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Equipment Discussions >> Cats & Casses

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crow
sage


Reged: 07/09/12

Loc: BC, Canada
EdgeHD White Paper, A Must Read.
      #5529695 - 11/20/12 09:49 AM

Nice white paper on the EdgeHD design, production etc available for those that are interested. Lots of good info on the Focal reducers too.

http://tinyurl.com/c4n69wn

under the more info button. (Celeston Webpage)

These are cool/capable scopes.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: EdgeHD White Paper, interesting read. new [Re: crow]
      #5529782 - 11/20/12 10:37 AM

Wow. An excellent white paper.

I have been complaining for years that US customers don't seem interested in having spot diagrams supplied for the equipment they are considering purchasing, and this paper has it all.

This is the kind of data US consumers should demand from suppliers.

I was an early champoin of the EdgeHD and remain convinced that it is one of the finest telescope I have ever owned. The quality is fantastic, and the visual off axis performance is as good as my 6" APO.

And now we have the data that shows exactly how good the performnace can be.

Of particular interest are the cutaway diagrams of the various desings that shows how rays comeing from the edge of the mirrors just miss the baffles. Many people struggle with how an SCT can loose aperture with back focus, and from these drawings, it is possible to see how moving the mirror forward only a few millimeters can cause the light cone to be cut off.

Anyway, an superb paper because of the technical content. I wish amateurs would insist on seeing spots for the equipment they are buying.

But because most do not truely understand how to read the spots, and because they often give the impression that a telescope does not perform as well as it does (you have to understand the distribion methods and density), I can understand why they have been reluctant to do so in the past.

But imagers want it more than ever, and we all should follow their example.

My hat is off to Celestron for both producing the perfect SCT, and for having the guts to publish the spots that show how excellent the design really is.


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herrointment
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Reged: 03/12/11

Loc: North of Hwy. 64
Re: EdgeHD White Paper, interesting read. new [Re: Eddgie]
      #5530967 - 11/20/12 08:18 PM

That white paper is answering questions I've had and questions I had not thought of.

I'll need a few read-thrus to digest it.

A must read IMO.

What a link! Thanks!


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crow
sage


Reged: 07/09/12

Loc: BC, Canada
Re: EdgeHD White Paper, interesting read. new [Re: herrointment]
      #5530979 - 11/20/12 08:26 PM

Yeah I'm still digesting it, lots of good info. I like how they go into the philosophy behind the design and their thinking behind it. They even produced a couple of CDK prototypes, before coming up the Edge.

Changed the title of the post.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: EdgeHD White Paper, interesting read. new [Re: herrointment]
      #5531092 - 11/20/12 09:34 PM

Yes, this paper is to me ground breaking in that it is the first time I have ever seen spots actually published by Celestron.

And it confirms what another forum member told me a couple of weeks ago, which is that the formula for the EdgeHD scopes had changed (f/1.9 primary). When the design first came out, other members of the Cats and Casses forum swore that the mirror forumala had not changed.

I was convinced that it had though, because the optimal back focus positions had changed, and I did not think it would have been possible without changing something in the mirror forumla. I was told that I was wrong though, and a year later someone told me that the forumla had indeed changed.

And now the paper confirms this.

But this is an excellent paper and I am thrilled to see that Celestron put it out there.

And it clearly shows the huge improvement in off axis performance.

The spots even shows that a sharp eyed observer can indeed see the expanded fresnel pattern of a defocused star.

I had people I think not believe me when I would tell them that using a 22mm Nagler, I could see that stars at the outside were defocused enought to show a couple of rings, and I think they simply thought I was daffy.

But the spots show it clearly. The Standard SCT has a couple of wavelenghts of defocus at the edge of a low power eyepiece, and with modern wide field eyepeices, a sharp eyed observer can easily see this.

Celestron should be praised by all of us for finally matching Takahasi by publishing spot diagrams for at least some of its scopes.

Once again though, I am not confident that enough people out there understand how spots are generated and how to interpret them. For example the spot diagrams for coma far overstate how much you can see at the eyepiece because the density at the point is many dozens of times more intense as it is even part of the way out in the fan.

But the fan will show in a long exposure picture, but not visually.

With this informatoin, I hope that more people start to take an interest in spot diagrams, how to read them, and the value they bring to the consumer.


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Gord
Carpal Tunnel
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: EdgeHD White Paper, interesting read. new [Re: Eddgie]
      #5531190 - 11/20/12 10:32 PM

Hey Eddgie,

The spots may have things exaggerated some in terms of the full extent that light can be seen. It does however though, show how much off axis aberrations will affect limiting magnitude of stars. You can really only go deep right in the center with the normal SCT and the Meade's, but the Edge can do it way off axis as well. That light being spread out is going to push the stars below the background light threshold.

The Edge is also showing tighter on axis as well, but only by a little. I think it's the tight spots all over the field that are contributing to the feeling (visually) that the Edge's are so sharp and contrasty.

Clear skies,


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Lord Beowulf
Pooh-Bah
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Reged: 10/13/08

Loc: Cedar Park, TX
Re: EdgeHD White Paper, interesting read. new [Re: crow]
      #5531265 - 11/20/12 11:05 PM

Pretty cool paper. And while I'm pretty pleased with my new 11" Edge HD, the one deficiency it has compared to my 11" NexStar GPS is that it doesn't have enough focus range. I only just discovered this a couple of days ago. I just bought a binoviewer and while it works great with my Denkmeier PowerXSwitch and the GPS, I can't reach focus with the reducer on the Edge HD. The mirror doesn't move nearly as far. I realize this is most likely due to the sweet spot for the Edge optics, but it's still disappointing, especially given the incredible results I'm getting with the binoviewer (should have bought one a LONG time ago!) and the NexStar GPS.

Beo


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Nicola
Vendor: Skymonsters.net.


Reged: 05/23/06

Loc: Milan, Italy
Re: EdgeHD White Paper, interesting read. new [Re: Lord Beowulf]
      #5531695 - 11/21/12 08:42 AM

It's a pity they haven't produced a focal reducer for the 800 and 925. Still I don't understand why.

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dobsoscope
sage
*****

Reged: 05/24/06

Re: EdgeHD White Paper, interesting read. new [Re: Lord Beowulf]
      #5532021 - 11/21/12 11:22 AM

Hello does that apply to the use of the EdgeHD with focal reducer?


Quote:

Pretty cool paper. And while I'm pretty pleased with my new 11" Edge HD, the one deficiency it has compared to my 11" NexStar GPS is that it doesn't have enough focus range. I only just discovered this a couple of days ago. I just bought a binoviewer and while it works great with my Denkmeier PowerXSwitch and the GPS, I can't reach focus with the reducer on the Edge HD. The mirror doesn't move nearly as far. I realize this is most likely due to the sweet spot for the Edge optics, but it's still disappointing, especially given the incredible results I'm getting with the binoviewer (should have bought one a LONG time ago!) and the NexStar GPS.

Beo




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Joe Cipriano
Entropy Personified
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Reged: 09/03/05

Loc: Uh... anyone have a GPS?
Re: EdgeHD White Paper, interesting read. new [Re: Nicola]
      #5532031 - 11/21/12 11:26 AM

Quote:

It's a pity they haven't produced a focal reducer for the 800 and 925. Still I don't understand why.




Explanation here, from the horse's mouth.
In light of the scaling issues, logic would seem to dictate that C would have designed the FR for the 8 next, rather than the 11. However, they may make more money from 11 sales. Didn't think to ask the question when I had Kopit's ear.

Celestron was handing out copies of that white paper at ASAE. Fascinating read.

Still wish they'd produce a 6, but with production costs close to an 8 Edge, I understand why they don't - I don't think I'd be willing to spend ~$1K for a 6.

Edited by Joe Cipriano (11/21/12 11:38 AM)


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Gord
Carpal Tunnel
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: EdgeHD White Paper, interesting read. new [Re: Joe Cipriano]
      #5532049 - 11/21/12 11:34 AM

I think Daniel mentioned even way back at NEAF that the 8 was next in line to get a reducer and it was being worked. The 9.25 seemed to be a bit of an unknown at that point, but would be after the 8.

Clear skies,


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: EdgeHD White Paper, interesting read. new [Re: Joe Cipriano]
      #5533964 - 11/22/12 01:23 PM

What I like about the white paper is the cut aways of both the scopes and the reducers. Even for non-experts on optics like me, it is easy to see why the reducers for the Edge HD are nothing like the old correctors for the standard SCTs and have clearly involved a lot of design and more difficult manufacturing and thus the price. Just those pictures have been the best explanation for the increased cost that I have seen (even though I hope that the cost comes down).

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crow
sage


Reged: 07/09/12

Loc: BC, Canada
Re: EdgeHD White Paper, interesting read. new [Re: EFT]
      #5534038 - 11/22/12 02:28 PM

Yes I balked a bit at the price of the FR, but having got it, the price made some sense. Its a seriously heavy duty piece of metal & glass. I'm curious about its visual use, I've only managed to try it once and couldn't bring the scope to focus. However I had the big Baader Visual back on the end of it and the Baader 2" diagonal. I think the stock diagonal attached directly to the FR may do the trick.

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ensign
professor emeritus


Reged: 12/16/08

Loc: Southwestern Ontario
Re: EdgeHD White Paper, A Must Read. new [Re: crow]
      #5535232 - 11/23/12 10:46 AM

Quote:

Nice white paper on the EdgeHD design, production etc available for those that are interested. Lots of good info on the Focal reducers too.

http://tinyurl.com/c4n69wn

under the more info button. (Celeston Webpage)

These are cool/capable scopes.




Thanks for posting that, crow. It's a good read.


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EddWen
Pooh-Bah


Reged: 04/26/08

Re: EdgeHD White Paper, A Must Read. new [Re: ensign]
      #5535356 - 11/23/12 12:06 PM

For those interested in Roland Cristen's thinking about the Edge and ACF scopes, you can find it here:

http://tech.groups.yahoo.com/group/ap-ug/message/62596


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Lee Jay
Pooh-Bah
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Reged: 02/27/08

Loc: Westminster, CO
Re: EdgeHD White Paper, A Must Read. new [Re: EddWen]
      #5535456 - 11/23/12 12:59 PM

Quote:

For those interested in Roland Cristen's thinking about the Edge and ACF scopes, you can find it here:

http://tech.groups.yahoo.com/group/ap-ug/message/62596




Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.

Second, is a whole host of images like this one and this one taken with a Meade 12" ACF bare (no reducer/flattener, etc.). I may be nuts, but I don't see any star bloat at the edges of those frames.

EDIT: Here's another one, taken with a roughly APS-H size sensor (bigger than APS-C, smaller than full-frame 35mm). Do you see signs of field curvature in that image?

Edited by Lee Jay (11/23/12 01:18 PM)


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coopman
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Reged: 04/23/06

Loc: South Louisiana
Re: EdgeHD White Paper, A Must Read. new [Re: EddWen]
      #5535457 - 11/23/12 12:59 PM

The image is being reflected off of several mirrors and thru a lens, several lenses if the FR is also being used. Does this effect the brightness of the image in a noticable way?

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Lee Jay
Pooh-Bah
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Reged: 02/27/08

Loc: Westminster, CO
Re: EdgeHD White Paper, A Must Read. new [Re: coopman]
      #5535497 - 11/23/12 01:20 PM

Quote:

The image is being reflected off of several mirrors and thru a lens, several lenses if the FR is also being used. Does this effect the brightness of the image in a noticable way?




Very unlikely.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: EdgeHD White Paper, A Must Read. new [Re: Lee Jay]
      #5535608 - 11/23/12 02:17 PM

Quote:

Second, is a whole host of images ... taken with a Meade 12" ACF bare (no reducer/flattener, etc.).



+1.
Just check images posted to 'CCD Imaging & Processing' forum by Rick J. The practical impact of the field curvature of the R/C and the ACF on the quality of pretty pictures is insignificant.


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Lee Jay
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Reged: 02/27/08

Loc: Westminster, CO
Re: EdgeHD White Paper, A Must Read. new [Re: Alph]
      #5535637 - 11/23/12 02:31 PM

Quote:

Quote:

Second, is a whole host of images ... taken with a Meade 12" ACF bare (no reducer/flattener, etc.).



+1.
Just check images posted to 'CCD Imaging & Processing' forum by Rick J. The practical impact of the field curvature of the R/C and the ACF on the quality of pretty pictures is insignificant.




Maybe I just figured out why?

The blur radii given at the edge of the f/10 coma-free scope in the Celestron white paper (which may or many not be the actual spot sizes of the Meade ACF scopes), are smaller than 2 arc seconds - the common limit of seeing for long-exposure photography. So, nearly all of the size of the smallest stars isn't coming from either diffraction or field curvature, it's coming from the atmosphere.


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Lee Jay
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Reged: 02/27/08

Loc: Westminster, CO
Re: EdgeHD White Paper, A Must Read. new [Re: Alph]
      #5535643 - 11/23/12 02:35 PM

Quote:

Quote:

Second, is a whole host of images ... taken with a Meade 12" ACF bare (no reducer/flattener, etc.).



+1.
Just check images posted to 'CCD Imaging & Processing' forum by Rick J. The practical impact of the field curvature of the R/C and the ACF on the quality of pretty pictures is insignificant.




Ooooo....show me the "bloat" in this image!


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: EdgeHD White Paper, A Must Read. new [Re: Lee Jay]
      #5535678 - 11/23/12 02:52 PM

Quote:

The blur radii given at the edge of the f/10 coma-free scope in the Celestron white paper (which may or many not be the actual spot sizes of the Meade ACF scopes), are smaller than 2 arc seconds - the common limit of seeing for long-exposure photography. So, nearly all of the size of the smallest stars isn't coming from either diffraction or field curvature, it's coming from the atmosphere.




CCD Inspector does show star bloat in ACF images. However you have to look really hard to find its effect in a pretty picture.


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Gord
Carpal Tunnel
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Reged: 01/06/04

Loc: Toronto, ON, Canada
Re: EdgeHD White Paper, A Must Read. new [Re: Lee Jay]
      #5535820 - 11/23/12 04:16 PM

Quote:


Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.




Actually, this is an area (scientific work) where things are very different than with pretty pictures, and a lot of aberrations/problems have little/no impact. For example, photometry doesn't even require the stars to be in focus, or the tracking to even be perfectly accurate. Field curvature or other problems that spread out the star light will not have an impact other than reduced sensitivity.

Another is the R/C design as it relates to astrometry. The inherit aberration in an RC is astigmatism, however this has one nuance that means it will have no effect on use in measuring positions in that the star centroid is always in the same spot relative to the blur of the spot plot. That is the reason the RC design was chosen for this role. Field curvature will not affect that (other than reduced sensitivity, as I understand it).

As for how images look in scopes with field curvature, it's hard to tell how they were focused as well. Could be going for a midway point to try to average things out (little less focused at center, more at the edge). This is often what people suggest to do with standard SCT's and visual observing. I find it just looks "soft" everywhere.

Of course, combine that with wider seeing disks and more average light in the first ring in an exposure lasting several minutes and things are going to take on a larger size in terms of star appearance. I wouldn't say any of those images that I looked at have particularly tight stars. We don't know how they were processed of course.

I think the last image in the quoted white paper is sharper (the single frame of M51 in b&w). Or for example this one taken with a newtonian:

http://www.dreamscopes.com/pages/2010/16inDAtube_MelHelm_05.htm

Clear skies,


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fred1871
professor emeritus


Reged: 03/22/09

Loc: Australia
Re: EdgeHD White Paper, A Must Read. new [Re: Gord]
      #5536611 - 11/24/12 03:03 AM

Quote:

Quote:


Two things keep me from accepting such analyses. First, the standard professional scope type is the R/C, curved field and all. Yet, they use flat sensors and get scientifically-valid results from those instruments.





Another is the R/C design as it relates to astrometry. The inherit aberration in an RC is astigmatism, however this has one nuance that means it will have no effect on use in measuring positions in that the star centroid is always in the same spot relative to the blur of the spot plot. That is the reason the RC design was chosen for this role. Field curvature will not affect that (other than reduced sensitivity, as I understand it).

As for how images look in scopes with field curvature, it's hard to tell how they were focused as well. Could be going for a midway point to try to average things out (little less focused at center, more at the edge). This is often what people suggest to do with standard SCT's and visual observing. I find it just looks "soft" everywhere.







R-C scopes have evolved over time. Starting in the 1960s with the invention of the Gascoigne corrector, which got rid of most of the astigmatism, then the addition of field-flattening optics, the professionals from the 1970s were no longer having big issues with field curvature in the way they did with the classic Schmidt camera (it's no fun bending glass plates).

If you have a look at the Sloan Digital Sky Survey Project Book, there's a section on the optical design of the 2.5 meter telescope, that discusses these matters and how they were dealt with.

to be found at: www.astro.princeton.edu/PBOOK/welcome.htm

The Sloan was not the first telescope to have modified R-C optics to improve performance.


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Lee Jay
Pooh-Bah
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Reged: 02/27/08

Loc: Westminster, CO
Re: EdgeHD White Paper, interesting read. new [Re: Eddgie]
      #5538779 - 11/25/12 02:01 PM

Quote:

And it confirms what another forum member told me a couple of weeks ago, which is that the formula for the EdgeHD scopes had changed (f/1.9 primary).




Does this mean that Hyperstar on edge scopes is f/1.9 not f/2? Not much difference, I realize, but I'm just wondering.


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Lance1234
sage
*****

Reged: 11/01/12

Loc: SoCal
Re: EdgeHD White Paper, A Must Read. new [Re: Lee Jay]
      #5543977 - 11/28/12 02:21 PM

Just wonderful...wish I had seen this white paper two weeks ago before I purchaced my non-HD 9.25. Nothing like buying a horse and buggy while the Ford factory is rolling out model A's. And so much for getting good advice, "oh the HD is really only of merrit for imaging; not really a significant benefit for visual observing" (my interest). Based on the paper this is obviouly not the case, with the HD being superior in all respects. And way to go Celestron for waiting how many years after the HD came out to really explain this? Even their marketing made it look like an imaging instrument. Oh well, at least I have a telescope again.

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rmollise
Postmaster
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Reged: 07/06/07

Re: EdgeHD White Paper, A Must Read. new [Re: Lance1234]
      #5544243 - 11/28/12 05:19 PM

I don't think Celestron has _ever_ promoted it just as an imaging instrument, and the basics, if not the details, were known from the beginning. I am sorry about your disappointment, but if returning/selling the standard SCT is out of the question, consider just buying a Celestron f/6.3 reducer/corrector. That takes an old kitty a long way toward the goodness of an HD.

Edited by rmollise (11/28/12 05:19 PM)


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: EdgeHD White Paper, A Must Read. new [Re: rmollise]
      #5544261 - 11/28/12 05:28 PM

If your going F2, it doesn't really matter..

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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: EdgeHD White Paper, A Must Read. new [Re: wolfman_4_ever]
      #5544267 - 11/28/12 05:34 PM

I still think the C9.25 Edge is priced too high vs. the others. $1000 more for HD! And you can always say "less glass".

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mistyridge
Post Laureate
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Reged: 10/28/05

Loc: Loomis, CA
Re: EdgeHD White Paper, A Must Read. new [Re: MrJones]
      #5544378 - 11/28/12 06:44 PM

Actually less than $1000 more since they include a very nice 23mm 82* EP that costs $200+ and a 2" diagonal instead of the 1.25". which you do not have to buy.

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astroricardo
sage
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Reged: 11/14/11

Loc: Marietta, GA
Re: EdgeHD White Paper, A Must Read. new [Re: mistyridge]
      #5545552 - 11/29/12 01:34 PM

my wife will be thrilled to know I want one even more now

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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: EdgeHD White Paper, A Must Read. new [Re: MrJones]
      #5545717 - 11/29/12 03:28 PM

Quote:

I still think the C9.25 Edge is priced too high vs. the others. $1000 more for HD! And you can always say "less glass".




You always have the option of not buying it...

Tanveer.


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: EdgeHD White Paper, A Must Read. new [Re: crow]
      #5545732 - 11/29/12 03:44 PM

Quote:

Nice white paper on the EdgeHD design, production etc available for those that are interested. Lots of good info on the Focal reducers too.

http://tinyurl.com/c4n69wn

under the more info button. (Celeston Webpage)

These are cool/capable scopes.




Thanks for the link, "crow". Some people are always harping about the inferiority of "mass produced" optics and it's interesting to see what current Celestron production is like. I find it striking that there is no difference, in principle, between what Celestron's doing and what, say, Astro-Physics does for their refractors:

http://www.astro-physics.com/about_us/optical-production.htm
YouTube video at: http://www.youtube.com/watch?v=Cc6Nj9R3NO4

till the figuring stage. For this final stage, A-P's Chief Cook and Bottle Washer, Roland, figures one surface (or perhaps two, I'm not sure) using an interferometer. Celestron uses double-pass auto-collimation to do the same during the final figuring step. A-P used to use the same method before they switched to interferometers and I've owned two specimens of A-P scopes produced this way and I can say that their optical performance leave little to be desired.

The variables in Celestron's process are obviously their acceptance criteria for the optical components, the time spent on the final figuring and the skill of the optician. I think they're completely capable of producing a "DX" version where the optics are guaranteed to a much higher level but I have no faith, going by the whinging on this forum about HD prices, that people would be willing to pay the premium.

Tanveer.


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: EdgeHD White Paper, A Must Read. new [Re: mistyridge]
      #5545758 - 11/29/12 03:57 PM

Actually the "worth" of the included eyepiece is what it will fetch on the used market, not what it sells for new, if you already have better eyepieces than the one bundled in. Consumers would be better off having Celestron skip the bundled eyepiece and drop the price by a couple hundred bucks IMO.

- Jim


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MisterBill
super member


Reged: 09/18/10

Loc: Geezerville aka S.FL, MILE 264
Re: EdgeHD White Paper, A Must Read. new [Re: Lance1234]
      #5545789 - 11/29/12 04:17 PM

Quote:

And way to go Celestron for waiting how many years after the HD came out to really explain this? Even their marketing made it look like an imaging instrument. Oh well, at least I have a telescope again.




This is from a couple years ago, when I was looking for a scope. From Celestron's website....

"As a visual instrument, EdgeHD optics deliver pinpoint images even with your widest field eyepiece."

This first sentence helped me decide to buy one. I think it's still on there or at least something to that effect. Sorry you overlooked it, but don't blame them.

Bill


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Slow Astronomer
member


Reged: 05/01/10

Loc: Milky Way Galaxy
Re: EdgeHD White Paper, A Must Read. new [Re: MisterBill]
      #5546107 - 11/29/12 07:32 PM

What an awesome document. I ordered a Edge HD 800 and can't wait for delivery. Got a deal on an open box sale from a well known supplier and saved $300. Just got the UPS delivery notice for delivery on 12/5. I can't stand it so much I already added it to my signiture!!

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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: EdgeHD White Paper, A Must Read. new [Re: Slow Astronomer]
      #5547080 - 11/30/12 12:11 PM

Quote:

What an awesome document. I ordered a Edge HD 800 and can't wait for delivery. Got a deal on an open box sale from a well known supplier and saved $300. Just got the UPS delivery notice for delivery on 12/5. I can't stand it so much I already added it to my signiture!!




Slow down, Slow Astronomer!

Gale


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Jeff B
Carpal Tunnel
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Reged: 12/30/06

Re: EdgeHD White Paper, A Must Read. new [Re: crow]
      #5547291 - 11/30/12 02:03 PM

Quote:

Nice white paper on the EdgeHD design, production etc available for those that are interested. Lots of good info on the Focal reducers too.

http://tinyurl.com/c4n69wn

under the more info button. (Celeston Webpage)

These are cool/capable scopes.




Thanks, a nice read. My hat's off to Celestron.

Two things jumped out at me.

1. The C9.25 is the best corrected on axis, with even the spherochromatism reduced to below the airy disk.

2. No mention of the effects on image quality with the use of bino-viewers. These critters add about 4" more backfocus past the "optimum" back focus of 3.5". Curious about that.

Jeff


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Eddgie
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Reged: 02/01/06

Re: EdgeHD White Paper, A Must Read. new [Re: Jeff B]
      #5547334 - 11/30/12 02:40 PM

Quote:

The C9.25 is the best corrected on axis, with even the spherochromatism reduced to below the airy disk.





I am not disputing at all what you are saying, but on the other hand, this may give a misleading impression that somehow the other scopes are not going to provide as good on axis performance, and that really isn't the case.

This is the danger of spot diagrams.... Unless people know how many spots were used, it can make a tiny difference in distribution seem much more meaningful than it really is.

The difference depicited by the spots regarding spherochromatism seems to suggest that the correction is perhaps much better, but chances are that the actual percentage of spots falling outside of the circle represented by the Airy Disk diameter is very small.

In fact, Celestron says as much with this quote for the 8" version and this quote is lifted directly from page 18 where the spots for the EdgeHD 8" are published.

Quote:

And because blue rays are strongly concentrated inside the Airy disk, the 8 inch EdgeHD is diffraction limited in blue light.




Again, I am not at all disputing what you are saying, but trying to make sure that people don't get the impression that this means the C9.25 will somehow offer better visual performance than the other scopes. All of them are diffraction limited in red, green, and blue at the center of the field.

The EdgeHD 8" is totally color free in visual use, and for imaging, I doubt that the amount of color distribution is worse than for an 8" APO.

Also (and I am sure that you already know this), the spot patterns are geometric and do not at all include diffraction induced by secondary obstructions. This means that in actual use, more rays would fall outside of the Airy Disk of the C9.25 than the other scopes because of the larger central obstruction.

Again, I what you say is true, but I am just trying to ensure that people who don't work with spot diagrams don't get a mistaken impression that this subtle difference would be meaningful when using the scopes.


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Eddgie
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Re: EdgeHD White Paper, A Must Read. new [Re: Eddgie]
      #5547378 - 11/30/12 03:14 PM

And a followup to my note.

The reason I felt so compelled to comment on this is because I believe Celestron took a bold step in presenting this information.

I believe that manufacturers have been hesitent to provide spots in the past because people with no experinece with them might infer something negative about how a telescope could perform based on the fact that perhaps not all spots would fall into the circle represented by the Airy Disk (and I am not speaking here about the poster of the note I responded to, becuase this was not really intended for him, but rather for people that do not have much experience with spot diagrams).

Now that Celestron has published spots for these instruments, we the consumers should do our due dilligence to study how spots are generated and distributed so that we can better assess the meanings.

Not picking on the poster. What he said is 100% correct. My intention was solely to say only what I said... That the spots in this case might suggest to someone that did not know how spot diagrams work that the other SCTs would somehow be inferrior to the C9, and that really isn't the case because as Celestron says, the instrument was still diffraction limited even in blue light.

We should all applause Celestron and demand this data from other manufacturers, but we should also do our own due dilligance to make sure that we know how to use this data properly.

I would be very sad that someone would think that the EdgeHD 8" or any of the other EdgeHD scopes that don't put all of the dots of one color into a circle represented by the size of the Airy Disk and it resulted in their feeling that the EdgeHD scopes are in some way inferrior to other types.

And that is why I think manufacturers don't want to publish spots.

And my guess is that the on axis spots from any number of ED refractors would look scary as compared to these...

The book "Telescope Optics," (a book that I can't praise highly enough) spends several pages discussing how spot diagrams are produced (there is actually a lot to it) and the effect that the number of spots can have on how the distrubution looks.

Our own web sites and media (astronomy magazines and such) tend to ignore this kind of important info but to really get the best use from spots, it is important to study the topic a bit to get a decent understanding of how to read them.

And finally, one more time to say that the person I responded to was 100% correct. The C9.25 shows the best spots.

But the varience between it and the other scopes (at the center of the field) is so small as to be totally meaningless in terms of performance differences. And that was my point. You can't just "look" at the dots. You have to know the distribution to see if enough dots fall outside of the circle to make a meaningful difference. And the fact that Celestron said it was still diffraction limited suggests that it was a tiny fraction of all of the dots that were plotted.


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Jeff B
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Reged: 12/30/06

Re: EdgeHD White Paper, A Must Read. new [Re: Eddgie]
      #5547623 - 11/30/12 06:17 PM

And what you say is true too Ed.

Spot diagrams assume perfectly made optics perfectly installed. One an absolute basis, they are interesting but may not accurately represent how the optic will perform in use.

However, from a design point of view, as long as the use of the analysis tools are identical, the differences between the diagrams are meaningful and certainly say, again, from a design point of view, the Edge C9.25 is the best of the bunch even with its very slightly larger % central obstuction. The C9.25 already had that reputation before the Edge came along due to its optimization around its slightly slower primary mirror, which Celestron did for a reason.

Whether or not it's visible outside the test stand is another story but there is subjective testimony that the C9.25 is visibily the best of the line, but I've a feeling that sample to sample variations could make validating any such subjective evidence a challenge. But the subjective claims persist and there is, assuming identical analytical tools, supporting evidence for that in Celestrons white paper.

Jeff


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: EdgeHD White Paper, A Must Read. new [Re: Jeff B]
      #5547933 - 11/30/12 09:44 PM

Quote:


2. No mention of the effects on image quality with the use of bino-viewers. These critters add about 4" more backfocus past the "optimum" back focus of 3.5". Curious about that.

Jeff




The specs say that for best performance you have to stay within +/-0.5mm of spec'd BFL (146mm for all except the 8"). However, they don't say how performance degrades with increased BFL. Personally, I've not noticed anything terribly wrong with decreased BFL such as when using a 1.25" diagonal. I suspect, however, that were you to use a BV in "native" mode, i.e., without a barlow/corrector, the added 125mm or so of BFL would noticeably degrade images. With a barlow/corrector in place, it's very hard to tell whether the scope is operating at or close to its design BFL or not.

Tanveer.


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DesertRat
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Reged: 06/18/06

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Re: EdgeHD White Paper, A Must Read. new [Re: TG]
      #5547937 - 11/30/12 09:51 PM

Tanveer where did you get +/-0.5mm figure? Also it does not match my experience. That would be difficult to realize in practice.

Glenn


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: EdgeHD White Paper, A Must Read. new [Re: DesertRat]
      #5548308 - 12/01/12 05:21 AM

Quote:

Tanveer where did you get +/-0.5mm figure? Also it does not match my experience. That would be difficult to realize in practice.

Glenn




It says that right there in the white paper. This is probably just the optimum design tolerance but as users we're interested in the rate of change of the spherical correction with change in BFL. Unfortunately, the white paper does not specify this. I once calculated this using OSLO for an 8" stereotypical SCT but Celestron hasn't published the spec for the HD series. I suppose I could use the prescription in the new book by Berry, Cerargioli and Hallock (?). But that'll have to wait for a lazy Sunday afternoon

Tanveer


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DesertRat
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Re: EdgeHD White Paper, A Must Read. new [Re: TG]
      #5549035 - 12/01/12 04:10 PM

Thanks Tanveer, I should have read the paper more closely!

I think Celestron meant to say +- 0.5 inches. That I could believe. My own tests showed at least in red light the optimum BFL was at least an inch closer than the stated 5.75 inches.

Glenn


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Dan McConaughy
sage


Reged: 11/11/06

Loc: LA
Re: EdgeHD White Paper, A Must Read. new [Re: DesertRat]
      #5550003 - 12/02/12 09:29 AM

If you can put a good crayford focuser on the back and have enough BFL for what you need, then you can adjust the scope to the nominal BFL and leave it, using the focuser to focus whatever you are using. Then you can be certain you are getting the best performance. That is what I did with my C14, and then I locked the mirror.

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