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Meade vs. Celestron

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#51 Jon_Doh

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Posted 22 May 2014 - 01:40 PM

According to Meade's website their ACF optics reduces field curvature.


Interesting. Can you point me to where it says that? I couldn't find the reference. All that is mentioned at www.meade.com/acf is the reduced coma--nothing about curvature.

Other owners running tests with different size scopes and different cameras get different CCDInspector results. Look here, for example:

http://www.lx-850.com/?p=296

Of course, this is an f/8 model with a larger chip, so I would expect field curvature to be worse than the example previously given

It's not a flat field scope. It's a coma free scope. I can't find anything other than hearsay that shows reduced field curvature. Not even Meade's own marketing literature--at least what I could find on their website--suggests reduced curvature.



They've changed their website and it's not on the first page like it used to be. Still, there is a reference to "flatter field" on several pages. Here is one explaining the ACF design: http://www.meade.com/history

I've copied the text below:

Advanced Coma-Free optical system, the first affordable optics delivering Ritchey-Chrétien-like performance, in 2005. A traditional Ritchey-Chrétien (RC) is a type of reflector that delivers a coma-free view via hyperbolic primary and secondary mirrors. Because the mirrors in these telescopes have always been very expensive to make, few amateur astronomers could enjoy them. Suspecting that there was a better way to design this optical system, Meade engineers developed a radical new Advanced Coma-Free design by combining a hyperbolic secondary mirror with a corrector-lens-and-spherical-primary-mirror combination that performs as one hyperbolic element. This ACF design produces a coma-free, flatter field of view that equals traditional RC telescopes at a fraction of the cost. The design also eliminates diffraction spikes and improves astigmatism, both of which are inherent in the traditional RC design. No other optical design delivers both the level of performance and affordability as ACF.

#52 SandyHouTex

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Posted 22 May 2014 - 02:56 PM

The post for the Meade says:

"This ACF design produces a coma-free, flatter field of view that equals traditional RC telescopes..."

If it indeed equals traditional RC telescopes then it has significant field curvature. The:

"flatter field of view that equals traditional RC telescopes..."

comment makes no sense.

#53 Jon_Doh

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Posted 22 May 2014 - 03:07 PM

Elsewhere they say they use a modified RC design. The modification is to flatten the field perhaps?

Anyway, the one I had did seem to have a flatter field than my friends's non HD Celestron. But then again, it just could be that my old eyes can't tell the difference. :confused:

#54 Live_Steam_Mad

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Posted 22 May 2014 - 05:25 PM

If you get the book "Telescope Optics" by Rutten and Vanrooij you just need to look at the SCT spot plots with the field curvature numbers and the Ritchey - Chretien spot plots and field curvature numbers.


True. Look at this spot diagram from Telescope Optics for a conventional all-spherical older Meade and Celestron SCT of 8" F10, Radius of curvature for the optimally curved field is 275mm (Field Curvature). Then take a look at the Ritchey-Chretien of 8" F8 and it has a field curvature of 199mm, which is stronger (since the radius is smaller so more curvature) and finally look at the "optimised" 8" F10 SCT where the secondary is aspherised to remove Coma (ACF style) and the field curvature is 155mm, the most strongly curved field of any Cassegrain design.

http://www.cloudynig...ber/6537287/...

Which means I think mean that stars at the edge of the field on the ACF require a decent eye to provide the accomodation necessary, or you need a curved piece of film to see the stars as perfectly round at the field edges on the ACF. Otherwise I think (some small amount of?) Astigmatism would be a problem at the field edges? Much like how the RC is in that respect.

The standard SCT has a flatter field than the ACF but more coma. The Edge HD has a much flatter field and no coma. So the best for imaging or for very old eyes at the field edge would be the Edge HD, no doubt. The Edge HD outperforms the RC in that respect, as I understand it. But the EdgeHD natively is a slower system than the average commercial RC in small to medium sizes.

For what it's worth, after looking at the Double Cluster with 39 year old eyes in a 32mm Plossl with an F10 ACF 8" I saw that the stars were fairly sharp all over the eyepiece. But I was not really specifically looking for sharp stars, I just noticed no aberrations of note. However I am not the best judge visually as I have 3.5 diopters of Astigmatism in my observing eye and some very significant uncorrected Astigmatism even with my glasses on with larger exit pupils.
I just found the view to be very nice LOL.

Regards,

Alistair G.

Attached Thumbnails

  • 6540316-SCT_spot_diagram.jpg


#55 Jared

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Posted 22 May 2014 - 07:31 PM

According to Meade's website their ACF optics reduces field curvature.


Interesting. Can you point me to where it says that? I couldn't find the reference. All that is mentioned at www.meade.com/acf is the reduced coma--nothing about curvature.

Other owners running tests with different size scopes and different cameras get different CCDInspector results. Look here, for example:

http://www.lx-850.com/?p=296

Of course, this is an f/8 model with a larger chip, so I would expect field curvature to be worse than the example previously given

It's not a flat field scope. It's a coma free scope. I can't find anything other than hearsay that shows reduced field curvature. Not even Meade's own marketing literature--at least what I could find on their website--suggests reduced curvature.



They've changed their website and it's not on the first page like it used to be. Still, there is a reference to "flatter field" on several pages. Here is one explaining the ACF design: http://www.meade.com/history

I've copied the text below:

Advanced Coma-Free optical system, the first affordable optics delivering Ritchey-Chrétien-like performance, in 2005. A traditional Ritchey-Chrétien (RC) is a type of reflector that delivers a coma-free view via hyperbolic primary and secondary mirrors. Because the mirrors in these telescopes have always been very expensive to make, few amateur astronomers could enjoy them. Suspecting that there was a better way to design this optical system, Meade engineers developed a radical new Advanced Coma-Free design by combining a hyperbolic secondary mirror with a corrector-lens-and-spherical-primary-mirror combination that performs as one hyperbolic element. This ACF design produces a coma-free, flatter field of view that equals traditional RC telescopes at a fraction of the cost. The design also eliminates diffraction spikes and improves astigmatism, both of which are inherent in the traditional RC design. No other optical design delivers both the level of performance and affordability as ACF.


Since Meade has chosen to remove that sentence from the current description of the ACF design, And since there is ample evidence out there--both empirical measurements using CCDInspector and in textbooks describing aplanatic SCT designs--that it is NOT a flat field scope, I'd consider the current description a correction of the historical description you linked to. It's a good scope that the original poster should give strong consideration to, but its field is not flatter than that of a traditional SCT. Better edge performance--definitely. Flat field like in a Takahashi FSQ or Edge HD? Definitely not.

To those who say stars are sharp to the edge of the field, I have no problem believing that. The ACF aplanatic SCT is definitely a big step up on the traditional spherical secondary SCT, but it does not have a flatter field. Just an improved field due to the removal of coma. For visual use at f/10 on a 10" or 12" model I expect it is every bit as good as a Celestron Edge HD. It just doesn't happen to have a flat field. Neither does my Astro-Physics 130, and I use that as a rich field scope!

#56 Jared

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Posted 22 May 2014 - 07:36 PM

One very minor correction to your post, Alistair... I believe that the ACF has vey little astigmatism, so stars at the edge of a wide photographic field would look slightly bloated but would not be football shaped as they are in an RC. In this respect an ACF is actually an improvement on an uncorrected RC.

#57 GeneT

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Posted 22 May 2014 - 08:06 PM

I am surprised that I am in the minority--but if I were choosing between a Meade or Celestron, I would go with Celestron. I owned a C8 as my primary viewing instrument for about 10 years. It was an excellent instrument. I tend to stick with the horse that got me there.

#58 Live_Steam_Mad

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Posted 22 May 2014 - 08:20 PM

One very minor correction to your post, Alistair... I believe that the ACF has vey little astigmatism, so stars at the edge of a wide photographic field would look slightly bloated but would not be football shaped as they are in an RC. In this respect an ACF is actually an improvement on an uncorrected RC.


Yep, bloated stars but round. Thanks. I am trying to understand all this stuff as best I can, every little helps,
believe me.

Cheers,

Alistair G.

#59 Jon_Doh

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Posted 23 May 2014 - 08:32 AM

Jared, they haven't removed that statement from their website. It's just not on the first page, but on the page of definitions. Also, on several pages describing their SCT's, like the Max.

#60 Jared

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Posted 23 May 2014 - 09:25 AM

Interesting, thanks. I couldn't find it anywhere, but didn't check the Max page. It doesn't mention flatter field on the "About ACF" page or on the individual page for the LX90, but you are correct that it says so on the Max and LX200 pages. I suspect it is an error since the optical textbooks showing spot diagrams for aplanatic SCT's as well as CCDInspector tests of actual scopes still show significant curvature. Just for the heck of it I will ask Meade for a clarification--perhaps they will respond. Tighter stars at the edge--yes. I suspect that is all they meant when they referred to a flatter field since most people think of field curvature as the primary cause of poor edge performance for everything but Newtonians. Thanks for the correction on the website content.

#61 Jared

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Posted 23 May 2014 - 10:39 AM

Oh, just so people don't think I'm making this up or hating on Meade, here is a quote from page 254 of Telescopes Eyepieces Astrographs by Smith et. al. after analyzing the performance of the aspheric secondary coma free SCT,

"The field curvature of all Cassegrains--including Schmidt-Cassegrains--is due to the mirror curvatures. Because the field curvature is essentially independent of the corrector plate and the figures of the primary and secondary mirrors, it cannot be corrected easily. Although the aspheric Schmidt-Cassegrains that we have explored benefit enormously from correction of coma, when you examine the off-axis images on a flat field focused for field center, the outer spots blow up badly. Field curvature is, therefore, the limiting factor for these SCT's."

#62 Spacetravelerx

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Posted 23 May 2014 - 11:52 AM

There are many references out there on the pros/cons of the ACF optics along with the Edge scopes.

There are a variety of optical systems out there for a variety of needs and uses. Yep, that is true!

The ACF has numerous advantages out there, and shockingly some people do prefer the ACF over the standard Celestron telescopes - heck even prefer it over the Edge!

Here is a good link discussing the ACF optics Skypoint Info, however there are many out there.

What I can say is the views from my f/8 ACF Meade are truly wonderful. Counter to all the claims, visually the views are stunning of the night sky. I can wax poetically about the details, but the bottom-line I find the ACF optics to be a winner. Guests and other astronomers when taking a look are very hooked on the view.

The OP is welcome to swing by New Mexico and Logan, Utah (first week of August) to take a look. Heck, I would even welcome side by side views.

For what the op is looking for regarding dabbling in astrophotography the ACF is quite sufficient. I have been ramping up learning this digital ap world, and I am quite impressed with the ACF. It works very well with my Canon 60Da. For Video astronomy the OP will be easily pleased with the ACF.

And if the OP wants to go to the "pro" level in astrophotography? Then we are talking different mounts, different optics and a different price range. For my set-up I am now running about $19k, and it will easily hit $50k with the next planned projects.

My suggestion? Go with the LX90-ACF 10" you will be covered for all the basics. For mostly visual I would take the fork over a GEM any day, especially if you don't plan on any heavy use. And for some light ap, get the wedge with it. Or to cover all the bases get the LX200 10" ACF. A very good all around telescope, easy to setup and take down, and it is a solid telescope. Mine (original 10" SCT) has lasted over 20 years and has traveled well. It was only replaced because of the LX850.

Now, if you are a collector of OTAs and mounts there are other options there too. Still, go with the 10" LX90 or LX200 and you are good.

Oh, I would take the Meade controller over the Celestron any day.

#63 Alph

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Posted 23 May 2014 - 12:47 PM

Oh, just so people don't think I'm making this up or hating on Meade, here is a quote from page 254 of Telescopes Eyepieces Astrographs by Smith et. al. after analyzing the performance of the aspheric secondary coma free SCT,



That book does not describe the optical prescription of the Meade ACF. The design described in that book was made up on the fly for the book.

Also Meade claim that the ACF has a flatter field than a conventional SCT. They said it multiple times and they stand behind it.

BTW. The authors of the book made a very poor decision in choosing ZEMAX for their examples. They made it easy for themselves not us amateurs and I really dislike this type of attitude.

#64 Jared

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Posted 23 May 2014 - 06:53 PM

I have faxed Meade to ask for clarification. Perhaps they will respond. I agree that the sample in the book is based on a guess as to the design of the ACF rather than being based on information shared by Meade. Still, the authors stated that field curvature is "essentially independent of the corrector plate and the figures of the primary and secondary mirrors". Since there is no field flattener included in the ACF design, either the authors are profoundly mistaken as to how aplanatic SCT's work, or Meade is wrong.

What is wrong with ZEMAX? I understand is lower end than OSLO, but don't know why it would be a worse choice for illustrations for amateurs. Never mind. That's probably too far off topic.

#65 Jared

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Posted 23 May 2014 - 07:00 PM

Despite my pushing back on whether the ACF scopes have a significantly flatter field than a traditional SCT, I certainly agree it's a good match for the original poster's requirements. The field of a 10" f/10 scope is well enough corrected that it is extremely unlikely field curvature would be the limiting factor in image quality for photographic use (unless the scope were de-forked, put on a much more expensive mount, etc.), and the scopes are very good visually. It fits the requirements very well.

#66 bigj

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Posted 23 May 2014 - 08:39 PM

The R design was to be better than the SC but not as flat as the RCX F-8. The F-8 was designed to perform as a Ritchey-Chrétien this is why they named it RCX F-8. The objective of this design was to give the average astronomers to get equipment that was once only the elite could afford. Because of a lawsuit Meade changed the name.
In the past
• The R is an F-10 flat not as good as a RCX. Much better than a SC
• The RCX is a F-8 it is flat the design was made to mimic the Ritchey-Chrétien performance
Now today
• ACF F-10 is the R optical design
• ACF F-8 is the RCX optical design
From Wikipedia
ACF Telescopes[edit source | editbeta]
ACF (Advanced Coma-Free) is an altered version of the Meade's previous schmidt-cassegrain telescopes that replaces the traditional spherical schmidt-cassegrain secondary mirror with a hyperbolic secondary mirror. In the new design the full aperture corrector is slightly altered in shape and combined with a spherical primary mirror.[8] Meade's literature originally describe their ACF as a variation on the Ritchey-Chrétien telescope, although it does not use the two hyperbolic mirror combination in that design (being more of an aplantic design).[9] After a legal settlement Meade dropped the claim.

#67 Alph

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Posted 23 May 2014 - 08:53 PM

What is wrong with ZEMAX? I understand is lower end than OSLO, but don't know why it would be a worse choice for illustrations for amateurs.


If the prescriptions were in the OSLO format then it would be easier to tinker with them using the free version of OSLO. There is no free version of ZEMAX.

#68 Rand Barthel

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Posted 23 May 2014 - 11:04 PM

We're talking fall of 2013 here, and at that time the research I did (a lot of it here on these forums) indicated that the LX80 mount had a lot of bugs and was really not solid enough to count on as an AP mount. I knew of no bugs in the LX850, which was way out of my price range. Meade did not have an intermediate, AP-capable GEM with the bugs mostly worked out of it, to compare to the Celestron CGEM, so that's what I bought. I'm not bashing Meade or saying that Celestron is better. Both companies have strong points and weaknesses in their products lines, and at the time I was in the market, Celestron's strengths aligned better with my needs. It also didn't help Meade that the financial buzzards were circling over them at the time, which caused me to wonder if they would be around if my scope needed service.

#69 freestar8n

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Posted 24 May 2014 - 04:57 AM

Since there is no field flattener included in the ACF design, either the authors are profoundly mistaken as to how aplanatic SCT's work, or Meade is wrong.



Hi Jared-

There is a subtlety here that may or may not be relevant. When the authors talk about "field curvature" they are talking about Petzval curvature - and the only thing that matters for that is the curvature of the element surfaces - and the refractive indices of any lenses. The spacings and aspheres don't matter. This surprising fact has been known since the 1800's - but somehow many people in CN circa 2014 don't know it.

But there is another type of field curvature that is affected by other factors - and that is the curvature of the surface of "best focus." If you combine intentional astigmatism with the Petzval curvature, you can make the image "flatter" in terms of the best image surface. But even though it is flat, the field curvature will cause the stars to swell up as you go away from the center. If everything is tuned right the stars may be approximately round - but they have to swell because the Petzval curvature is always there.

Another way to put it is that having stars swell out from the center is neither necessary nor sufficient to tell if you have a best image surface that is curved. But it is sufficient to tell you the image is hurt by aberrations.

So a better description would be that you know immediately that the ACF has done nothing to alter the Petzval curvature - except going from f/10 to f/8 changes curvatures and will help. The field may or may not be additionally flattened by astigmatism - but in terms of stars getting bigger away from the center - you know they will, and they have done nothing to reduce that because it requires added lenses, as seen in professional instruments with wide and corrected fields.

Or you use a curved sensor, as in Kepler.

Yet another case of - no free lunch.

Frank

#70 GJJim

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Posted 24 May 2014 - 07:04 AM

No, Rodger is correct that all the expected field curvature is still there on the ACF scopes. However, most of what causes soft stars near the edge of field on a regular SCT is coma, not field curvature, and the ACF design DOES remove that. No question you can get sharp stars across a much larger field with an ACF scope than with a traditional SCT which is really the point. Also remember that the larger the scope, the smaller the amount of field curvature for a given eyepiece, so the fact that you see little or none in your 14" scope does not mean none would be visible in, say, an 8" scope.


IIRC this was confirmed several years ago in an optical analysis of the f/10 ACF design by Roland Christen. He bought a 10-inch ACF to play with and mentioned that the optical design seemed a compromise in many ways. It trades larger (bloated if you will) spot sizes in the center of the field for much less coma. The higher optical power of the ACF corrector also adds some chromatism, something that is typically negligible in a standard SCT.

The authors of TEA also prescribed a two-element field flattener for their reverse-engineered, f/10, ACF design. Anyone can try their hand at tweaking this simple corrector design to make the newer f/8 ACFs better astrographs.

#71 BillP

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Posted 24 May 2014 - 10:47 AM

That is an interesting post you refer too. He said the hyperbolic secondary of the ACF would make collimation more sensitive (but he also mentioned how fussy the Celestron was in that regard anyway). He also said that the ACF would have more on-axis sphero-chromatism so its off-axis would be sharper than its on-axis, and for the Celestron just the reverse with its on-axis sharper than its off-axis. And oddly he said that the off-axis coma correction on the ACF would not really be visible to the visual observer. This latter point seems off-base given the user reports. Not so sure how much of that post I would take to the bank.

#72 SandyHouTex

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Posted 24 May 2014 - 10:50 AM

Once again Ritchey-Chretiens do not have flat fields. If you think so please cite a reference. In "Telescope Optics" the field curvature for a R-C is similar to a standard SCT. Only the Celestron Edge HDs have flat fields. A standard SCT and a R-C do NOT have enough optical elements to correct for it. Geez.

#73 Alph

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Posted 24 May 2014 - 11:12 AM

The authors of TEA also prescribed a two-element field flattener for their reverse-engineered, f/10, ACF design.


Again, they did not reverse-engineer the Meade ACF. They have just given a description of a similar but different design.

#74 Jared

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Posted 24 May 2014 - 11:13 AM

Yes, I do recall that Roland tried out a 10" ACF. I also know that there is some spherochromatism inherent in the design that is almost non-existent in a traditional SCT. Why is everyone confused by the fact that the Petzval surface (to incorporate Frank's clarifications) is as curved as in a regular SCT? I'm not saying anything bad about the scope. All scopes are compromises. My beloved A-P 130 has a much smaller radius of curvature than a 10" ACF as well as chromatic aberration and likely more spherochromatism. Yet it's an excellent scope--one of the best for its size and weight. We seem to take the marketing hype as gospel for some reason. I think we all know Meade can overdo it a bit in some of their marketing copy. Hubble in your back yard, anyone? Heck, there are even examples in this thread--where Meade originally claimed the ACF design was a Ritchey Chretien in order to benefit from the excellent reputation of high-end, expensive Ritcheys. In fact, the scope just has some of the benefits of a Ritchey--such as no off-axis coma. It took a lawsuit, a rather silly one if you ask me, to get Meade to remove that claim and modify their marketing literature and naming. It's a great scope. I just don't want people thinking it's something it's not--a flat field scope. It has significant field curvature just like any SCT without a flattener.

#75 Jared

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Posted 24 May 2014 - 11:26 AM

Frank:

I've been thinking about it and it may be an even simpler explanation than you provided. If Meade changed the relative powers of the two mirrors very slightly it would could make the radius of curvature of the Petzval surface slightly longer and they could claim it's "flatter" without it being very MUCH flatter. Might be as simple as that. I don't know, of course.

The subtleties you mentioned--surface of best focus vs. Petzval surface--are not unlike choosing a point of best focus for the field in an image rather than just focusing in the center. That's a trick many astro photographers use with astrographs that aren't quite flat enough for the required field. Yours also seems a plausible explanation for making a claim of "flatter". In either case we are talking about a subtlety. Field curvature in an aplanatic SCT remains in approximately the same levels as in a traditional SCT.


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