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ACF central obstruction

SCT Meade
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#1 Nicole Sharp

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Posted 30 August 2021 - 10:09 PM

Following up on this closed discussion:

 

https://www.cloudyni...-the-meade-acf/

 

Does anyone know what is the diameter of the central obstruction on the Meade ACF 6 and 8?

 

Meade doesn't publish this information and they haven't replied to my emails :-( .

 

From the photos, I would most like to know if the Meade ACF 6 has a larger central obstruction than a Celestron C6.

 



#2 Sky Muse

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Posted 30 August 2021 - 10:42 PM

Hmm...

 

https://i.imgur.com/R1RMEw9.jpg

 

If I had to bet the farm, I'd have to choose the Celestron.

 

I chose a 127mm f/15 Maksutov instead...

 

secondary obstruction.jpg

 

But I see that you've already been there and done that.


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#3 ihf

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Posted 30 August 2021 - 11:15 PM

I just measured. The central obstruction in my ACF 6" is 69-70mm out of 152mm. That makes it 46% by diameter or 21% by area.


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#4 ihf

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Posted 30 August 2021 - 11:49 PM

I had to look it up. The C6 central obstruction is 56mm or 37% (14%). The C8 and EdgeHD 8" are 64mm or 31% (10%) and the C9.25 and EdgeHD 9.25 are both 85mm or 36% (13%).

 

Now I do run my 6" ACF (central obstruction 70mm or 46%/21%) with a 2 inch diagonal. For what it is worth visually I don't notice vignetting in ACF with the Buddha at night. I wonder if that would still be the case with the smaller central obstructions in the C6 or EdgeHD 8"?


Edited by ihf, 31 August 2021 - 12:08 AM.


#5 MrJones

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Posted 31 August 2021 - 09:45 AM

There are a few differences between the C6 and Meade 6" ACF to be aware of:

 

The clear aperture of the C6 is 150mm vs. 152mm for the Meade.

 

The baffle size of the C6 is 26mm vs. 31mm for the Meade.

 

The housing obstruction is 56mm for the C6 vs. 70mm vs. the Meade. The Meade also has an oversized secondary mirror that was masked down a few mm with a black adhesive ring in my sample.

 

So the Meade can actually get a wider FOV with higher illumination but pays for it with the larger obstruction.

 

https://www.cloudyni...ct-maximum-fov/


Edited by MrJones, 31 August 2021 - 09:45 AM.

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#6 ihf

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Posted 31 August 2021 - 12:01 PM

Thats great news for me! I think I would have been less happy with the smaller FOV of the C6.



#7 jgraham

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Posted 31 August 2021 - 03:40 PM

Yeppers, the ACFs tend a run a bit larger to give a wider fully illuminated field. They went through a lot of trouble to give a nice wide well corrected field, might as well take advantage of it. The view through my 6" f/10 ACF is super sharp. Luv it!


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#8 aa6ww

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Posted 31 August 2021 - 04:26 PM

I spent last night going over the specs on the Meade 6" ACF vs my C6SCT, an AstroTech 6" Classical Cassagrain (I like the white tubes for solar), and the 6" SW Mak. 

I think of the Meade as an Edge C6, which makes it a nicer option in my opinion in some ways. I do like to use my C6 SCT with a focal reducer to get the focal length below 1000mm. It makes the pointing accuracy easier on my smallest GoTo mount.

I wonder how these scopes work with big 2" eyepieces. I only use my SCT with 1.25" eyepieces to keep things more balanced.

All 4 of these have their strong points and weaknesses. I'm curious how the 6" Classical Cassagrain would work with 2" eyepieces, since its already set up for that. 

I find myself using my smaller scopes like these more in the winter time, so the open tube classical cassagrain has a winter appeal to me to curb cool down times.

I've never used an ACF 6" Meade but I would like to own one and see how it performs. I bet its very nice with a binoviewer if the field is corrected over the full range of the optics.

 

Ralph



#9 Nicole Sharp

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Posted 31 August 2021 - 04:37 PM

For an EQM-35, an EdgeHD 8, C8, or MC150 would be my top choices.  Unfortunately, those all weigh over 11 pounds (out of a 22-pound payload capacity).  Accessories on the OTA might add another 5 pounds, so the lighter the better.

 

For OTAs under 11 pounds, the two largest possible apertures are either an ACF 6 or a C6.  Though the Meade 1900/127 MCT (with UHTC) might also be a good alternative for imaging of bright planets, and high-magnification views of the Lunar and Solar surface (with a Solar filter).


Edited by Nicole Sharp, 31 August 2021 - 04:38 PM.

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#10 ihf

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Posted 31 August 2021 - 05:49 PM

Ralph, some experiences of the ACF6 with 2 inch eyepieces and here some extra pictures on the Scoptech Zero mount.


Edited by ihf, 31 August 2021 - 05:51 PM.


#11 carolinaskies

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Posted 31 August 2021 - 07:17 PM

The CO of Meade's have always been slightly larger on there scopes but not to any actual performance hit.  The ACF beats the standard SCTs for pinpoint stars.  I'm on the hunt for a 5" or 6" eventually to put on my LX65.   As far as 2" eyepieces they can be used up to a certain field stop size cutoff because the central baffle tube ID will limit the light cone coming out to the diagonal.  I'm sure it's documented somewhere here on CN the varying sizes for the different models. 



#12 ihf

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Posted 31 August 2021 - 07:37 PM

Even though the hole in the back of the Meade ACF6 is only 31-32mm I did not visually notice vignetting with 46mm field stop eyepieces during the day or at night. I can photograph vignetting, but the CO and SA make a mess out of most pictures with the small phone camera aperture.



#13 Bill Barlow

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Posted 31 August 2021 - 07:44 PM

I’ve owned both C6 and 6” Meade ACF scopes and the Meade has a much larger CO as others have said.  This would give the C6 a bit more clear aperture when subtracting the area of the CO vs. the primary mirror.  But both put up very nice views with the Meade being a bit heavier. 
 

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#14 SandyHouTex

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Posted 01 September 2021 - 05:23 PM

The CO of Meade's have always been slightly larger on there scopes but not to any actual performance hit.  The ACF beats the standard SCTs for pinpoint stars.  I'm on the hunt for a 5" or 6" eventually to put on my LX65.   As far as 2" eyepieces they can be used up to a certain field stop size cutoff because the central baffle tube ID will limit the light cone coming out to the diagonal.  I'm sure it's documented somewhere here on CN the varying sizes for the different models. 

You're incorrect on the "pinpoint" stars.  Go here:

 

https://s3.amazonaws...paper_final.pdf

 

Figure 2 shows that a standard SCT has coma, but the ACF has astigmatism that's just as bad.  No pinpoint stars off-axis for the ACF.



#15 ihf

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Posted 01 September 2021 - 06:24 PM

Nice Celestron whitepaper. If I read this correctly then the ACF will beat the standard SCT for a visual observer as long as they eyes are still young and can still adjust (figure 4). For a camera this may not be the case, but it is hard to rank the scatter plots in figure 2. Of course the EdgeHD is best, best, best. Except it is not available in a 6 inch size.


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#16 Chris Johnson

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Posted 01 September 2021 - 06:35 PM

You're incorrect on the "pinpoint" stars.  Go here:

 

https://s3.amazonaws...paper_final.pdf

 

Figure 2 shows that a standard SCT has coma, but the ACF has astigmatism that's just as bad.  No pinpoint stars off-axis for the ACF.

That’s hardly an unbiased paper.


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#17 Nicole Sharp

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Posted 01 September 2021 - 06:44 PM

Someone pointed out in another thread that "coma free" does not mean the same thing as "flat field" but I'm not sure I understand the difference.  The EdgeHD is supposed to have a flat field, but the ACF is merely coma-free.


Edited by Nicole Sharp, 01 September 2021 - 06:46 PM.


#18 luxo II

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Posted 01 September 2021 - 09:33 PM

... and flat field doesn't not mean coma free. either.

Field curvature and coma are two independent aberrations, there are others.



#19 carolinaskies

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Posted 01 September 2021 - 09:40 PM

Celestron's white paper politely refuses to plainly state they took the standard SCT primary and corrector, added a flattener, and only tinkered with tolerances.  The primary is not improved at all, the corrector is not improved, they do say the secondary is slightly massaged to improve performance, but that is prior to coating and would be normal for any company making a non-flat secondary and bench testing it. 

 

You're incorrect on the "pinpoint" stars.  Go here:

 

https://s3.amazonaws...paper_final.pdf

 

Figure 2 shows that a standard SCT has coma, but the ACF has astigmatism that's just as bad.  No pinpoint stars off-axis for the ACF.

An ACF is NOT a standard SCT and Celestron uses the term 'coma-free' as if it's code for ACF but the ACF is not implicitly mentioned because it would be a legal gotcha they could be sued over. 

Coma is predominant in newtonian designs but has always been mitigated for the most part in the modern SCT as manufacturing processes have improved and machine standardized in the 90s.  Standard SCTs tend to display curvature at the extreme edges of the field particularly with cameras because sensors are flat and the focal plane image is actually slightly curved due to the very fast F/2 primary.  Earlier SCTs and modern XLT SCTs can display field curvature with extremely wide 82-100 degree eyepieces but do not tend to do so with ACF telescopes. 

The ACF design uses a hyperbolic rather than spherical secondary and improved corrector plate profile compared to it's older SCT and the XLT and Edge.  The hyperbolic secondary takes care of coma and the improved corrector shape re-aligns the incoming light rays to help give edge to edge point stars rather than curves.  The net effect of the improved corrector is stars are uniform across the field because the wavelengths of light all are converging more uniformly than older SCTs.  

Currently I have seen little evidence to suggest the ACF field suffers significant degradation photographically.  The white paper is not a science paper as much as it's a nice piece of propaganda to convince buyers the Edge is better than a standard SCT.  


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#20 fred1871

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Posted 01 September 2021 - 10:06 PM

The Celestron White Paper makes no outlandish claims. Everything fits well for standard optical understanding. That's the design. If you want more detail, I'd suggest the book Telescopes, Eyepieces, Astrographs by Smith, Ceragioli and Berry (2012). Plenty there.

 

Once you've outlined the design, the next issue is how well its made so as to present the design qualities at a high level. Here there's always a problem - mass produced scopes vary a lot in how well they present the intended qualities. So it's important for the manufacturer to try to avoid outright bad examples - poorly made, or poorly assembled. They can't afford the costs of making each individual scope to a very high standard.

 

Then there's how well the observer can see the quality of the images. Some folk have low standards, possibly because they haven't met better quality scopes. Others notice some issues but not others. Quite a few have little awareness of off-axis coma, for example, or astigmatism near the field edge. That will affect their evaluation of a scope.

 

And of course imagers will have some different requirements from visual observers. Big CO in a scope is not an issue with imaging for deep sky; it can make a visible difference for visual observers especially for planets and double stars. Flat field? - how wide. Much less of an issue with smaller sensors. And field curvature can impact older visual observers more than young ones - the eye gradually loses accommodation with age.

 

So it may well be that a Meade ACF would be better than older design Meade and Celestron scopes for the visual observer due to the coma-free design; but it would need to be otherwise optically good as well. I've seen good and bad in both Meade and Celestron, but more bad Meades than bad Celestrons, and more good Celestrons than good Meades. Most over the past 40-odd years I've looked through were middling adequate. What I'd expect from mass production. The occasional one a stand-out.


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#21 Nicole Sharp

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Posted 01 September 2021 - 10:13 PM

Celestron's white paper politely refuses to plainly state they took the standard SCT primary and corrector, added a flattener, and only tinkered with tolerances.  The primary is not improved at all, the corrector is not improved, they do say the secondary is slightly massaged to improve performance, but that is prior to coating and would be normal for any company making a non-flat secondary and bench testing it. 

 

An ACF is NOT a standard SCT and Celestron uses the term 'coma-free' as if it's code for ACF but the ACF is not implicitly mentioned because it would be a legal gotcha they could be sued over. 

Coma is predominant in newtonian designs but has always been mitigated for the most part in the modern SCT as manufacturing processes have improved and machine standardized in the 90s.  Standard SCTs tend to display curvature at the extreme edges of the field particularly with cameras because sensors are flat and the focal plane image is actually slightly curved due to the very fast F/2 primary.  Earlier SCTs and modern XLT SCTs can display field curvature with extremely wide 82-100 degree eyepieces but do not tend to do so with ACF telescopes. 

The ACF design uses a hyperbolic rather than spherical secondary and improved corrector plate profile compared to it's older SCT and the XLT and Edge.  The hyperbolic secondary takes care of coma and the improved corrector shape re-aligns the incoming light rays to help give edge to edge point stars rather than curves.  The net effect of the improved corrector is stars are uniform across the field because the wavelengths of light all are converging more uniformly than older SCTs.  

Currently I have seen little evidence to suggest the ACF field suffers significant degradation photographically.  The white paper is not a science paper as much as it's a nice piece of propaganda to convince buyers the Edge is better than a standard SCT.  

 

An internal field flattening lens makes sense since otherwise I never really understood why the EdgeHD has the same central obstruction as the Celestron SCT.  Otherwise I figure the central obstruction should be larger, like it is with an ACF or a Ritchey-Chretien (to get the optics faster than an MCT with a coma-free field).

 

It is curious though if all Celestron does is add a field flattening lens, then why isn't the EdgeHD available in smaller apertures?

 

My original question about central obstruction then I think should also apply to the ACF 8 versus the EdgeHD 8, with the ACF 8 having a larger central obstruction than the EdgeHD 8.  Not sure if anyone has compared these two yet.

 

Whether or not the ACF 8 or the EdgeHD 8 is "better" is up for debate (and already the subject of another thread), but there isn't really any equivalent for a 6-inch OTA, other than an MC150 (smaller CO) or an RC6 (larger CO).  I think the ACF 6 still has a smaller CO than an RC6, yes?  Though the biggest advantage I think is the weight (lighter than an MCT, an RC, a Newtonian, or a refractor of the same aperture).


Edited by Nicole Sharp, 01 September 2021 - 10:19 PM.


#22 carolinaskies

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Posted 01 September 2021 - 10:48 PM

My first SCT experience was an 80s AC driven 8". Very nice optics.

As has been stated many times on CN... anecdotal performance of 'bad' SCTs must be asterisked for cool down and collimation issues. And with age also comes a seeming mfg bias when neither company has truly had atrocious optics except during the Haley comet era which is documented well enough. Rather than optics, mount variations more often effect overall opinion.

As I stated, the white paper IS more of a sales pitch than a scientific document. The material creation was garnered from the book publication, to promote the EDGE as the ACF was the well established improved SCT in the market. As stated, in general the performance is not visually a stark difference except when compared to the earlier SCTs.

For all the hoopla of the white paper, I don't believe the EDGE is honestly worth more $$$ unless someone wants to also use Hyperstar. The central obstruction size differences are actually nominal and no significant factor on comparative performance. Weight differences are less critical for 8" and smaller OTAs because stated values mainly differ by only a few pounds or not at all depending on aperture and considering fully dressed as shipped with accessories.

Realistically any modern SCT is going to be significantly benefited by ACF design when compared to non-improved versions.
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#23 SandyHouTex

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Posted 02 September 2021 - 10:04 AM

That’s hardly an unbiased paper.

How exactly do you”bias” a raytrace?



#24 SandyHouTex

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Posted 02 September 2021 - 10:11 AM

Someone pointed out in another thread that "coma free" does not mean the same thing as "flat field" but I'm not sure I understand the difference.  The EdgeHD is supposed to have a flat field, but the ACF is merely coma-free.

ACFs are supposed to be Ritchy-Chreiten like telescopes.  If that’s true, they are corrected for coma at the expense of worse astigmatism off-axis.  Since you’re only dealing with three optical elements, you can’t correct field curvature.  The field curvature of an ACF, and a compact SCT design as well, both have significant field curvature.  That’s why Celestron, with the Edge optics, put a couple lenses in the optical path.  They both correct field curvature and coma for a typical compact SCT design.



#25 Bill Barlow

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Posted 02 September 2021 - 10:47 AM

Even Al Nagler said that if you correct coma then astigmatism is also reduced.  This is what the ACF design does.  I have owned many Celestron and Meade SCT’s and the ACF optics and prefer the ACF design.  But I have never got to observe through an Edge scope yet.

 

Bill




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