From: "Marco Miglionico" 
To: "William Cook" ,
        "ATM Archives" 
Reply-To: "Marco Miglionico" 


You have written some seriously interesting stuff here. I've noted all you
have said and probably agree with most. I am still learning. Any ideas I
had about SCT's have come from the fact that most of the best hi-res
photo's of jupiter and saturn that one sees in the magazines have come from
a CCD coupled to an SCT.  But maybe that is because of the 'muscle' of the
added aperture and the fact that not everyone has 25000 dollars, UK
sterling etc.. to spent on an 8in Apo!

I am interested on one point you made though. - The fact that a large
central obstruction reduces the size of the airy disc.

Marco.

----- Original Message -----
From: William Cook 
To: 
Sent: Wednesday, March 05, 2003 7:21 PM Subject: ATM Re: SCTs Top Performers


>
> 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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