56 replies to this topic

[NinePlanets]

For VISUAL USE ONLY, does a C11 EdgeHD have any advantage over a standard C11 - other than the cell cooling vents and mirror shift focuser thingies?

 

[gstrumol]

I don't believe so.

[bokemon]

Ed Ting has a video about it

[f430]

Here's what they say about the Edge HD,

 

"See the Universe in HD. EdgeHD is an aplanatic, flat field Schmidt-Cassegrain telescope that produces aberration-free images across a wide visual and photographic field of view. The optical system was designed to reduce more than just off-axis star coma; it also provides an astrograph-quality flat focal plane all the way to the edge of the field of view.

 

I had the 11" Edge HD, and I agree with the above. 

[carolinaskies]

For VISUAL USE ONLY, does a C11 EdgeHD have any advantage over a standard C11 - other than the cell cooling vents and mirror shift focuser thingies?

'Advantage' is dependent on what eyepieces you use and whether you tend to notice field curvature at all.   If you focus mainly on small targets then a C11 is up to the same performance as the Edge.  If you have experienced regular distraction from aberrations at the outer regions of a field of view then the Edge will help.  If you use  many eyepieces that are greater than 72 degree AFOV you may experience field curvature on the C11 but shouldn't on the Edge 11.  

The difference in cost of 1200 is pretty hefty visually, but if money's no object there is one further consideration.  If you desire to use non-Edge focal reducers visually to decrease focal length and improve field of view, whether with one eyepiece or with a binoviewer, only the C11 is going to perform well using them.  The Edge telescopes require the specific reducer to work with the specific aperture and only come in .7x variant.  You can use a binoviewer with the Celestron specific .7x, but many people use .5x reducers.  

FWIW the cooling vents aren't nearly so useful when using reflectix to negate internal temperature of Edge scopes.  

Finally, if you ever did decide to use the telescope for imaging, unless you're using a very large sensor, the Edge flattener becomes less important the smaller the chip you use.  And of course using Hyperstar negates the effect of the field flattener making the C11 a logical choice.   

To sum up, yes you can use an Edge 11" for visual and it will perform as good as the C11 in most applications, and better in a few specific situations.  Its not a slam dunk choice visually though, so your mileage out of the Edge advantages will vary. 

 

[dustyc]

Yes. Pinpoint stars to the extent of your eyepiece view. 

[carolinaskies]

Here's what they say about the Edge HD,

 

"See the Universe in HD. EdgeHD is an aplanatic, flat field Schmidt-Cassegrain telescope that produces aberration-free images across a wide visual and photographic field of view. The optical system was designed to reduce more than just off-axis star coma; it also provides an astrograph-quality flat focal plane all the way to the edge of the field of view.

 

I had the 11" Edge HD, and I agree with the above. 

Which is salesperson-engineer provided spiel, for laymen description "It was designed to perform better toward the outer edge of the field where occassional users noticed aberrations" sums it up. 

[gfstallin]

Despite being for a C9.25, I think this might be enlightening: 

https://www.youtube....AI14pzOs&t=352s

 

At least for the C9.25, visual observers didn't notice much of a difference between the standard and HD versions. I had a standard C11 for planetary imaging and visual and had never felt a need to buy the HD version. Eventually, I bought the C11 Edge for DSO imaging, but that is not a jump everyone makes nor was it necessary to purchase the Edge version for DSO imaging. I've seen excellent DSO images from a standard C11 (check out Astrobin, though here's an example from a local club member). I did look through the Edge a couple times, but I was not wowed by the difference, if such a difference even existed using a 32mm 2" eyepiece. The standard C11 is already pretty well behaved visually in terms of edge sharpness, at least for my relative (in)experience and my eyes. Perhaps there would have been a noticeable difference with a side-by-side in a blind taste test, but I didn't have the ability to conduct a Pepsi Challenge and I wasn't inclined to do so. I was plenty happy with the standard C11 for visual and expected the same from the Edge version. It was at least just as good, which was the only confirmation I needed. Refractor connoisseurs and folks with more expensive eyes and discerning tastes than mine might argue differently, but if I were using a C11 for visual only, I wouldn't buy an Edge over a standard SCT unless money really wasn't an object. Nobody who is buying either version of the C11 is expecting 5-degree-wide sweeping vistas of open clusters with pinpoint stars at the field edge. That's what small refractors are for. With the cost savings of a C11 over and Edge 11, attach a fine 60-80mm APO atop your standard C11 and enjoy the best of both worlds. 

 

George

 

Edit: I should also add that I am still rather immune to/unbothered by slight field curvature. Some people have more expensive tastes for reasons they can see and cannot ignore. 

Edited by gfstallin, 25 April 2025 - 02:56 PM.

[dhferguson]

Cheers,

 

I bought a C8 Edge for AP, and now I use it for visual. I am nearly a septuagenarian, which means my eyes have essentially no focus accommodation anymore. I use quality 2" eyepieces that provide up to a 1.2 deg field of view in object space. I get point-like stars at the field-of-view edges and across the entire fields, even at low power (x68), which I really like and appreciate.

 

I also taught astronomy at a local university where a number of straight, non-Edge C8s were used, and so I have direct experience comparing them. I did notice significant degradation of star appearance due to defocus at the edges of the C8, and that is with even the smaller fields of view afforded with ~50 deg AFOV Plossls at low powers.

 

Happy observing always,

 

Don

[dhferguson]

Cheers,

 

Oh, by the way, regarding refractors vs the C8 Edge star sharpness, I also have floaters. That means the smaller exit pupils (for a given magnification) of the refractors get distorted much more (it's roughly by area of the exit pupil) than with the larger aperture C8s.

 

Further, attached is a graph of the MTF curves for a 5-inch unobstructed aperture (red) and an 8-inch aperture with an obstruction ratio (diameter) of x0.34 (black). This reflects diffraction performance only, and the equations are from Dan Schroeder's book: "Astronomical Optics." Note that angular resolution DECREASES with INCREASING spatial frequency (the abscissa). What we see is that the two MTFs are essentially identical down to angular resolution of ~ 1.4 arcsec. Below this, the SCT has the better MTF. Seeing being what it is, one expects both scopes to provide identical contrast down to visual seeing of about 1.5 arcsecond or greater. This is the normal condition for the sky above my backyard but maybe 5-10% of the time it is better. Then, the SCT will allow me to see planetary detail at higher angular resolution. This is also why the 8" SCT is to be far preferred for planetary AP: use of high SNR averaged "lucky" imaging of better quality overall than the ambient seeing allows one to tease out higher contrast than the human eye can detect.

 

Additional optics discussion regarding sharpness:

First, as so many DPAC results have shown, many "fracs" are corrected in the red, at 0.6321um because most interferometers use a HeNe laser of this wavelength. Results at this wavelength often correspond to Strehl ratios in excess of 0.95; i.e., 1/8 wave peak-to-valley wavefront error at 0.63um (or 1/7 wave peak-to-valley at 0.55 um, the peak of human color vision) if only the usual main aberration of spherical aberration is considered. Unfortunately, DPAC results usually degrade as one moves to shorter wavelengths, so that at blue the Strehl may be well less than 0.95, maybe 0.92 or some such. This translates directly into reduced MTF. However, this must be convolved with the wavelength response of the human eye (I assume color response, for bright planets), which peaks in the yellow. Further, correction for longitudinal color must be balanced with correction for spherochromatism and spherical aberration, so that these latter aberrations would be less well corrected at yellow than at red if the manufacturer corrected for red (which is what we usually suspect, including for the Chinese scopes). The result is that the "as-observed" (by us humans) MTF is less than the results computed for diffraction.

 

SCTs too suffer from aberrations. Both of the two major manufacturers compete on price, and so neither can manufacture to perfection. For a variety of commercial reasons--compactness and suitability for the large birder community, for example--both settled on f/2 spherical primary  and x5 secondary mirrors. This leads to an inherent spherochromatism of about (neither provides their optical prescriptions) of about 1/10 p-v wf at 0.63um. That is as good as these SCTs can ever be, unless of course one or both mirrors are aspherized or additional elements added into the optical train. Meade aspherized the secondary and used the extra design degree-of-freedom to remove coma, not spherochromatism, in their ACF series. Celestron added a refractive corrector group inside the baffle tube, and we don't know what-all it corrected for (except we do know both coma and curvature-of-field are corrected). My personal C8 Edge star tests by eye to Strehl=0.94 by carefully using the methods described in Suiter's book: Star Testing Astronomical Telescopes. 

 

One other thing, I suspect all the non-premium manufacturers, of both refractors and SCTs, use bulk quality commercial polishing compound instead of better. The result is higher surface microroughness. I see this with my otherwise very nice C8 Edge as manifested in a slight small angle-scattering "haze" around bright stars. For example, this made detecting Sirius B under very good seeing difficult in my C8 Edge and simply not detected with a Stellarvue SV125 Access (a telescope Stellarvue used to import from KUO, probably about the same scope sold today as the Astro-Tech 125EDL) I once owned. 

 

So BOTH types of scopes suffer from aberrations that lower the MTF and reduce contrast below that expected from diffraction. These will vary instrument-by-instrument and not type-by-type.

 

Summary: I am thrilled with the performance of my C8 Edge, especially considering I bought it on sale in 2019 from our sponsor, Astronomics.

 

Happy observing always,

 

Don

 

[NinePlanets]

Are EdgeHD optics selected and matched any better than standard C11 optics?

[carolinaskies]

Are EdgeHD optics selected and matched any better than standard C11 optics?

Both types are figured on the secondary in relation to the corrector-primary initially.   

However, the Edge goes through an assembly and test with master corrector lenses and the secondary mirror undergoes a correction in relation to the field corrector lenses used in the Edge optics.  It's this second pass at the secondary which is a further step not done with a standard XLT.  This extra testing is where the Edge is more finetuned to it's specific optical components.    

HD White Paper pg 9 excerpt
"If the combined optics set shows any slight residual under-or
over-correction, zones, astigmatism, upturned or downturned
edges, holes, or bulges, the optician marks the Foucault test
shadow transitions on the secondary mirror, then removes the
secondary mirror from the test fixture and translates these
markings into a paper pattern. The pattern is pressed against
a pitch polishing tool, and the optician applies corrective polishing
to the secondary mirror—as we show in Figure 11—until the
optical system as a whole displays a perfectly uniform illumination
(no unwanted zones or shadows) under the double-pass
Foucault test and smooth and straight fringes under the doublepass Ronchi test. The in-focus Airy disk pattern is evaluated for
roundness, a single uniform diffraction ring, and freedom from
scattered light. In addition, the intra- and extra-focal diffraction
pattern must display the same structure and central obscuration
on both sides of focus, and it must appear round and uniform."
 

[quilty]

To me the C11 does well for visual. I don't know why but I think it shows less coma than the 8 and little enough not to bother me.

[Patrick]

For VISUAL USE ONLY, does a C11 EdgeHD have any advantage over a standard C11 - other than the cell cooling vents and mirror shift focuser thingies?

Yes, absolutely for visual as well as imaging.  The Edge has a much flatter field of view.  If you look at the edge of the fov in a standard SCT you will see that the stars start to become elongated/fuzzy about 2/3rds of the way to the edge of the fov.  If you adjust the focus to bring the edges into focus, then the central portion will become slightly out of focus.  In an Edge HD the field is much flatter so things at the center of the fov are closer in focus to the edge areas. 

 

I think one of the reasons there is disagreement in this effect is because of the observers eyes.  Younger eyes are better able to accommodate this difference in focus regions, but with my 73 year old eyes it is very obvious.  When imaging the camera sees all so the flat field becomes much more important.

 

If you only use the central portion of the fov then the difference between a standard SCT vs the Edge will not be as significant (as in planetary observing).  However, I enjoy observing more extended objects with the entire object in focus. 

 

Also I don't like having to constantly make sure the object I'm observing be placed in the center of the fov.  With the Edge you can observe with an object off center and have it still be in focus.  With a standard SCT, you will always want to observe with the object in the center of the fov for sharpest focus.

 

Patrick

[ABQJeff]

Buy once, cry once.

+1 to all recommendations for C11 Edge, especially for DSO viewing.

Get the 0.7x reducer and you get an 11” F/7, 2” eyepiece capable, full coma free corrected, flat field DSO machine!

[fred1871]

Quote:  At least for the C9.25, visual observers didn't notice much of a difference between the standard and HD versions.

 

Quite true, but the C9.25 does not have an F/2.0 primary mirror, it's somewhat longer focal ratio. 

This reduces somewhat both field curvature and, more significantly, off-axis coma. 

 

The C11 and C8 do have an F/2.0 primary mirror, so the difference with them between the XLT version and the HD Edge version is greater. 

Therefore you can't translate C9.25 experience into C11 or C8 experience for comparisons. 

 

Also agreed, if your interest is mainly or solely planetary observing (or imaging) you may not get much benefit from the better-corrected wide field.

Obviously. But individual eyesight having a lot of variety, person to person, quite a few of us will notice a plain difference between the HD and non-HD versions - especially if we can see coma effects without any straining to see them. 

 

Personally? - I'm still okay with my XLT version of my C9.25. But the extra difference that the Edge version makes for a C8 or C11 would cause me to prefer the Edge versions of those.

[Procyon]

For VISUAL USE ONLY, does a C11 EdgeHD have any advantage over a standard C11 - other than the cell cooling vents and mirror shift focuser thingies?

Tried both, kept my XLT.

[thierry martin]

SIMULATIONS WITH OSLO

 

C11 versus C11 EDGE. 

We  can noticed lot of difference. The C11 worls verry well for few millimetres . Enough for planets, After the spots are getting bigger , Energy is not well focused, The C11 is far better. But much more expensive.

 

 

The cheap solution to improve the C11    is  the Respaced C11 for  visual  or and the C11 respaced with a meniscus for astro photo. The last  version  gives nices spots at least up to 14mm radius ( diffraction limited and  flat field   the simulations is in the next post

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[thierry martin]

SIMULATIONS WITH OSLO

 

C11 versus C11 EDGE. 

We  can noticed lot of difference. The C11 worls verry well for few millimetres . Enough for planets, After the spots are getting bigger , Energy is not well focused, The C11 is far better. But much more expensive.

 

 

The cheap solution to improve the C11    is  the Respaced C11 for  visual  or and the C11 respaced with a meniscus for astro photo. The last  version  gives nices spots at least up to 14mm radius ( diffraction limited and  flat field   the simulations is in the next post

SIMULATION  WITH OSLO  RESPACED C11 FOR ASTRO PHOTO. this will work at F10 flat field with the meniscus .

But the RESPACED C11 will work fine with the Lepus  0.62  with 0.65 x reduction < 1800mm FL,  flat field

It will work at F12.  > 3360mm FL  flat field  with the inverted CCTV67. 

The respaced C11 remain CLOSED TO  diffraction limited in all cases at least up to 14mm off focus, because the coma is killed before the ligth arrives to the primary with the Schmidt corrected at the good position. 

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Edited by thierry martin, 26 April 2025 - 12:44 AM.

[quilty]

Quote:  At least for the C9.25, visual observers didn't notice much of a difference between the standard and HD versions.
 
Quite true, but the C9.25 does not have an F/2.0 primary mirror, it's somewhat longer focal ratio. 
This reduces somewhat both field curvature and, more significantly, off-axis coma. 
 
The C11 and C8 do have an F/2.0 primary mirror, so the difference with them between the XLT version and the HD Edge version is greater. 
Therefore you can't translate C9.25 experience into C11 or C8 experience for comparisons. 
 
Also agreed, if your interest is mainly or solely planetary observing (or imaging) you may not get much benefit from the better-corrected wide field.
Obviously. But individual eyesight having a lot of variety, person to person, quite a few of us will notice a plain difference between the HD and non-HD versions - especially if we can see coma effects without any straining to see them. 
 
Personally? - I'm still okay with my XLT version of my C9.25. But the extra difference that the Edge version makes for a C8 or C11 would cause me to prefer the Edge versions of those.

The 11 shows less coma than the 8, the 2080 at least. To me it's half the way towards the C9. And that's enough.
My eyes are old and have completely lost their ability to focus. Nevertheless to me no significant drawback to a 100% correct FOV
I know ther're purists who claim to use only the perfect part of the FOV, but that's not true. We benefit from a large FOV even when only the middle third is sharp (to me in the 11 it's the middle half)

[bobhen]

For VISUAL USE ONLY, does a C11 EdgeHD have any advantage over a standard C11 - other than the cell cooling vents and mirror shift focuser thingies?

Folks were "happily" using SCTs for visual for 50-years before the Edge versions came along. So, a standard C11 should make you happy as well. I mean, how many visual observers observer objects at the edge of the field. When I had my Standard C11, which I owned for 15-years, I don't remember one time when I said to myself, I wish the edge of the field was flatter.

 

If you are a deep sky imager, the flat field of the Edge version offers a useful improvement.

 

IMO, one big advantage of the Edge versions are the vents. One can install fans that can speed-up acclimation and also chase heat plumes that reform during the night as temps drop. Chasing heat plumes is important for high power observing. Of course, one can wrap a standard C11. But I find fans to be the  better overall solution. YMMV.

 

If Celestron was smart, which is questionable, they would incorporate vents in their standard version SCTs and supply a fan kit as an accessory. That would make their SCTs better performers and the fan kits would be another revenue stream for Celestron.

 

Bob

[Illinois]

I remember to use around 200 power to see double double stars in Lyra in my C8 but I can see all 4 stars at only 100 power in my 100 mm ED refractor.  Refractors seem is more sharper pinpoint stars than SCT.  I wonder, which C11 and C11 edge is better for sharp tight double stars? Which is better at 100 power to see double double in Lyra? 

[ex-Bubblehead]

I remember to use around 200 power to see double double stars in Lyra in my C8 but I can see all 4 stars at only 100 power in my 100 mm ED refractor.  Refractors seem is more sharper pinpoint stars than SCT.  I wonder, which C11 and C11 edge is better for sharp tight double stars? Which is better at 100 power to see double double in Lyra? 

Well, the question at that point for the SCTs becomes: where do they fall on the continuum of mush-dog to freaky-sharp?  Assuming comparable optical quality, the center FOV of each should show similar performance, EdgeHD should show increasingly better comparative performance as you get further away from the center.  I am assuming here that "better" means smaller, rounder stars.

[quilty]

when same mirror and corrector quality supposed the non edge shows better stars.

doubledouble does better in 4" fracs, in smaller apertures in general. But at dim and tight doubles the 11 is way above Like ST 1932 or 72Peg and dimmer.
Or count the pinpoint stars in M13 through the 4"frac and the C11. Any star cluster like M 36, 37, 38, just count the pinpoint stars in both scopes to find 2 times more in the C11
Back to doubledouble: When you reduce brightness to 25% the 8" SC easily shows all 4 stars at 100x (good eyesight provided) The stars then are not as pinpoint as in the frac but pinpointer than at full brightness

Edited by quilty, 26 April 2025 - 06:43 AM.

[thierry martin]

The 11 shows less coma than the 8, the 2080 at least. To me it's half the way towards the C9. And that's enough.
My eyes are old and have completely lost their ability to focus. Nevertheless to me no significant drawback to a 100% correct FOV
I know ther're purists who claim to use only the perfect part of the FOV, but that's not true. We benefit from a large FOV even when only the middle third is sharp (to me in the 11 it's the middle half)

The C11 is getting worse than the C8 from Oslo simulations. It is normal because  due to Celestron choice .The seconday mirror works less on C11 to reduce coma from primary than the secondary from the C8. 

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