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Celestron F6.3 Reducer for visual with C8?

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#26 orion61

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Posted 06 January 2013 - 07:59 PM

I use my MIJ R/C all the time, It makes a great dust seal and gets rid of quite a bit of coma!
My opinion is it is a must have for any Standard Schmidt.
I have noticed it does induce a small bit of Spherical Aberration to my C8. I did have a C8 a few years ago that had a bit of SA and it cancelled most if it out!
I'd like to see someone build a corrector into a 2" Diagonal barrel, Inserted inside not screwed on sticking out.
That could eliminate 2 problems!!

#27 Joe Cepleur

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Posted 06 January 2013 - 08:12 PM

I'd like to see someone build a corrector into a 2" Diagonal barrel, Inserted inside not screwed on sticking out.


That strikes me as a truly great idea. Would the right design allow a standard SCT with an inexpensive accessory to pretty well mimic the performance of the fancy new EdgeHDs and LXs? Optically, anyway. I know there is a lot about their focusers and mounts contributing to their overall performance.

#28 orion61

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Posted 06 January 2013 - 08:26 PM

I'd like to see someone build a corrector into a 2" Diagonal barrel, Inserted inside not screwed on sticking out.


That strikes me as a truly great idea. Would the right design allow a standard SCT with an inexpensive accessory to pretty well mimic the performance of the fancy new EdgeHDs and LXs? Optically, anyway. I know there is a lot about their focusers and mounts contributing to their overall performance.

Thats what gave me the idea, the corrector lens in the focuser baffle,
But then they couldnt sell those 2K tubes SHHH...LOL

#29 mclewis1

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Posted 07 January 2013 - 08:34 AM

Moonlite sells an external focuser that has the option of placing a modified SCT focal reducer into the barrel. There are also other sources on the web of repackaged focal reducers and kits that fit inside many 2" barrels.

http://www.optecinc....talog/17408.htm

http://www.observatory.org/vpacs.htm (low profile adapters ... no focal reducers)

#30 Joe Cepleur

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Posted 09 January 2013 - 10:55 AM

In the last sentence of the Overview, B&H continues the notion that Celestron sells an f/6.3 focal reducer because, "a f/6.3 Schmidt-Cassegrain telescope... is impractical with a large central obstruction and a resulting loss in contrast." That is not the source I intended to quote, so I'll keep looking for a better source, yet it is interesting to see this notion out there, especially in light of Meade's new LX600. Maybe fast focus Schmidt-Cassegrains require fancy, modern optics, or is this entire notion really just a myth?

I'll continue searching for a primary source stating that Celestron originally intended its f/10 SCT as the fastest natively practical, with a reducer intended to reach faster speeds with a reasonable obstruction.

#31 Joe Cepleur

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Posted 09 January 2013 - 11:11 AM

Is this notion beginning to sound familiar? Same exact wording! This still does not prove or disprove that the focal reducer was intended to be part of the C8 kit from the beginning, but it suggests that recycled advertising copy could distort the actual history. This site is British, so the notion is spreading world-wide:

"Celestron doesn’t offer a f/6.3 Schmidt-Cassegrain telescope, because the design is impractical, with a large central obstruction, and resulting loss in contrast. This, added to the inability to utilize longer focal ratios for certain objects, led Celestron to design the four-element, fully multicoated f/6.3 Reducer/Corrector."

#32 rmollise

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Posted 09 January 2013 - 11:31 AM

This still does not prove or disprove that the focal reducer was intended to be part of the C8 kit from the beginning


Well, what disproves that is that the reducer/corrector did not appear till nearly 20 years after the C8 was first introduced...

There had been reducers (not reducer/correctors) available before then, most of them at f/5, from Celestron and other sources.

Once again, the impetus for the r/c was Meade's release of the LX-6 in the late 80s.

;)

#33 Joe Cepleur

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Posted 09 January 2013 - 11:49 AM

Okay, thanks, Uncle Rod; time for me to quit hunting for history I can not find because it does not exist, except maybe in some convincing sounding error I read someplace.

After reading an earlier post about the Meade LX-6, I get it... maybe. Meade introduced a natively f/6.3 SCT, with all the attendant advantages of faster exposures for photographers, but with iffy optics, at least in the early models. To compete with the faster exposures while beating the coma-shaped stars at the edge of the Meade's field of view, Celestron introduced the f/6.3 focal reducer for its f/10 scopes. Along with offering better contrast than the competing Meade, due to the smaller central obstruction, this would have made an arguably better f/6.3 system, at least one well tuned to Celestron's advertising copy.

What's the deal with Celestron's eariler f/5 reducer? Presumably it was great for reducing exposure times, but wreaked havoc on the sharpness of stars toward the edge of the wider field of view? That would explain why Celestron needed to introduce a reducer/CORRECTOR.

#34 rmollise

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Posted 09 January 2013 - 02:14 PM

It was like any other reducer then or now. Yeah, shorter exposure times, but edge of field performance no bettter than--and usually worse than at f/10. ;)

#35 JamesL

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Posted 09 January 2013 - 05:24 PM

My focal reducer came today. I would like to try it out but clouds and rain will prevent me for the next several days.
In the mean time I,ll install this sliding counter weight system and just admire my cat untill the weather clears!

#36 Eddgie

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Posted 09 January 2013 - 05:45 PM

Hello Joe,

There may be some confusion and some of it may be mine.

At one point, you could buy a "Rich Field Adapter" for SCTs and this was indeed f/5.

It was in essence a special 1.25" diagonal with a reducer built right in.

Horrible to use in my opinion. I used to own one maybe 25 years ago. And that would in iteself explain why it is no longer sold. Really terrible. Loaded with astigmatism as I recall, though to be fair, it could have been the eyepeices made at the time were just not working well with the f/5 light cone.

But the coma from the scope and the astigmatism where so severe that I used it once and threw it in my junk drawer.

So, maybe this is what the f/5 was in reference to...

#37 Eddgie

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Posted 09 January 2013 - 05:46 PM

Speak of the devil... Regarding my previous post, here is an example:

Celestron f/5 Rich Field Adapter

And by the way... The focal reducer does not make the obstruction any larger unless it is used in a configuration with a lot of back focus so that the scope is loosing aperture.

The Celestron marketing positioned the Celestron focal reducer as a better alternative to an Native f/6.3 SCT, which was made by Meade for a while. The Meade f/6.3 SCT had a 40% obstruction, and Celetron's position was that it was better to use the reducer and keep the smaller CO than go to the Meade f/6.3 with the bigger obstruction.

So, it was a marketing thing.

And it must have worked. The Meade f/6.3 SCT bombed in the marketplace. But this was well before digital imageing came into the picture. Hypered film was the leading edge technology, and because of the expense (processing the film was the big cost) the hobby was not as popular as today, and I doubt that they sold enough scopes to make it worth the production time.

But if you can do it wite a telecompressor, then why bother with a native f/6.3 SCT which had wicked field curvature.

#38 rmollise

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Posted 09 January 2013 - 06:13 PM

Hello Joe,

There may be some confusion and some of it may be mine.

At one point, you could buy a "Rich Field Adapter" for SCTs and this was indeed f/5.

It was in essence a special 1.25" diagonal with a reducer built right in.

Horrible to use in my opinion. I used to own one maybe 25 years ago. And that would in iteself explain why it is no longer sold. Really terrible. Loaded with astigmatism as I recall, though to be fair, it could have been the eyepeices made at the time were just not working well with the f/5 light cone.

But the coma from the scope and the astigmatism where so severe that I used it once and threw it in my junk drawer.

So, maybe this is what the f/5 was in reference to...


There were numerous reducers at f/5 available, including from Lumicon, Orion, etc., etc. The non-correcting reducer was around since the beginning and even Celestron produced a couple of different iterations. ;)

Many were quite useable for imaging, and that was all we had unless you wanted to shoot the deep sky at f/10.

#39 Joe Cepleur

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Posted 09 January 2013 - 06:38 PM

Celestron marketing positioned the Celestron focal reducer as a better alternative to an Native f/6.3 SCT, which was made by Meade for a while. The Meade f/6.3 SCT had a 40% obstruction, and Celetron's position was that it was better to use the reducer and keep the smaller CO than go to the Meade f/6.3 with the bigger obstruction.


Thanks, all -- Now the story makes perfect sense. Kinda like the game of "telephone." By the time it got to me thirty years later, either it had morphed or I had morphed it. While a C8 with a focal reducer is not the ultimate rich-field scope, being able to swap between f/10 and f/6.3 is an excellent perk in a flexible system, whatever its origins. Carrying a reducer/corrector beats carrying and setting up a second optical tube!

Celestron's marketing must have been effective. I'm sure I would have bought the pitch. Sounds as though it was true, anyway.

Thanks for the tip on the old f/5 diagonals. I'm a trinkets guy, and love collecting old accessories. I like the feeling of tangible history. I'm happy with my recently purchased, Japanese-made Meade f/6.3 refucer/corrector, happier than with a new Synta! I was the only bidder on eBay, and almost felt sorry for the seller, but fair was fair, he did okay. But -- I've seen these curious f/5 diagonals for sale, and have erased them from my wish list!

#40 Joe Cepleur

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Posted 09 January 2013 - 06:53 PM

There were numerous reducers at f/5 available, including from Lumicon, Orion, etc., etc. The non-correcting reducer was around since the beginning and even Celestron produced a couple of different iterations.


From a marketing point of view, it would be advantageous for Celestron to let people believe that the better solution to Meade's native f/6.3 had been available from the beginning. It sounds as though the truth was that it became truly viable only with the introduction of Celestron's f/6.3 reducer/corrector, but what a great marketing blitz it must have been! Smothered Meade's native f/6.3 in its infancy, with a seemingly objective argument about the size of the central obstruction. The competition between these companies must have been savage.

#41 rmollise

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Posted 10 January 2013 - 07:51 AM


From a marketing point of view, it would be advantageous for Celestron to let people believe that the better solution to Meade's native f/6.3 had been available from the beginning. It sounds as though the truth was that it became truly viable only with the introduction of Celestron's f/6.3 reducer/corrector, but what a great marketing blitz it must have been! Smothered Meade's native f/6.3 in its infancy, with a seemingly objective argument about the size of the central obstruction. The competition between these companies must have been savage.



Celestron never suggested that they had an "answer" to the LX-6 Meade from the beginning. If you go back to the advertisements of the day, you will find ads announcing the new 6.3 reducer (desgined by Jim Riffle), and explaining that it gave you TWO TELESCOPES IN ONE. :lol:

#42 careysub

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Posted 04 March 2013 - 07:16 PM

...
You get a alightly wider field of view, but the brightness will only be that of a 6.5" telescope, and the contrast for things like planets will be reduced somewhat.

The apture is reduced because to get the focal plane to go far enough to the back to reach the field stop of the eyepiece, the mirror must be moved very far forward using the focuser. When you do this, the outside of the light cone converging on the focal plane is cut off by the baffles in the telescope. And because the outside of the light cone is cut off, rather than being a 34% obstruction, the seconary acts as a 37% obstruction...


I'm not sure how you are arriving at the figure of 6.5", given the CO figures you provide.

If the calculation addresses contrast through the (aperture-CO) rule-of-thumb, then:
(1-0.37)*8 = 5.04
but
(1-0.34)*8 = 5.28

If the calculation is about light grasp then:
((1- 0.37^2)*8^2)^0.5 = 7.43
and
((1- 0.34^2)*8^2)^0.5 = 7.52

In neither case is there a large difference (4.5% and 1.2%), in fact neither is detectable to the eye.

#43 rflinn68

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Posted 05 March 2013 - 02:31 PM

I cant reach focus with my C8 using the Celestron f/6.3 reducer/corrector using my 2" diagonal. I have to use my 1.25" diagonal, however, I assume the reason is I have a refractor style 2" diagonal and use an adapter. I just use mine for imaging until I buy a dedicated 2" SCT style diagonal.

#44 Geo31

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Posted 09 April 2013 - 10:52 PM

This still does not prove or disprove that the focal reducer was intended to be part of the C8 kit from the beginning


Well, what disproves that is that the reducer/corrector did not appear till nearly 20 years after the C8 was first introduced...

There had been reducers (not reducer/correctors) available before then, most of them at f/5, from Celestron and other sources.

Once again, the impetus for the r/c was Meade's release of the LX-6 in the late 80s.

;)


I'm just getting back into this after 35 years away, but, while the current style R/C may have been designed 20 years after the C8, I know for certain that "tele-compressors" have been with us since at least 1975. I know that date as that is the first year I went to Stellafane and it was there that I first encountered (or even heard of) one. And while it's possible that it was homemade (hey, it WAS Stellafane), nothing in this guy's gear appeared to be anything but store bought.

I cannot contribute to any idea that the C8 was designed with a reducer/compressor in mind. I devoured Sky & Telescope (and about any other books/magazines I could get my hands on) from 72-77 and never heard of this.

Now, that said, I would personally like to thank you Uncle Rod for your SCT buyer's guide. It was invaluable for me as I sifted through the SCTs available for sale. It became my buyer's bible and I'm happy to say I have a nice Super C8 with Starbright coatings and Byers drive on its way to me as I write this. I'm totally certain you kept me from making some foolish mistakes. Big thanks! Now if I could just figure out which eyepieces to start with (value minded, but not beginner). Things have changed a LOT since I've been gone. Orthos ruled the day, but Plossls just started becoming popular (and were twice the price of Orthos - go figure).

George Roffe
Kingwood, TX

#45 jrbarnett

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Posted 09 April 2013 - 11:01 PM

"Just be aware that when you use a 2" diagonal in a C8 behind the 6.3 focal reducer, it will have the effect of turning your telescope into a much smaller telescope."

What if you use a 1.25" diagonal behind the FF/FR?

Regards,

Jim

#46 rmollise

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Posted 10 April 2013 - 08:09 AM

"Just be aware that when you use a 2" diagonal in a C8 behind the 6.3 focal reducer, it will have the effect of turning your telescope into a much smaller telescope."

What if you use a 1.25" diagonal behind the FF/FR?

Regards,

Jim


Jim...don't worry about it. I guarantee that the 6.3 works well, even with a 2-inch diagonal, and most assuredly does NOT turn your telescope into a much smaller one. :cool:

#47 Eddgie

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Posted 10 April 2013 - 11:07 AM

I will give you the answer, but I agree with everyone else. Don't worry about it.

With the standard focal reducer in place, just about all of the Celetron SCTs will start loosing aperture once you go past about 100mm of back focus, and most 2" diagonal configurations will be longer than this.

A 1.25" configuration will usually be less than 100mm and the system should still work at full aperture.

But don't worry about it. If you can't tell any difference, then it doesn't matter.

If you would like Ken Hutchinson's SCT vitnetting analysis though, I am happy to send it to you. Just PM me your email address. It has all of the information regarding things like focal ratio change, aperture change, spot size change at the edge of the field, and other such data.

#48 cn register 5

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Posted 10 April 2013 - 06:12 PM

AIUI it isn't the size of the diagonal that matters. How can the light cone care about what's around it? What matters is the separation between the FR and the EP. The optimum is 105mm and this is achieved with the visual back and 1.25" diagonal.

A 2" diagonal is bigger and so give more separation, especially the refractor style that needs a 2" visual back to connect it to the FR but the type of 2" diagonal that screws directly to the FR may have a short enough light path that the FR to EP separation may be what's required. In this case I can't see a reason why an aperture reduction would be seen.

Hope this makes sense,

Chris

#49 orion61

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Posted 10 April 2013 - 07:30 PM

Well the F6.3 FR/FF is still around and the F6.3 optical system isn't... :applause:
I have mine on my C14 and it pretty much never comes off unless I am using my binoviewer.
for any std SCT they are a must have! I may be biased but the origonal (Celestron Japanese units) are the best. I have owned 2 other brands, and can tell the difference in truer Star colors and sharper stars. I think of it as my Edge with the correcting lens on the outside LOL...






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