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What makes the C9.25 so special?

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#1 jerry10137

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Posted 25 February 2013 - 01:19 PM

Curious minds want to know. The reason I ask is because the used ones literally get scooped up really fast.....almost instantly

#2 MikeBOKC

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Posted 25 February 2013 - 01:21 PM

I suspect it is because they are an excellent sweet spot in aperture, portabiity and versatility for either visual or AP, or both. They will ride comfortably on EQ mounts like the CG-5, or larger if one wants more stability. Kind of a Goldilocks SCT, not too big, not too small, just right.

#3 dweller25

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Posted 25 February 2013 - 01:22 PM

I think this is mostly hype - which I fell for - my C9.25 was nice but nothing "special".

My FS128 on the other hand ..........

#4 wolfman_4_ever

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Posted 25 February 2013 - 01:29 PM

lol!

:gotpopcorn:

#5 Dave Venne

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Posted 25 February 2013 - 01:37 PM

Good question, because I fell for the mystique--not that I regret getting one in the least. I like the size, it is a sort of sweet spot thing for me. The 9.25 is about the biggest SCT I can handle without worrying about damaging it or myself. At f/10 its field is nice and flat for imaging, and the scope works well on my CGEM. I like it, but is it special? Don't know. Give me some other SCTs for comparison and I'll get back to you. Eventually.

#6 MrJones

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Posted 25 February 2013 - 02:23 PM

This old topic? :) As per some posts above what makes it special for me is that it's the biggest GOTO aperture that is easy to manage and will go on the lightweight mounts. I keep mine fully assembled in the garage on the LXD75 with counterweights even and it's easy to pick it up and carry to the driveway or backyard.

#7 jerry10137

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Posted 25 February 2013 - 02:30 PM

Sorry guys, I'm not trying to stir a pot here. I've seen several of them for sale in the last few months and they just get scooped up fast. I purchased one myself but only in that size because I knew it would be a happy medium for me. I'm going to upgrade and was curious if I should stay at 9.25 or go 11 in HD of course. I'm very anxious to look thru a 9.25HD

#8 vahe

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Posted 25 February 2013 - 02:31 PM

If you look at the high end Maks you will notice that they all come with +/-F/3 primaries, for a given aperture their tubes are obviously much longer than SCT’s.

SCT’s use F’2 primaries, a major compromise in the interest of compactness, there is one exception and that is C9.25, not F/3 but midway.

In theory F/2 or F/3 should not make any difference, in practice it is the opposite.

Vahe

#9 Eigen

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Posted 25 February 2013 - 02:34 PM

I've read quite a few theories on the 9.25's mirror configuration being slower than that of other SCTs (F2 vs. F2.5-3.0) contributing to a less crucial critical collimation.

While I am unsure as to whether the above holds, I am quite sure in the fact that "most" SCT's out there are never critically collimated. I base this on countless SCT's at star-parties that I have had a chance to look through.

As most of you on here will know critical collimation is pretty tight on an SCT and the difference in the percieved optical quality changes quite dramatically when it is achieved.

What I am trying to say is that if the first statement holds, the likely factor contributing to the reputation of ethereal optical quality of the 9.25 series could simply stem from the fact that more folks got them properly collimated and compared them to less than perfectly collimated standard SCT's.

Just a theory though.

#10 orion61

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Posted 25 February 2013 - 02:47 PM

I think it was the fact for both the 9.25 and 6" were a totally new size for Celestron in some time, adding that to
the new computer control system that was guaranteeing very good optics in all Celestron scopes (and Meade) they hit the floor running and gained a reputation of bullet proof optics.
Both the 6 and 9.25 are very user friendly sizes.

#11 bob midiri

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Posted 25 February 2013 - 03:11 PM

Curious minds want to know. The reason I ask is because the used ones literally get scooped up really fast.....almost instantly

I don't know some say hype, some swear by them. I own a C14 orange, and a Orange Super C8 both with in my mind exceptional optics. I also own an Ultima 9.25 with the digital focuser. If I was forced to keep just one it would be the Ultima....very refractor like images and quite portable. One nine volt battery runs it for hours. This one is something special, optically, mechanically....but I don't why it just is. Bob

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#12 crsrs

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Posted 25 February 2013 - 05:43 PM

My Celestron Nexstar 9.25 GPS gives exceptional views and things like cool down times are quicker. The view is just beautiful. I like it better than any other scope I had.

#13 GeneT

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Posted 25 February 2013 - 06:33 PM

I suspect it is because they are an excellent sweet spot in aperture, portabiity and versatility for either visual or AP, or both. They will ride comfortably on EQ mounts like the CG-5, or larger if one wants more stability. Kind of a Goldilocks SCT, not too big, not too small, just right.


Well said.

#14 jerry10137

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Posted 25 February 2013 - 11:29 PM

Can you get a good clear focus with a quality 10mm eyepiece?

#15 bob midiri

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Posted 26 February 2013 - 05:44 AM

When the atmosphere is steady i like using my 9mm Nagler, so yes. But that is with my particular scope

#16 freestar8n

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Posted 26 February 2013 - 07:23 AM

The regular C9.25 is definitely special because, as mentioned above, the primary is f/2.5 (I think) instead of f/2 - making the tube noticeably longer. Although this theoretically would have no impact on the on-axis performance for planets, it would have a big effect on flattening the field and reducing aberrations across a largeish detector.

But for the EdgeHD version, the field correction is taken care of by additional correction lenses - so the benefit of the slower primary would be greatly reduced. The overall field may still benefit from it, though, but all the Edge designs should be well corrected across the image - when collimated anyway.

Frank

#17 Eddgie

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Posted 26 February 2013 - 09:03 AM

In theory F/2 or F/3 should not make any difference, in practice it is the opposite.


Correct me if I am wrong, but wouldn't the f/3 help flatten the field as compared to the f/2 configuration?

It was my understanding that the C9 was slighly compromised toward imaging where the larger secondary obstruction would provide a bit better off axis illumination and the flatter field would give a bit better off axis sharpness.

#18 Eddgie

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Posted 26 February 2013 - 09:20 AM

Curious minds want to know. The reason I ask is because the used ones literally get scooped up really fast.....almost instantly


This is just an opinion, like so many CN responses, so take it for what it is worth.

And for what it is worth, I have owned C5s, C8s, C9, C11, and C14.

I believe that this is an urban legand that started with a web review site put up by Ed Ting about 12 or 13 years ago.

In the site, Ed Ting reviewed the C9. I don't know what other SCTs Ed Ting had in depth experince with prior to the C9 test, but Ed praised the C9 as the "SCT for people who don't like SCTs" or some such accolade.

He praised the scope for having good contrast. Of course if his only experience was with C8s, that would be true because when you move up in aperture with SCTs, the contrast natrually improves (contrast transfer improves with aperture all else being equal).

Along the way, somehow the rumor evolved to say that the C9.25 optics got special treatment.

This is another urban myth because the bench testing I have seen on C9.25s (Rohr in Germany has tested three I think) seem to show the same generaly variations in quality that many other Celestron scopes have shown over the years, (some good, some very good, some excellent... I personally think the C8 enjoys the most consistent optical quality in the Celestron lineup).

I don't choose to debate the merits of the C9. I owned one and found it to be nothing special in any way and more or less between the C8 and C11 in performance.

I do attribute the myth to Ed Ting's review though, and the "Optics are treated special" myth (which Rorh's testing shows is completely unfounded) as just an outgrowth of that.

We don't really know though if people still buy these becasue they think they are "Special" in any particular way.

Perhaps they are just popular because of the balance of size, weight, and aperture. That is why the C8s are so popular and why a lot of people stay with the C11 rather than move to a C14.

We can only speculate.

And of course my answer is only speculation, but having been on CN and the other forum for 15 years or so, I correlate the popularity to the publishing of Ed Tings review.

Heck, I bought one after reading his review. I was disappointed though and quickly sold it to repalce it with a C11.

#19 freestar8n

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Posted 26 February 2013 - 09:26 AM

See my note just before yours.

I believe the primary is f/2.3 - according to some web sources. f/3 would make the tube a lot longer. But f/2.3 is a big change from f/2 since f/2 is so fast.

I don't think the secondary size is much bigger - they are all around 32-36%.

They could have flattened the field by keeping the primary f/2 and used a much larger and closer secondary - with a correspondingly lower overall f/number. But making the primary slower and the tube longer, while keeping the f/ratio the same, means they can flatten the field without increasing the secondary much. The downside is that the tube is longer. That's not so bad in the 9.25 size - but for C14 it would get pretty big, and a big attraction of the sct is that it is compact.

I guess the choice dates back to film days and they targeted this size specifically for long exposure 35mm work. So they knew they needed a flatter field. Now with CCD's and better guiding - they need a *really* flat field - but now provided by additional corrective lenses.

Frank

#20 Eddgie

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Posted 26 February 2013 - 10:22 AM

Ok, apologies for the mistake. My main point was that the design was a bit different most likely to slightly enhance imaging performance.

And I agree, there are a dozen different ways to configure an "SCT."

The "SCT" we buy is only one of several possible configurations, but being a commercial product, the designers clearly looked for the perfect middle ground in terms of design compromise, and I think they nailed it.

The optical compromises were fine when the designs were originated (Naglers didn't exist either when hypered Pan was being used), but I agree that they fell behind the times.

This is why I have said in the past (and still believe) that the EdgeHD design has made the standard SCT obsolete.

But your point is correct. The book Telescope Optics shows for different basic configurations for SCTs, but all of them make a compromise in either packaging or obstruction size.

You can't get too far away from the standard forumla without taking a hit in these other areas.

#21 Stelios

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Posted 26 February 2013 - 01:06 PM

I have owned two 9.25" scopes (still own the 2nd), and two C8's (both gone). Performance-wise, the difference is what the aperture difference would indicate, no more, no less.

As someone else mentioned, the 9.25" is the largest scope that is relatively easy to handle for those of us who are getting up there in years. It can be comfortably handled by the CG-5 and equivalent mounts for visual astronomy, whereas the 11" is iffy on such mounts. I have been thinking of buying an 11" scope, but haven't pulled the trigger because I worry that if I'm unhappy with how it sits on a CG-5, it will be used far less.

#22 TG

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Posted 26 February 2013 - 02:21 PM

Edggie, we've heard more than once that you've been burned by your C9.25 experience. We all like to say that a sample size of 1 proves nothing and I think it applies in your case. Given the positive experience people generally have with a C9.25, there *is* something to be said for the design. As somebody said above in the thread the primary going from f/2 to f/2.3 is a big deal and it was the reason I ditched my otherwise terrific NexStar 11 GPS: I simply couldn't withstand the field curvature. In comparison, the curvature in my C9.25 was unnoticeable. On top of that my C9.25 had excellent optics (see the attached pic), not the case the with NS11, which had run of the mill ones (not bad, but not excellent either). You could say that a sample size of 1 again proves nothing in this case but we've heard very little about bad C9.25s (apart from you and 1 at that French site) and generally people say good things about them. I may be naive but I like to think people aren't generally idiots and know a good image when they see it.

From Piekiel's book, here's how the C9.25 came about: Celestron's earliest method of figuring correctors was to bend the glass onto a master block using vacuum and then grind and polish them flat. When released, the flat side took on the aspherical shape of the master block. To facilitate the vacuum, they had radial grooves running partway from the edge. This often caused the "print thru" that you so often hear reported in older C11s. For reasons I don't understand, they had to use oversized master blocks, e.g., a 10in block for grinding the 8in correctors. Then they developed a new kind of "catenary" master block which had the aspherical curve superimposed on a spherical (?) curve. This kind of block didn't need the radial grooves to facilitate the vaccum and didn't need to be oversized as well. This meant that the 10in master blocks they had refigured to the catenary form would be able to produce a larger corrector than 8in and they chose the 9.25in size. I don't know whether the new master block then forced them to go to a slower primary/secondary combination but I strongly suspect that the newer master blocks improved the overall quality of the C9.25s compared to the other (earlier) SCTs. This coupled with a flatter field and less susceptibility to miscollimation can indeed have resulted in a better SCT.

Tanveer.

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#23 Eddgie

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Posted 26 February 2013 - 10:47 PM

Not saying the design improvements are not without merit.

My own C9 did not have much more than so-so optics.

Rohr has tested three. One was so-so, one was good, and one was excellent.

Given my own sample was only so-so and given the fact that the ones Rorh tested varied, it would be hard to accept that the scopes are built to a higher optical standard than other Celestron scopes because if that were the case, one would expect the quality distribution of four different samples to be more like two excellent, one good and one so-so.. Or 3 excellent and one Good.

But based on four samples, it appears that the quality varies and that indicates that no special attention was given to the design in serial production.

It matters little if you improve your tools and processes or the scope. What matters is what you ship to the customer.

And from a sample of four scopes, only one had excellent optics.

This was typical of Celestron quality.

They only guaranty a diffraction limited scope. Most are good to excellent, but not all.

And from the three bench tests published by Rohr and my own experience, I have a great deal of difficult accepting that the quality of the C9 was "Special."

It is like the Tak Mewlon. Everyone thinks that because they say Takahshi on the side, they must be perfect.

And yet Interferometer testing shows that they are not all that special. At least the ones Rohr has tested.

We believe what we choose to believe, but the interferometer takes no prisoners.

In my case I believe that the quality was nothing special, and as it happens, the optical bench seems to support my position.

As for the design being better, perhaps for imaging. For visual use, it is a mixed bag.

For someone that wants an SCT (packaging preference.. the main reason I like them), if the C8 is to small and the C11 is to large, then it is the perfect scope.

This doesn't make it "Special." It just makes it something between the other choices.

#24 freestar8n

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Posted 27 February 2013 - 05:02 AM

I think there are several different types of "special" being discussed here:

1) Something different about the design that benefits performance: yes - its slower primary yields flatter field, but this may not be noticeable visually. It will also likely improve collimatability, i.e. it would be less sensitive to small errors in collimation. The field flattening follows directly from the Petzval curvature due to the two mirrors.

2) Extra care spent in manufacturing. There is no evidence either way for this.

3) Superior wavefront quality in the assembled and delivered product. There is no real evidence either way for this in a statistical sense, but due to (1), the tolerances in the design are likely more forgiving to achieve a given wavefront quality target. As a result, even without extra care being given, there is good reason to think the wavefront quality in the delivered product would be better - just as an f/2 primary is more forgiving than an f/1.5 primary. The few tested samples and anecdotal evidence either way aren't enough to nail down anything statistically - but nonetheless there is good reason to think that even if the same care is given in manufacturing, the end result with the slower primary will be better.

Again - the downside is the longer tube, but I don't know any high end RC's, CDK's, or maks that use an f/2 primary. They all have the longish aspect ratio that the 9.25 has.

Large observatory telescopes use fast primaries, but they are driven by the need to keep the whole thing compact so it doesn't require a gigantic building. They can also make most any surface to any degree of accuracy they can afford.

With the Edge design, the benefits of the slower primary for a flatter field are moot - but there may still be a small benefit in the quality of the delivered product - without any extra care in manufacturing. But an Edge14HD with f/2.3 primary would be a lot harder to handle than f/2.

Frank

#25 roadi

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Posted 27 February 2013 - 07:17 AM

Rohr has tested three. One was so-so, one was good, and one was excellent.

It is like the Tak Mewlon. Everyone thinks that because they say Takahshi on the side, they must be perfect.

And yet Interferometer testing shows that they are not all that special. At least the ones Rohr has tested.

We believe what we choose to believe, but the interferometer takes no prisoners.


As you love to quote him, I surgest you read more about Rohr's testing and learn more about what determines optical excellence before you make such claims: "It is like the Tak Mewlon. Everyone thinks that because they say Takahshi on the side, they must be perfect."

A little of topic, Sorry.. but so was the claim!

About Strehl difinition and roughness:

http://translate.goo...&prev=_t&amp...






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