Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Edge HD SCT's

  • Please log in to reply
28 replies to this topic

#1 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 03:15 PM

Kind of wonder if the only difference in the optics of an Edge HD and a regular SCT is the flatner optics in the baffle tube?   I have not read anything that stated the primary, secondary or corrector plate was any different.



#2 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 03:20 PM

I did find this

 

The inspiration for the EdgeHD optics came from combining
the best features of the CDK with the best features of the
classic SCT. We placed two small lenses in the beam of light
converging toward focus and re-optimized the entire telescope
for center-to-edge performance. In the EdgeHD, the primary and
secondary mirrors retain smooth spherical surfaces, and the
corrector plate remains unchanged. The two small lenses do
the big job of correcting aberrations for a small increment in
cost to the telescope buyer. Furthermore, because it retains key
elements of the classic SCT, the EdgeHD design is compatible
with the popular Starizona Hyperstar accessory.



#3 dswtan

dswtan

    Viking 1

  • *****
  • Posts: 622
  • Joined: 29 Oct 2006
  • Loc: Morgan Hill, CA

Posted 07 May 2021 - 04:07 PM

Have you reviewed the white paper?

https://www.firstlig...9-25-Final_.pdf

https://nexstarsite....Whitepaper.pdf 



#4 Jinux

Jinux

    Messenger

  • *****
  • Posts: 432
  • Joined: 25 Jun 2007
  • Loc: Bay Area, CA

Posted 07 May 2021 - 04:15 PM

Optical system wise, they're identical if I understand whitepaper correctly, but there are some mechanical improvement (section 5) to achieve full frame coma free with flat field, they claim.

For example.

 

 

 

Because it covers a wide field of view, the optical elements of
the EdgeHD must meet centering and alignment tolerances
considerably tighter than those of the classic SCT design. For
example, because the corrector plate must remain precisely
centered, we secure it with alignment screws tipped with soft
Nylon plastic. The screws are set on the optical bench during
assembly while we center the corrector plate. Once this
adjustment is perfect, the screws are tightened and sealed with
Loctite® to secure the corrector in position. This seemingly small
mechanical change ensures that the corrector plate and the
secondary mirror mounted on the corrector plate stay in
permanent optical alignment.


#5 carolinaskies

carolinaskies

    Mercury-Atlas

  • *****
  • Posts: 2,748
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 07 May 2021 - 04:46 PM

Essentially an Edge is a hypertuned SCT with an internal 2-lens flatner.  

To achieve this the following was done

1. Corrector aligned and secured in place to perfectly match the primary mirror. 
2. The interior primary sled was redesigned to a higher precision so primary movement was reduced further than the standard XLT model. 
3. An internal flatner was specifically tuned in place in the baffle to meet specific tolerances of backfocus to achieve the flat field. 

Long term the question arises, if a consumer disassembles the telescope can they reassemble with the same expectations as shipped from factory.  

Meade approached the issue differently.  Rather than add a flatner they hypertuned the corrector shape and changed the secondary mirror from spherical to hyperbolic. In the new design the effect is to make all stars pinpoint which has the effect of making the field appear flat even if there is some curvature.  The classical SCT could appear to have curvature in some eyepiece designs where edge stars where slightly elongated. This is no longer an issue.  The curvature could also be noticed with full frame cameras in classical design. 



#6 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 05:17 PM

of course



#7 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 7,212
  • Joined: 24 Feb 2007
  • Loc: Ellensburg, WA

Posted 07 May 2021 - 05:18 PM

The EdgeHD has an optic in the baffle tube that corrects for coma and field curvature.  I don't know for sure whether the rest of the optics are identical to the XLT version or not.  I suspect that the folks at Starizona would know for sure.  They designed and market the Hyperstar lens for these scopes.  If the only difference between the EdgeHD and XLT versions is the lens group in the baffle tube, then the optics in the Hyperstar would be identical for both versions.  If there are other differences in the optics, then Starizona would have needed to alter the Hyperstar optics to accommodate.

 

The Meade ACF scopes use, an aspherized secondary to correct for coma, which accounts for the "pinpoint" appearance of the stars.  It does not correct for field curvature at all.  It behaves very much like an RC in that regard.  I don't know if they changed the figure on the corrector plate or not, but it's certainly possible.  Note that the stars are round across the field, because it's coma that affects their shape, and that is well corrected in the ACF scopes.  I have owned and imaged with an ACF scope and the stars look excellent across the field and into the corners.  If you were to critically measure them, though, you would see that the field curvature affects the FWHM such that it varies slightly across the field.



#8 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 05:21 PM

Optical system wise, they're identical if I understand whitepaper correctly, but there are some mechanical improvement (section 5) to achieve full frame coma free with flat field, they claim.

For example.

I have a couple of HD scopes and have been playing with the assemblies on the bench.   So far I think if I can figure out how to mount the achromat they use in a regular SCT I want to test it on the bench and see how it changes things.  I took one out of one Edge HD and noticed the DPAC did not change.  I was surprised at that.   However the backfocus on an edge HD changes a bit when the lens is removed.    



#9 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 05:29 PM

The EdgeHD has an optic in the baffle tube that corrects for coma and field curvature.  I don't know for sure whether the rest of the optics are identical to the XLT version or not.  I suspect that the folks at Starizona would know for sure.  They designed and market the Hyperstar lens for these scopes.  If the only difference between the EdgeHD and XLT versions is the lens group in the baffle tube, then the optics in the Hyperstar would be identical for both versions.  If there are other differences in the optics, then Starizona would have needed to alter the Hyperstar optics to accommodate.

 

The Meade ACF scopes use, an aspherized secondary to correct for coma, which accounts for the "pinpoint" appearance of the stars.  It does not correct for field curvature at all.  It behaves very much like an RC in that regard.  I don't know if they changed the figure on the corrector plate or not, but it's certainly possible.  Note that the stars are round across the field, because it's coma that affects their shape, and that is well corrected in the ACF scopes.  I have owned and imaged with an ACF scope and the stars look excellent across the field and into the corners.  If you were to critically measure them, though, you would see that the field curvature affects the FWHM such that it varies slightly across the field.

Yes I know about the optic in the baffle tube and what it does.  The white paper goes over this.   I am curious if this is the only optical difference.   There are mechanical differences.  The modified baffle tube to accept the lens to flatten the field is interesting.  It is quite different compared to the normal SCT.    I presume it has to be at a certain point if the optic to make the correction for a flat field.   What I also found interesting in the white paper they tell the truth about why a normal SCT is only decent in the center of the optics and the optic in the edge HD baffle corrects for field curvature that we all hate.



#10 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • Posts: 5,839
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 07 May 2021 - 05:35 PM

Essentially an Edge is a hypertuned SCT with an internal 2-lens flatner.  

To achieve this the following was done

1. Corrector aligned and secured in place to perfectly match the primary mirror. 
2. The interior primary sled was redesigned to a higher precision so primary movement was reduced further than the standard XLT model. 
3. An internal flatner was specifically tuned in place in the baffle to meet specific tolerances of backfocus to achieve the flat field. 

Long term the question arises, if a consumer disassembles the telescope can they reassemble with the same expectations as shipped from factory.  

Meade approached the issue differently.  Rather than add a flatner they hypertuned the corrector shape and changed the secondary mirror from spherical to hyperbolic. In the new design the effect is to make all stars pinpoint which has the effect of making the field appear flat even if there is some curvature.  The classical SCT could appear to have curvature in some eyepiece designs where edge stars where slightly elongated. This is no longer an issue.  The curvature could also be noticed with full frame cameras in classical design. 

The two lenses do way more than just flatten the field in the HD.  They also correct for coma and astigmatism.

 

In the case of the Meade ACFs, they still have significant field curvature and astigmatism.  To correct those two items, Meade would have to add more optical elements, as Celestron has already done.


Edited by SandyHouTex, 07 May 2021 - 05:37 PM.


#11 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 7,212
  • Joined: 24 Feb 2007
  • Loc: Ellensburg, WA

Posted 07 May 2021 - 05:49 PM

I am curious if this is the only optical difference. 

That's why I suggested contacting Starizona.  Since their Hyperstar lens depends on both the primary mirror and the corrector plate, they would need to know with certainty if those were the same between different versions or not.  It's still possible that Celestron has altered the secondary mirror, since that is removed when you attach the Hyperstar lens, but I believe that I've read that the secondary is the same.



#12 Tangent

Tangent

    Sputnik

  • *****
  • Posts: 47
  • Joined: 24 Feb 2021

Posted 07 May 2021 - 05:55 PM

The Meade ACF scopes use, an aspherized secondary to correct for coma, which accounts for the "pinpoint" appearance of the stars.

The LX3 manual from Meade mentions that the secondary is aspherical; did they switch to spherical at some point before they brought out the ACF line?

 

MeadeLX3.png



#13 carolinaskies

carolinaskies

    Mercury-Atlas

  • *****
  • Posts: 2,748
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 07 May 2021 - 06:40 PM

The LX3 manual from Meade mentions that the secondary is aspherical; did they switch to spherical at some point before they brought out the ACF line?

 

attachicon.gifMeadeLX3.png

Meade upgraded their design starting with the infamous "ritchey chretien" mod... replacing the secondary and modifying the shape of the corrector plate.  The last classical SCT from Meade were the standard GPS telescopes circa 2004 which were replaced by the -R and then full ACF.  

The secondary was changed to HYPERBOLIC which paired with the spherical primary allowed the field to have pinpoint stars.  

Despite SandyHouTex claim, the ACF is performing on par with the Edge.  Not sure whether his claim on astigmatism is coming from personal experience or not.  As for curvature... it IS NOT noticeable visually as it was in classical SCTs if certain eyepieces were in use.  These days the only people claiming curvature in an ACF seem to be non-ACF owners.  



#14 dustyc

dustyc

    Viking 1

  • *****
  • Posts: 716
  • Joined: 10 Oct 2014
  • Loc: Phoenix,AZ

Posted 07 May 2021 - 06:53 PM

Former C11 HD owner. Prior to buying mine I read the "white paper". I think the magic comes from the lenses in the baffle tube. They're a mass producer of telescopes so "cost effective" is the name of the game. If producing these needed a complete reengineering of the product I think it never would have gotten off the design screen. 

Same for Meade. The ACF gets you pretty close without major redesign.



#15 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 06:54 PM

Ok guys let's stick to the edge HD topic

#16 carolinaskies

carolinaskies

    Mercury-Atlas

  • *****
  • Posts: 2,748
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 07 May 2021 - 06:56 PM

Here's an interesting comment in an old thread on how the classic SCTs from Meade and Celestron approached reducing coma.  Refiguring vs best match.   What it indicates to me is that those who are noticing aberrations in their SCT likely have either misadjusted optics or a unit that slipped by with less than quality attention to detail.   

https://www.cloudyni...t/#entry6216191

 

I would say the Edge and ACF scopes are still receiving personal attention to detail in fitting the flattner and matching the secondaries to best performance.  

Personally I think either is a very fine instrument.  The Edge simply is as I said earlier a hypertuned optical set vs the classic XLT


Edited by carolinaskies, 07 May 2021 - 06:58 PM.


#17 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 7,212
  • Joined: 24 Feb 2007
  • Loc: Ellensburg, WA

Posted 07 May 2021 - 07:34 PM

 These days the only people claiming curvature in an ACF seem to be non-ACF owners.  

In order to keep this on the EdgeHD topic, the following is the last thing I will say about the ACF scopes in this thread, but I want to make sure that we are being accurate.

 

I am claiming field curvature in the ACF.  It is absolutely part of the optical design, regardless of how many times you claim the contrary.  The only way to correct field curvature in a Cassegrain scope is with refractive optics, which the ACF lacks (yes, the corrector plate is refractive, but it corrects spherical aberration and is not the right kind of optic to correct curvature).  That said, I fully agree with you that the ACF scopes can produce excellent images.  They don't benefit from false or misleading claims regarding curvature.

 

And despite your claim above, I was an ACF owner and user for many years.



#18 TG

TG

    Mercury-Atlas

  • *****
  • Posts: 2,725
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 07 May 2021 - 07:39 PM

Some time ago, an optical expert on CN tested a C14HD with a sophisticated commercial interferometer. The secondary turned out to be perfectly spherical. No aspherization, etc. on it.  Given the constraints of cassegrain designs, this means that the corrector+primary combination has to be the same as before to achieve full spherical correction, if the corrector lenses do not affect spherical aberration. Since there is little incentive for Celestron to change tooling, I surmise that the EdgeHDs are exactly the same as the classics, with the mirror spacing changed a bit to give a longer back-focus.

 

This is only a theory, of course.

 

TG



#19 TG

TG

    Mercury-Atlas

  • *****
  • Posts: 2,725
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 07 May 2021 - 07:42 PM

The only way to correct field curvature in a Cassegrain scope is with refractive optics, which the ACF lacks (yes, the corrector plate is refractive, but it corrects spherical aberration and is not the right kind of optic to correct curvature).  T

The alternative is to not have it in the first place. You can have flat field cassegrains without any corrective optics but they tend to have enormous secondaries.

 

TG



#20 davidc135

davidc135

    Surveyor 1

  • -----
  • Posts: 1,627
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 07 May 2021 - 07:54 PM

In order to keep this on the EdgeHD topic, the following is the last thing I will say about the ACF scopes in this thread, but I want to make sure that we are being accurate.

 

I am claiming field curvature in the ACF.  It is absolutely part of the optical design, regardless of how many times you claim the contrary.  The only way to correct field curvature in a Cassegrain scope is with refractive optics, which the ACF lacks (yes, the corrector plate is refractive, but it corrects spherical aberration and is not the right kind of optic to correct curvature).  That said, I fully agree with you that the ACF scopes can produce excellent images.  They don't benefit from false or misleading claims regarding curvature.

 

And despite your claim above, I was an ACF owner and user for many years.

I agree. Rutten and van Venrooij in Telescope Optics give the example of an 8'' sct corrected for coma. In spite of being strong, field curvature is barely noticeable at the edge of a 20mm field but is very obvious if the field is doubled. According to the diagram.

 

Astigmatism isn't corrected in the ACF because there is virtually no astigmatism to correct. Likewise with the Edge.

 

David

 

PS Astigmatism in the ACF example above was smaller than the Airy disc even at the edge of a 40mm field.


Edited by davidc135, 08 May 2021 - 03:14 AM.


#21 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • topic starter
  • Posts: 24,073
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 07 May 2021 - 07:56 PM

Some time ago, an optical expert on CN tested a C14HD with a sophisticated commercial interferometer. The secondary turned out to be perfectly spherical. No aspherization, etc. on it.  Given the constraints of cassegrain designs, this means that the corrector+primary combination has to be the same as before to achieve full spherical correction, if the corrector lenses do not affect spherical aberration. Since there is little incentive for Celestron to change tooling, I surmise that the EdgeHDs are exactly the same as the classics, with the mirror spacing changed a bit to give a longer back-focus.

 

This is only a theory, of course.

 

TG

I have some extra C14 optic sets.   Going to see if I take the baffle from a edge HD and use it to build a C14 with regular optics and install the corrector and see what the results are.   Might be a while before I get to this project.  Will be interesting. 


  • TG likes this

#22 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,524
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 07 May 2021 - 08:54 PM

This topic comes up periodically and although it used to require some speculation, nowadays there is more info to go by.

 

First it's good to know how these designs are related:

 

Normal spherical sct:  Corrected for spherical and chromatic aberration.  Has coma, astigmatism and field curvature

 

ACF: Corrected for coma but still has astigmatism and field curvature

 

RC: No chromatic aberration or coma but has astigmatism and field curvature.  Not intended for imaging but good for photometry and astrometry

 

EdgeHD: flat field aplanat - corrected for spherical, coma, astigmatism and field curvature.  Diffraction limited over large area - unlike all the above.

 

As for how the internals depart from the normal XLT versions - before the details were known I was confident they at least changed things a bit - and the obvious one would be the secondary focal length - and possibly spacings.  After EdgeHD came out a new good book on recent telescope designs came out - and it is by Smith, Ceragioli and Berry.  It's a little cagey because they say they got the exact design from Celestron - but what they show is slightly altered.  So technically it doesn't exactly match the design - but it sure makes sense to me.  If you compare the EdgeHD and XLT designs, the primary and corrector plate are unchanged, but the secondary focal length is different, as is the spacing from the primary.

 

If you have a design that operates well as-is and you want to add lenses to it to fix additional things - it is very different if you want those lenses to be optional or not.  If they are intended to be permanent then it would be crazy to stick to the original design and make the new lenses work with it - because by allowing changes in the original you get an additional degree of freedom that acts like a free lens.

 

When this is done to an RC so it works well with flattener, it is called a quasi-RC by some - because it doesn't have the exact design of a functioning RC, but it's close.  And it works very well with the added lenses.  Similarly the CDK is based on a quasi-DK design and wouldn't work well without the lenses.  But I don't know anyone using the term quasi-SCT - but I think it applies.

 

As for whether or not the secondary is retouched a bit to make it work well with each set of optics - all I know is that the whitepaper says they do that, so I have no reason to doubt they do - but they may do it on an as-needed basis.  Or that might be old info and tooling/suppliers have changed so it isn't needed anymore.  At the same time I think it's common sense that testing one or even several samples won't answer it definitively - particularly if the retouching amounts to making the secondary a better sphere, rather than an asphere.

 

Frank


Edited by freestar8n, 07 May 2021 - 08:57 PM.

  • bortle2 likes this

#23 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,524
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 08 May 2021 - 12:35 AM

I have some extra C14 optic sets.   Going to see if I take the baffle from a edge HD and use it to build a C14 with regular optics and install the corrector and see what the results are.   Might be a while before I get to this project.  Will be interesting. 

I take it as a given that the secondary radius is different in EdgeHD and the spacing from the secondary to the primary is different - but that spacing may just amount to shifting the primary a bit on the normal focusing shaft.  So there may not have been any changes to tube length or other kinds of tooling - the existing focusing range may be large enough to accommodate the change in spacings needed.

 

What this means is that if you stuck the flattener system in a normal XLT (somehow) you might still be able to come to focus.  But you would like have far from an optimal view.

 

If somehow you did it and everything was set up and exactly right and the full field was well corrected - based on an image - that would suggest the secondary radius isn't any different.  But I doubt that is the case.

 

If you have the two systems to play with, I would just compare the secondaries.  They are easy to remove.

 

This also means you can do the equivalent comparison just by swapping the secondaries.  You might need to move the primary a bit to reach focus - and that would reflect the change in spacing that is part of the design.

 

Hah - I just remembered I have an old C11 XLT and EdgeHD11 also.  I guess I will take a look.

 

Frank


Edited by freestar8n, 08 May 2021 - 12:43 AM.


#24 davidc135

davidc135

    Surveyor 1

  • -----
  • Posts: 1,627
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 08 May 2021 - 03:41 AM

It sounds from post#8 that the corrector doublet is altering the back focus and I guess the focal length just slightly in which case why alter the secondary?

The doublet (is it cemented?) has to do a lot: remove coma and field curvature whilst not introducing spherical aberration or colour error. Also, either cure astigmatism (or not introduce it) or alter the scope focal length significantly. 6 fronts which is pretty nifty.

 

David


Edited by davidc135, 08 May 2021 - 03:48 AM.


#25 davidc135

davidc135

    Surveyor 1

  • -----
  • Posts: 1,627
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 08 May 2021 - 04:52 AM

Had a look at page 10.1.2 of Telescope-Optics.net which pointed out some limitations of a doublet flat field corrector for Scts. In his example slight astigmatism was introduced to flatten the field and coma and spherical aberration couldn't be simultaneously completely removed without a small refiguring of the Schmidt plate. It would be interesting to see how the Edge compares. Maybe improved glass choice solved residual errors.

 

David


Edited by davidc135, 08 May 2021 - 05:12 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics