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Celestron 60’s SCT - 27% obstruction & 1/20 Wave?!

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#1 EverlastingSky

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Posted 13 November 2006 - 07:55 PM

Celestron Pacific 60’s SCT’s – 27% obstruction and 1/20 wave?

I noticed two extraordinary pieces of info in this old thread here
Someone said that those old Celestron Pacific’s advertised 1/20 wave mirrors and had a central obstruction of a mere 27%. Not to mention those 3 main mirror lock knobs on the back.

I can’t believe it! The performance of such an old SCT would start to approach that of a Maksutov-Cassegrain. The best of the modern SCT’s by Celestron (to say nothing of Meade) are advertised as 1/10 and of course have that huge 34-37% obstruction, (and a whopping 43% on an 8” Meade according to this chart here )

What happened? A retrograde de-evolution? Why is it that they didn’t continue to offer a limited edition high quality planetary SCT? Or did the Maksutov-Cassegrain make the effort redundant? Or is it the “same old same old” of trade off’s and the balancing act required in order to make the SCT’s that followed cheaper and easier to produce allowing affordability? Have I just answered my own question? Hmmm…

I Just feel like cracking some heads together at the big Orange C and shaming them with facts of their 40 year old SCT’s being Potentially unconfirmed higher quality. Would love to see the reaction – or more likely, a total lack thereof… and glazed eye stares.

I'd like to see some reviews and shoot out's between old 60's Celestron Pacific SCT's and their new models. This current ongoing cloudynights request for Cassegrain reviews would benefit immensely from a review... on such an old 60's SCT. I hope one of you guys writes one!
(The short review of the Opticon 10 is yet another interesting story in the potential of the SCT design)

#2 ultralord

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Posted 14 November 2006 - 08:33 AM

Thanks for the great info. I always thought that more performance could be squeezed out of a SCT.

#3 sarastro

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Posted 14 November 2006 - 12:36 PM

I was looking at some old Sky and Telescope magazines from the early 70's recently. One thing that struck me was that all the Ads claimed their mirrors to be 1/20 wave or even 1/30 wave! :jawdrop:
I doubt that many mirrors were up to those standards. :shrug:

#4 Jae

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Posted 14 November 2006 - 12:52 PM

The methodology used - RMS, PV, "wave front", etc were never clear in mind.....sometimes still isn't. Roland Christen used to make all kind of points that I can't fully recall.

Jae

#5 EverlastingSky

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Posted 14 November 2006 - 01:10 PM

It would be fun to see those old magazine ads. Maybe those 1/20 and 1/30 claims are just old advertising hype and make believe? It just seems stupendous any which way you think about it. Even if a few of those mirrors approached that level it would be earth shattering in my mind. Those longer focal lengths and smaller secondary obstructions are pretty amazing too.

#6 Jay_Bird

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Posted 14 November 2006 - 01:17 PM

I just hope the next few decades for Celestron / Synta make Torrance, California near as proud as the last 40+ years should!

#7 Clive Gibbons

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Posted 14 November 2006 - 01:39 PM

Celestron Pacific 60’s SCT’s – 27% obstruction and 1/20 wave?

I noticed two extraordinary pieces of info in this old thread here
Someone said that those old Celestron Pacific’s advertised 1/20 wave mirrors and had a central obstruction of a mere 27%. Not to mention those 3 main mirror lock knobs on the back.

I can’t believe it! The performance of such an old SCT would start to approach that of a Maksutov-Cassegrain.


Celestron used to make corrector plates back in the '60s the same way they make 'em today. That's really the weak link in the chain, when it comes to getting a Mak-Cass quality image.
The corrector lens is ground and polished using a vacuum-forming jig. It results in a decent approximation of the complex aspheric curve needed to correct the image, but it's not perfect. The residuals are responsible for the less-than-classic diffraction image we often see in SCTs.

#8 Jon Isaacs

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Posted 14 November 2006 - 10:49 PM

>>The residuals are responsible for the less-than-classic diffraction image we often see in SCTs.
---

Clive: There you go, ruining a party with the facts. :ooo:

What do you think about a 27% CO on these scopes? Thinks that's real? What about placement of the corrector, ie at the center of the radius of curvature??

Jon

#9 Clive Gibbons

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Posted 15 November 2006 - 12:03 PM

>>The residuals are responsible for the less-than-classic diffraction image we often see in SCTs.
---

Clive: There you go, ruining a party with the facts. :ooo:

What do you think about a 27% CO on these scopes? Thinks that's real? What about placement of the corrector, ie at the center of the radius of curvature??

Jon


Sorry about that, Jon! :lol:

IIRC, the older Celestron SCTs (like the C10), were longer f/ratio (f/13.5 or so?), so a smaller c.o. would make sense.
However, the corrector plate was still near the primary mirror's first focus, rather than the centre of curvature, so off-axis performance wouldn't be much better compared to more recent SCTs.

#10 Jon Isaacs

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Posted 15 November 2006 - 12:12 PM

IIRC, the older Celestron SCTs (like the C10), were longer f/ratio (f/13.5 or so?), so a smaller c.o. would make sense.
However, the corrector plate was still near the primary mirror's first focus, rather than the centre of curvature, so off-axis performance wouldn't be much better compared to more recent SCTs.
-----

Thanks... Just wondered how they got that 27% CO.

Jon

#11 EverlastingSky

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Posted 15 November 2006 - 01:46 PM

Yup, long focal length “planetary specialized” SCT is the name of the game… and was more so then back in the 60’s when planetary was more popular (that’s my understanding anyway). So Maksukov Cassegrains win out today for long focal length small obstruction telescopes. If only everyone else was planet crazy like me… then they might start making more/cheaper Mak’s and SCT’s for the job. I don't like the "jack of all trades master of none" label on SCT's. That may be true with todays SCT's that try to be all things... but if they wanted, a SCT could be so much more specialized. Well, you get my point :p

#12 Jon Isaacs

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Posted 16 November 2006 - 06:29 PM

>>> That may be true with todays SCT's that try to be all things... but if they wanted, a SCT could be so much more specialized. Well, you get my point..
----

A while back when the issue of "premium" SCTs was raised over on S.A.A. it was suggested that it would be very expensive to produce SCTs that were of consistent high quality, the number that was suggested was around 5K for an 8 incher.

My limited understanding is that a MAK is better corrected across the FOV.

But this is my thinking:

An SCT is not really a long focal length telescope, rather it is a short focal length scope/primary mirror with a convex magnifying mirror. A 27% Central obstruction is small for a SCT but large for a Newtonian, whether it be a MAK-NEWT or a standard Newt. Both of these can have CO's less than 20% and typically an F/6 or F/8 general purpose Newt will have a 22% CO...

So, put that all in the coffee ginder and one has to ask whether one prefers a relatively large CO and a magnifying mirror or a smaller CO and Barlow. Of course I am ignoring tube length which is favor of the compound design...

It seems to me, again ignoring tube length, that a MAK-Newtonian has about the most to offer as a purely planetary scope.

jon

#13 ABC

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Posted 19 November 2006 - 01:17 PM

27% obstruction & 1/20 Wave?!


I can imagine...
I have had a vintage 1978 orange C8 f/10 for appr. 25 years. Secondary mirror assembly was rather small that unambiguously resulted in a central obstruction of less than 30%.
PTV was 1/10 lambda and Strehl was 0.99 as determined by Fizeau interferometry at 532 nm wave lenght.
The scope was a planet bomb capable of outperforming 6-7" apos.

#14 Clive Gibbons

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Posted 21 November 2006 - 11:45 AM

PTV was 1/10 lambda and Strehl was 0.99 as determined by Fizeau interferometry at 532 nm wave lenght.
The scope was a planet bomb capable of outperforming 6-7" apos.


WOW!! :bow:

That one should have gone straight from the assembly line, into the Celestron museum. :waytogo:


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