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What makes an old Achromat better than a modern ED scope at low magnification

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#1 Princess Leah

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Posted 14 April 2024 - 03:10 PM

Living in the city I often have a thermal ceiling of 50--60X.

Due to this I have the dubious merit of gathering knowledge of what works best at this magnification.

Although 50-60X is low for planetary/lunar work; the planets and the moon are my friends and I miss them if I don't observe them.

 

What I have discovered that over time, by repeated practice, I am able to see more and more detail even at this modest magnification.

 

I have the same experience terrestrially, where there is a similar thermal limit.

 

Surprisingly the best scopes I have tried at this magnification are traditional achomats.

 

Why is this?

What makes a traditional achromat perform better than my ED scopes?

At low power CA is not a problem for these achromats. But what makes them sharper?

Is the glass used in ED scopes softer and therefore harder to figure/polish?

 

My experience so far with refractors for best viewing for planets/lunar at 50-60X are as follows.

 

Carton 70mm F15

TS Optics 60mm F15

Celestron/Vixen C80, 

Vixen F15 (older Japanese model- can't remember aperture).

Skywatcher 70/500 (old blue model).

Televue Pronto/Ranger

Towa 60mm F11.4

 

 

 

 


 

#2 Jethro7

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Posted 14 April 2024 - 03:25 PM

Hello Pricess Leah,

Usually it does not but in your circumstance If anything, I would believe that the long focal ratios and small apertures of these Achromats are what is favoring you. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro


Edited by Jethro7, 14 April 2024 - 03:34 PM.

 

#3 havasman

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Posted 14 April 2024 - 03:37 PM

For me, nothing.


 

#4 CHASLX200

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Posted 14 April 2024 - 04:17 PM

For me, nothing.

Same here other than money wise since they are cheaper.


 

#5 ButterFly

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Posted 14 April 2024 - 04:29 PM

I would first check to see whether you have smudges on your ED scopes.

 

Longer f/ratios can also use less well corrected eyepieces, and higher focal length eyepieces to get to the same power.  Getting to the power of the objective diameter in mms requires an eyepiece with a focal length equal to the f/ratio.  For example, an f/15 scope would require a 15mm eyepiece, whereas an f/6 scope would require a 6mm eyepiece.  Based on the information you provided, one can't distinguish whether it's the objective, or the objective plus eyepiece combination that makes your achromats "sharper".

 

 

 

At low power CA is not a problem for these achromats.

 

8x is a fairly low power, and standard for birding.  CA is easily noticeable at that "low" power when one knows what to look for.  There is a good reason why birders choose ED glass when available.

 

All optics show CA to some degree, regardless of price, when one knows what to look for.  Birch twigs against a bright blue sky is a rather harsh test, and will show all the CA there is to be seen.  Again, all optics show CA to some degree.  Whether the CA is a "problem" is in the eye of the beholder.
 


 

#6 Princess Leah

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Posted 14 April 2024 - 05:15 PM

Found the answers in post 4 here.

 

https://www.cloudyni...pticsrefractor/


 

#7 AstroApe

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Posted 14 April 2024 - 05:25 PM

The way the lens is figured and the quality of the finished polish has a lot to do with it. Some newer ED scopes may have great glass, but it may not be figured to the same high handlevel quality, especially in older Japanese achros vs newer Chinese made ED doublets (although "newest" has gotten a lot better) 


 

#8 KBHornblower

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Posted 14 April 2024 - 05:25 PM

Perhaps your ED scopes are duds.  I see no reason why a simple achromat would outperform a good ED of the same aperture and magnification in poor seeing.


 

#9 CHASLX200

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Posted 14 April 2024 - 05:48 PM

Perhaps your ED scopes are duds.  I see no reason why a simple achromat would outperform a good ED of the same aperture and magnification in poor seeing.

My past insane freaky sharp C102 will give any ED a run for it's money on Jupiters moons at 400x to 600x as far as showing sharp balls, But color wise on Jupiter it would lose.


 

#10 jgraham

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Posted 14 April 2024 - 06:30 PM

I wonder if field curvature plays a roll. Classic achromats tend to have relatively long focal lengths and high f/ ratios giving a relatively flat, well corrected field. Modern EDs (and achromats) tend to have shorter focal lengths and lower f/ ratios giving comparatively curved fields.

Just a thought...
 

#11 Bomber Bob

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Posted 15 April 2024 - 11:40 AM

We need to keep these comparisons straight -- aperture vs aperture, f/ratio vs f/ratio.

 

Much as I like the wide fields & high-quality (almost TAK level) hardware & views in my Mizar GT-80S F7 Achro Doublet, that Meade Series 5000 80mm F6 ED Triplet would eat the GT's lunch at any magnification, with any size or type of accessories, in the one area I look for at Low Power -- Natural Star Colors.  CA is better controlled in the GT than you'd expect, but.... it's still an achromatic.  (I haven't owned an 80mm ED Doublet, and now that I have a near-perfect FC-76 F8, I don't see me buying one -- ever.)  

 

Ditto for the AT-102ED F7 versus either my Edmund 4" F15 or my Dakin 4" F10.  That "cheap" ED Doublet trounced both high-quality achromats for color correction at low powers on Deep Sky objects -- especially open clusters.  Like the GT-80, my Dakin has a very well made (probably hand-finished) Baker lens that'll present micro-dot stars -- I just have to accept that orange stars are gonna be reddish, and red stars are gonna be RED.  On Jupiter & Mars, the Dakin could out-resolve the Edmund, which is why I kept it, and sold what had been my favorite scope - ever.

 

IOW:  I haven't seen any of my achros trounce any of my APOs.  But, since all mine are Fluorite & Takahashi, I can't say for certain that some cheap ED Doublets are lackluster refractors...


 

#12 jgraham

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Posted 15 April 2024 - 12:37 PM

I need to take a closer look. I was so surprised by the image quality through my Unitron 155 and 510 that I spent a fair amount of time with several other scopes to convince myself that this result was real or just my imagination. This initial evaluation included my C80ED and Orion 100ED. I don't recall the specifics, but my impressions of the Unitrons held up (they deliver a beautiful field for objects within their wheelhouse) and the EDs held their own quite well. When I get a chance I'll take another look and this time include a Meade 152ED in the mix.

I usually don't like to compare scopes 1:1, but sometimes it's good to have a reality check.

Neat stuff.
 

#13 RichA

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Posted 15 April 2024 - 12:46 PM

Living in the city I often have a thermal ceiling of 50--60X.

Due to this I have the dubious merit of gathering knowledge of what works best at this magnification.

Although 50-60X is low for planetary/lunar work; the planets and the moon are my friends and I miss them if I don't observe them.

 

What I have discovered that over time, by repeated practice, I am able to see more and more detail even at this modest magnification.

 

I have the same experience terrestrially, where there is a similar thermal limit.

 

Surprisingly the best scopes I have tried at this magnification are traditional achomats.

 

Why is this?

What makes a traditional achromat perform better than my ED scopes?

At low power CA is not a problem for these achromats. But what makes them sharper?

Is the glass used in ED scopes softer and therefore harder to figure/polish?

 

My experience so far with refractors for best viewing for planets/lunar at 50-60X are as follows.

 

Carton 70mm F15

TS Optics 60mm F15

Celestron/Vixen C80, 

Vixen F15 (older Japanese model- can't remember aperture).

Skywatcher 70/500 (old blue model).

Televue Pronto/Ranger

Towa 60mm F11.4

Modern achromats die on the vine for viewing because they are all too fast.  The so-called "Strehl" ratio for a "perfect" 6 inch f5 achromat is about 0.57 because of SA and CA, utterly unacceptable except for very low power deepsky.  However, I'd venture that even with residual colour, a long, small achro well made is at least 0.90 strehl.


Edited by RichA, 15 April 2024 - 02:26 PM.

 

#14 Lagrange

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Posted 15 April 2024 - 04:46 PM

I wonder if field curvature plays a roll. Classic achromats tend to have relatively long focal lengths and high f/ ratios giving a relatively flat, well corrected field. Modern EDs (and achromats) tend to have shorter focal lengths and lower f/ ratios giving comparatively curved fields.

Just a thought...

That would be my guess for why they often look better. At low magnifications the field curvature of the short focal length scope will be much more apparent and contribute to the view looking worse. Scopes like the various TeleVue Petzval models are renowned for the spectacular views they provide at low magnifications due to their flat field design.


 

#15 CHASLX200

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Posted 15 April 2024 - 06:14 PM

I wonder if field curvature plays a roll. Classic achromats tend to have relatively long focal lengths and high f/ ratios giving a relatively flat, well corrected field. Modern EDs (and achromats) tend to have shorter focal lengths and lower f/ ratios giving comparatively curved fields.

Just a thought...

I can't stand FC about as much as Coma in fast Newts. FC is not bad in my AT80ED but in a ST80 i had to dumb it down to a older style 16mm Nag for sweeping to tame FC.  I just can't deal with out of focus stars coming and going thru the FOV while sweeping as i get sea sick.


 

#16 DAVIDG

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Posted 16 April 2024 - 09:13 AM

 You have a number of variables that need to  be eliminated before one can make a generalization that  old achromats and better then modern ED lens under certain conditions. Here are just a few. 

  1)  How many scopes are you comparing ? If sample is too small you can't make any generalization 

  2) What is  the ACTUAL optical quality of the scopes  your are comparing. If the ED lens is poorly figured that would be the reason and it has nothing to do with it being an ED vs classic achromat

  3)  Was the comparison done  at the same magnification using the same eyepiece and same diagonal. If not that could be the reason 

  4) Was the comparison done  side by side  ? If not observing condition can be the reason. 

  5) Are the apertures the same. Since different apertures produces different resolutions  

  6) What is the actual optical design ? There are  large variety of  achromats made of all different types of glass and that is also true of ED lens and they have different optical characterization.  For example in the design of "old " achromats a  Fraunhofer design is fully corrected for coma and astigmatism but a Littrow design is not. 

 

  The only conclusion one can make is that for some reason in the observer's opinion,  telescopes in group  A  seems to perform better then telescopes group  B. The actual reason is unknown.

 

              - Dave 


 

#17 deSitter

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Posted 16 April 2024 - 10:00 AM

Modern achromats die on the vine for viewing because they are all too fast.  The so-called "Strehl" ratio for a "perfect" 6 inch f5 achromat is about 0.57 because of SA and CA, utterly unacceptable except for very low power deepsky.  However, I'd venture that even with residual colour, a long, small achro well made is at least 0.90 strehl.

IIRC Conrady standard (f/ratio = 5 D or longer) is certainly high 90s Strehl if accurately figured to spec.

 

FPL-53 EDs are 3x better, and fluorite 6x better. So a 5" ED can be f/9 instead of f/25 and a 5" fluorite f/5. I can confirm that the color error in my Meade 127ED at f/9 is almost identical to that of a 3" f/16 achromat.

 

ED optics have much steeper curves so much harder to make, and hard to align and to stay aligned unless the mechanics are very good. So ED scopes have more places they can misfire.

 

-drl


 

#18 CHASLX200

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Posted 16 April 2024 - 10:33 AM

IIRC Conrady standard (f/ratio = 5 D or longer) is certainly high 90s Strehl if accurately figured to spec.

 

FPL-53 EDs are 3x better, and fluorite 6x better. So a 5" ED can be f/9 instead of f/25 and a 5" fluorite f/5. I can confirm that the color error in my Meade 127ED at f/9 is almost identical to that of a 3" f/16 achromat.

 

ED optics have much steeper curves so much harder to make, and hard to align and to stay aligned unless the mechanics are very good. So ED scopes have more places they can misfire.

 

-drl

Watch out for 7" Meade ED's.


 

#19 DAVIDG

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Posted 16 April 2024 - 10:54 AM

" IIRC Conrady standard (f/ratio = 5 D or longer) is certainly high 90s Strehl if accurately figured to spec."    No it is around 0.5 as stated since it has huge amount of chromatic aberration.  Strehl is measure of how a wavelength (mono) or multiple wavlengths (Poly) come to focus. It is  a combination off all aberrations including spherical and chromatic. The problem is that refractor Strehls are given for only one wavelength which can be very high but that doesn't tell the full story. A singlet lens can have Strehl of 1.00 ie perfect in one wavelength but as we know has a very large  amount of chromatic aberration and drop the total Strehl to a very low amount.  

 

  Even a 3 f/15 achromat is just barely diffraction limited over visual wavelengths.  What happen is you have a sharp image over some range of wavelengths floating inside a  blurry image and your eye focuses on the sharp one. When the F-ratio gets faster the range of wavelengths that come to sharp focus becomes less, hence you see chromatic fringe around bright objects.

 

 

3 f15.JPG

 

         

                    - Dave 

 

 


 

#20 deSitter

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Posted 16 April 2024 - 12:41 PM

" IIRC Conrady standard (f/ratio = 5 D or longer) is certainly high 90s Strehl if accurately figured to spec."    No it is around 0.5 as stated since it has huge amount of chromatic aberration.  Strehl is measure of how a wavelength (mono) or multiple wavlengths (Poly) come to focus. It is  a combination off all aberrations including spherical and chromatic. The problem is that refractor Strehls are given for only one wavelength which can be very high but that doesn't tell the full story. A singlet lens can have Strehl of 1.00 ie perfect in one wavelength but as we know has a very large  amount of chromatic aberration and drop the total Strehl to a very low amount.  

 

  Even a 3 f/15 achromat is just barely diffraction limited over visual wavelengths.  What happen is you have a sharp image over some range of wavelengths floating inside a  blurry image and your eye focuses on the sharp one. When the F-ratio gets faster the range of wavelengths that come to sharp focus becomes less, hence you see chromatic fringe around bright objects.

 

 

attachicon.gif 3 f15.JPG

 

         

                    - Dave 

Ok right polychromatic Strehl - forgot. That apparently doesn't accurately characterize contrast transfer - Roland Christen talked about it. Something about spatial frequencies not being accurately represented because of an averaging process. I honestly don't remember. If the star test is good I'm happy smile.gif

 

-drl


Edited by deSitter, 16 April 2024 - 12:41 PM.

 

#21 RichA

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Posted 17 April 2024 - 01:57 AM

 You have a number of variables that need to  be eliminated before one can make a generalization that  old achromats and better then modern ED lens under certain conditions. Here are just a few. 

  1)  How many scopes are you comparing ? If sample is too small you can't make any generalization 

  2) What is  the ACTUAL optical quality of the scopes  your are comparing. If the ED lens is poorly figured that would be the reason and it has nothing to do with it being an ED vs classic achromat

  3)  Was the comparison done  at the same magnification using the same eyepiece and same diagonal. If not that could be the reason 

  4) Was the comparison done  side by side  ? If not observing condition can be the reason. 

  5) Are the apertures the same. Since different apertures produces different resolutions  

  6) What is the actual optical design ? There are  large variety of  achromats made of all different types of glass and that is also true of ED lens and they have different optical characterization.  For example in the design of "old " achromats a  Fraunhofer design is fully corrected for coma and astigmatism but a Littrow design is not. 

 

  The only conclusion one can make is that for some reason in the observer's opinion,  telescopes in group  A  seems to perform better then telescopes group  B. The actual reason is unknown.

 

              - Dave 

Most differences in telescopes when it comes to performance are due to residual errors of figure and polish.


 

#22 Jon Isaacs

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Posted 17 April 2024 - 03:52 AM

IIRC Conrady standard (f/ratio = 5 D or longer) is certainly high 90s Strehl if accurately figured to spec.

 

FPL-53 EDs are 3x better, and fluorite 6x better. So a 5" ED can be f/9 instead of f/25 and a 5" fluorite f/5. I can confirm that the color error in my Meade 127ED at f/9 is almost identical to that of a 3" f/16 achromat.

 

ED optics have much steeper curves so much harder to make, and hard to align and to stay aligned unless the mechanics are very good. So ED scopes have more places they can misfire.

 

-drl

 

An FPL-51 doublet is about 3.6 x better than an Achro, a FPL-53 doublet is more than 5 times better, a Fluorite doublet is better than that.

 

Jon


 

#23 Jon Isaacs

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Posted 17 April 2024 - 04:10 AM

What makes a traditional achromat perform better than my ED scopes?

At low power CA is not a problem for these achromats. But what makes them sharper?

Is the glass used in ED scopes softer and therefore harder to figure/polish?

 

 

That has not been my experience.  

 

You did not list the ED scopes you were using nor the eyepieces.  I've owned a number of the scopes on your list including several Celestron/Vixen 80 mm x 900 mm achros. They were decent but my 80 mm F/7 WO FPL-53 doublet was superior at all magnifications.

 

I'm not sure what you mean by the city and the thermal ceiling.  My best seeing occurs in my urban backyard and on the worst day on the 25 years I've been here, I've never seen a night that didn't support 100x.. 

 

Jon


 

#24 Princess Leah

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Posted 17 April 2024 - 08:08 AM

That has not been my experience.  

 

You did not list the ED scopes you were using nor the eyepieces.  I've owned a number of the scopes on your list including several Celestron/Vixen 80 mm x 900 mm achros. They were decent but my 80 mm F/7 WO FPL-53 doublet was superior at all magnifications.

 

I'm not sure what you mean by the city and the thermal ceiling.  My best seeing occurs in my urban backyard and on the worst day on the 25 years I've been here, I've never seen a night that didn't support 100x.. 

 

Jon

Well I don't think you live in Dublin by some heavy industry Jon. If you do your welcome to come round and see how sharp my eyes are ;)

Last night the 'off gassing' of the neighbouring industry meant I was 50X max on Hercules, but managed 120 near Virgo.

 

But why am I always explaining myself?

 

Why am I always receiving private messages that concur with what I am experiencing, and yet people are too scared to post publicly for fear of ridicule.

It's hardly a triumph to win an argument, when there is a silent majority (or minority). A lot of people are too scared to contribute to these discussions, for fear of a few stalwarts shooting them down. Where's the enjoyment in that. I can tell you no one would stand for such a show of disrespect in Ireland.


 

#25 jragsdale

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Posted 17 April 2024 - 08:38 AM

CTE is also a factor when comparing modern ED lenses with standard crown/flint achromats.

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