Marco Miglionico wrote:

>>>Here is the problem though. Commercial telescopes of the cassegrain
>>>veriety
often have central obstructions approaching 37% by diameter which Texereau
may well have baulked at the idea of. There is however no question about
these telescopes' ability to resolve very fine detail on the planets. They
are in fact among the top performers. So my question is (in the words of a
true potiticain) who is almost right and who is almost wrong?<<<

Marco, you covered a lot of ground in your post; I will address one facet
of your query.

You say: “There is however no question about these telescopes' ability to
resolve very fine detail on the planets.”

That may be correct—depending on how you define “fine.” However, when you
equalize apertures, questions—hard questions—start falling out of the sky.
Yes, a 10-inch SCT will resolve more than, say, a 6-inch refractor. The
difference here is not based as much on diffraction as much as the muscle
in aperture. Add a couple of inches to the refractor and, barring concerns
for chromatic aberration, the SCT will lose the match—quality in design,
construction and baffling being equal.

You continue with: “They [SCTS] are in fact among the top performers.”

If this is your personal observation of the matter, I will totally support
you in your belief. Amateur astronomy is supposed to be fun. Thus, if you
are happy, that’s all that matters. However, if you are looking into the
matter to gain more knowledge on the subject, I will proceed.

In fact, they are not . . . never were . . . and never will be “top
performers.” Glance through the photo sections of any of the major color,
glossy astronomy magazines. Chances are that when you see an amateur photo
with pinpoint star images, that photo was take with a refractor; When you
see one in which the star images look like they were drawn with crayons,
those were probably taken with an SCT.

I have owned a few SCTs. I have a couple, now. I will undoubtedly own more
in the future. I LOVE the convenience of SCTs on nights when I want an
instrument that tracks. But they are ALL compromise instruments.

Bernard Schmidt did not design a telescope; he designed a camera. In the
late 50s and early 60s, men like Wiley and DeVany tinkered with the idea of
bastardizing a camera that performed MARVELOUSLY if you were willing to
fiddle with all of its tedium into a telescope that performed ADEQUATELY,
but was very convenient to use. By the mid-60s Alan Hale and Tom Johnson of
(then) Celestron Pacific predicted that consumers would fall in love with
this Schmidt-Camera derivative. The rest, as they say, is history.

When you buy an SCT today, unless it has NASA, Boeing, Livermore or the
like on the side, it is a mass produced instrument with all the problems or
potential problems one might expect to encounter in a mass produced
instrument. This does not bother me as it does all those who strive for
non-existent, “perfection.” Still, to call any SCT a top performer without
factoring in the inherent weaknesses of the design is not really fair.

If, for example, your primary interest is in deep-sky objects, the
telescope should be considerably longer (slower primary) and more
cumbersome. Conversely, to get better performance for lunar and planetary
observations, the secondary would need to be much smaller. This, of course,
could lead to a light drop-off at the edge of the field far greater than
the 40% standard for visual work.

As an aside, the following excerpt from Telescope Optics Evaluation and
Design (Rutten and van Venrooij, Willmann-Bell, April 1999 edition, p.
219.)

“There is clearly a loss of contrast in obstructed systems. But
surprisingly, for higher resolutions, contrast appears somewhat enhanced in
obstructed systems. The reason for this is that the Airy disk diameter is
slightly reduced when a large central obstruction in introduced.”

Well, I will crawl back in my hole, now. I hope I have offered at least
something worth while to think about.

Kindest Regards,

Bill Cook, Chief Opticalman, USNR-Ret. Manager, Precision Instruments &
Optics, Captain’s Nautical Supplies, Seattle
Former editor-in-chief, Amateur Telescope Making Journal


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