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APO Doublet vs Triplet

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#1 Cometeer

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Posted 14 June 2015 - 09:32 PM

Hey all, I'm a greenbean when it comes to refractors. I appreciate your help.

 

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

For example the Orion EON 110ED f/6.0 Apochromatic Refractor sold for about $2000, a price comparable to a triplet's price (like the larger EON 130ed). 

Why is this APO doublet so expensive?

 

Hopefully someone can clear my mind. Thanks!


Edited by astronomylife, 14 June 2015 - 09:37 PM.


#2 chasing photons

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Posted 14 June 2015 - 10:29 PM

The Orion EON 110mm ED doublet sold for $1300 and the EON 130 APO triplet sells for $3000.  I will let others discuss the differences between an ED doublet and an APO triplet.



#3 Peter Besenbruch

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Posted 14 June 2015 - 10:56 PM

Hey all, I'm a greenbean when it comes to refractors. I appreciate your help.

 

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

For example the Orion EON 110ED f/6.0 Apochromatic Refractor sold for about $2000, a price comparable to a triplet's price (like the larger EON 130ed). 

Why is this APO doublet so expensive?

 

Hopefully someone can clear my mind. Thanks!

 

The 110 used FPL-51 glass, and as an f6 doublet, probably showed some false color. I have never seen one in person, but mechanically, the scope looks nice. It comes with a heavy duty, rotating focuser, and a sliding dew shield. In many ways, the scope is built for astrophotography, but I have my doubts about an FPL-51 doublet.

 

Orion has now discontinued it.



#4 GShaffer

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Posted 14 June 2015 - 11:21 PM

Hey all, I'm a greenbean when it comes to refractors. I appreciate your help.

 

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

For example the Orion EON 110ED f/6.0 Apochromatic Refractor sold for about $2000, a price comparable to a triplet's price (like the larger EON 130ed). 

Why is this APO doublet so expensive?

 

Hopefully someone can clear my mind. Thanks!

 

Lots of opinions you will see on this my young Jedi :)

 

There are a number of ED or FL doublets that are for all practical purposes true visual APO's at least where color is concerned and are for all practical purposes almost visually indistinguishable from a high end triplet. There are a number of them that are quite distinguishable also LOL........If you aren't going to do imaging then buy a quality doublet and spend that extra money on some great EP's........


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#5 Erik Bakker

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Posted 15 June 2015 - 02:54 AM

For 3 pieces of glass to be made to the same degree of perfection as 2, it will take 50% more time and effort than with 2. 

 

For 3 pieces of glass to be mounted and aligned to perform as designed and stay that way over time takes a lot more effort on the lens cell than with 2 pieces of glass.

 

With 3 pieces of glass, it is a little easier to correct for chromatic aberration than with 2 pieces.

 

With 2 pieces of glass, especially with 2 good pieces of glass working at f/8 or slower, you reach a level of perfection with regard to sharpness and contrast at the eyepiece more easily than with 3 pieces of glass.

 

Ultimately, a triplet can reach a higher degree of perfection with respect to color correction at the eyepiece. But at the price of weight, cost, increased sensitivity to getting out of alignment and longer time to cool down (takes longer to reach it's optimum performance at high magnification).

 

Best contrast? Tough one. The best doublets are superb performers that most triplets cannot reach in sharpness and contrast under real world observing conditions in most climates. But under optimum conditions and given enough time to cool down, the very best triplets will do very well because of their better color correction. On some objects they will pull slightly ahead, on others the best doublets do a little better. I'd call it a draw.

 

Summary:

  • A good triplet scope will always be more expensive than a similar quality doublet.
  • Great visual performance comes cheaper in a doublet and good photographic color correction tends to be more affordable in a triplet.
  • Cameras like fast optics that are easier to correct for color and tend to prefer triplets
  • Retinae are more sensitive to contrast that is easier to achieve in a good doublet.

As always, choose the optimum for your personal needs. Both a doublet and a triplet can do wonderful things.


Edited by Erik Bakker, 15 June 2015 - 04:47 AM.

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#6 russell23

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Posted 15 June 2015 - 05:17 AM

Hey all, I'm a greenbean when it comes to refractors. I appreciate your help.

 

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

For example the Orion EON 110ED f/6.0 Apochromatic Refractor sold for about $2000, a price comparable to a triplet's price (like the larger EON 130ed). 

Why is this APO doublet so expensive?

 

Hopefully someone can clear my mind. Thanks!

 

Can somebody confirm this telescope sold for $2000?  That seems significantly out of place for a 110mm f/6 fpl-51 doublet.  Orion originally sold their f/7 fpl-51 doublet for ~$1200.    The SW120ED f/7.5 with fpl-53 glass sells for $1549. 

 

Dave


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#7 junomike

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Posted 15 June 2015 - 07:00 AM

I doubt it ever want for $2G's.   Looks like it's still available for $1300 USD.

 

Mike



#8 Eddgie

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Posted 15 June 2015 - 07:47 AM

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

 

There is not an easy answer to this question.

 

I will just say that the performance in not just about how many lenses, but also the kinds of glass used.

 

Here is an aproximate ranking of how several 4, " f/7.5 telescopes using two or three lenses would rank (assuming optical quality and coatings were the same, which they never really are but this is just to compare).

 

In last place would be a doublet using non-ED glasses.  This will have the most color on bright stars and planets will have visible chromatic aberration.   There will be a pale violet non the surface of the moon and Jupiter, and you may see a heavy fringe n the limb of the moon.

 

Next up would be an FPL 51 doublet.  This will have much less color on the same targets, but will be an improvement on the non-ED doublet.

 

The next type would be an FPL 53 doublet. This is a very exotic (and expensive) kind of glass with better dispersion characteristics than FPL 51 (or equitivlent glasses).

 

Now all the hype is about ED glass, but here is the reality..   A third lens is a very powerful tool in controlling chromatic aberration.   

 

At 4", and f/7.5 you could get correction that is perhaps as good or even slightly better using non-ED glass by making the scope a triplet!!!   Thats right.  Just using a third lens gives the designer far more power in correcting chromatic aberration.

 

Next to this, a triplet using FPL 51 would be almost perfect.  It would be almost impossible to see any chromatic defocus even on the brightest targets when used visually.  For visual use at 4", an FPL51 triplet is sensibly perfect.

 

Ah, but what if you want to image.  The camera lens is far more sensitive to red and blue light, so now, the super-perfection that imagers demand might mean that for some, the price of the FPL 53 triplet makes sense.

 

Hope this helps.


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#9 Eddgie

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Posted 15 June 2015 - 08:00 AM

And here is a great link to show you how different refractors using different glasses and different focal lengths would compare..   

 

Hope this helps further:

 

Look at figure 74 on the page at the link I am sending you:

 

http://telescope-opt...romatic_psf.htm

 

The top curved line on this page shows how much contrast transfer a "Perfect" aperture would provide, and the plots show how much contrast would be lost for a given type of telescope.

The left side of the plot represents detail like the planet against the background sky (even a very poor telescope easily shows a planety against the sky) and the further right you move, represents how the scope would do with progressively smaller and smaller detail.

 

All scopes loose contrast as the angular size of the detail decreases.. Even perfect ones.. This is caused by diffraction of the aperture (which is what makes the Airy Disks of a star fatter or bigger around in smaller scopes than in larger scopes).  This plot shows how much contrast is lost for the perfect aperture.

For example, a perfect 6" will have lost 50% contrast at about .4 times its limiting frequency, while the 6" f/8 achromat will have lost about 70% contrast and for detail this size, will not be that much better than the best 4" Apos..

 

Hope you find this useful. 

 

Sorry for the long response, but your question did not have a simple answer, and I thought a bunch of examples would help.

 


As you can see, a 4" FPL 53 f/9 doublet (Skywatcher 100ED f/9) can have better contrast on some larger detail than a 6" f/8 non-ED scope (Achromat)!!!!! (Scopes 41 and 67 respectively, but remeber, the 6" scope would have a much brighter image and this usually helps with being able to resolve fine detail)

 

Anyway, this chart is very useful for helping to understand how the aperture, focal ratio, number of elements, and types of glass change the performance of the system...


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#10 Peter Besenbruch

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Posted 15 June 2015 - 02:28 PM

Can somebody confirm this telescope sold for $2000?  That seems significantly out of place for a 110mm f/6 fpl-51 doublet.  Orion originally sold their f/7 fpl-51 doublet for ~$1200.    The SW120ED f/7.5 with fpl-53 glass sells for $1549.

 

It listed for $1300.



#11 Cometeer

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Posted 15 June 2015 - 03:13 PM

Perhaps I was mistaken about the $2k price point. $1300 seems more reasonable.

 

Thanks Eddgie, that cleared a lot up for me. The graph was extremely helpful.



#12 KaStern

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Posted 15 June 2015 - 04:52 PM

Hi astronomylife,

Sven, a friend of mine, does own a non-Orion 110mm f/6 FPL51 doublet.
I could observe with it and was not impressed at all.
First, the collimation was not good.
At high magnification the stars were not round.
The colour correction was not good as well.
There was a red fringe around brighter stars.
Later on the scope has been collimated, using an interferometer.
Now the stars are round, but the red fringe has not disappeared.

This scope does a much better Job as an 120mm f/5 achromat,
but it is far away from beeing an apochromat.

Cheers, Karsten

Edited by KaStern, 15 June 2015 - 05:43 PM.

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#13 Bill Barlow

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Posted 15 June 2015 - 07:44 PM

I recently compared a SV102ED FPL-51 doublet to a SV90T FPL-53 triplet side by side viewing Jupiter at similar magnifications.  It wasn't even close, the 90T clearly was noticeably sharper and showed more contrast and detail than the 102ED.  I was surprised that it was such a big difference.  But some of the newer doublets using FPL-53 glass would probably perform close to the triplet at similar apertures.  

 

Bill


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#14 BillP

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Posted 15 June 2015 - 08:32 PM

I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).

 

Actually, there is not sufficient information to make a determination if all you know is that they are same aperture and one is a doublet and the other triplet.  Color correction also involves the glass types for mating elements, oiled vs air, focal ratio.  So without knowing all the parameters, will be hard to tell. 

 

One thing you can do to get some ideas, is look at similar aperture Apos from premium dealers.  So you know the reputation of Takahashi and their Apos are among the very best.  They sell a 102mm aperture scope.  It is a triplet of some of the best glasses, and they make it an f/8!  So now look at the Orion 110 and it is only a doublet and at a very fast f/6...going to be impossible for it to be colorless. 

 

If you want a 100mm class scope that is an Apo, not so expensive, and visually fairly colorless, the 100mm Sky-Watcher f/9 Doublet is the way to go.  I think they are around $750.


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#15 Element79

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Posted 16 June 2015 - 11:05 AM

The various lines of 100mm f/9 ED scopes sold by Orion, Vixen, Celestron, Sky Watcher and probably others, and all made by Synta, are superb performers - especially for their price.  From Eddgie's graph you can see that they are almost at the level of a perfect 4-inch telescope.  They cool down rapidly, are light enough in weight to mount easily, and once again, THEY ARE CHEAP!  Everybody should have one of these for not only great view but for knocking around without a lot of worry!


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#16 BigC

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Posted 16 June 2015 - 12:13 PM

The various lines of 100mm f/9 ED scopes sold by Orion, Vixen, Celestron, Sky Watcher and probably others, and all made by Synta, are superb performers - especially for their price.  From Eddgie's graph you can see that they are almost at the level of a perfect 4-inch telescope.  They cool down rapidly, are light enough in weight to mount easily, and once again, THEY ARE CHEAP!  Everybody should have one of these for not only great view but for knocking around without a lot of worry!

Cheap is obviously relative to one's situation. Cheap to me is a $50 ST80 or 80f11 OTA.A $750 OTA is something warranting care and consideration.

It may be that a $750 OTA is an excellent price for that size and type.It would have to be only a couple hundred before I'd call it cheap.$500 would be a great deal.

 

Now if one could have a CG4/100EDf9 delivered to the door for $500 I would call it cheap and expect a new Golden Age of backyard stargazing.



#17 Element79

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Posted 16 June 2015 - 12:30 PM

Few of us are made of money and certainly not me either!  But when I say cheap I am comparing these scopes to $4000 and up Takahashi's, Astro Physics, TECs, etc.  No, they don't perform at the levels of these telescopes but they probably perform at 90% or maybe even 95% of those scopes levels!  That is what my comments meant to reflect...



#18 skysurfer

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Posted 16 June 2015 - 12:40 PM

I have a used ED 110 f/7 doublet (Kunming branded 'Robtics' same as Stellarvue or Astro Professional) which is surprisingly good. OK, faint blue fringes with Venus at night or branches against a bright background, but furthermore no disturbing color errors and razor sharp at 240x (seeing allows). Of course a fluorite doublet or triplet will also eliminate these residual blue fringes but ten I have to pay at least two grand.

#19 BillP

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Posted 16 June 2015 - 03:40 PM

 

Cheap is obviously relative to one's situation. Cheap to me is a $50 ST80 or 80f11 OTA.A $750 OTA is something warranting care and consideration.

It may be that a $750 OTA is an excellent price for that size and type.It would have to be only a couple hundred before I'd call it cheap.$500 would be a great deal.

 

Now if one could have a CG4/100EDf9 delivered to the door for $500 I would call it cheap and expect a new Golden Age of backyard stargazing.

 

 

Well don't know if what you are saying is a reasonable qualification for "cheap" since it is sliding too much.  So not cheap when it is not consistent IMO.  ST80 new is about $120.  An 80/F11 new is about $240.  To make $50 the price point for two widely varying price class instruments is not a criteria anyone could get a handle on.  If you want an ST80 cheap for $50, you are saying 50% off street price.  So the 80 f/11 in a "cheap" categorization would be $120 and not $50.

 

On the example of the $750 Skywatcher 100f/9, well it is almost as colorless as a top line TSA and that goes for $3300 new or a StellarVue goes for $2400, so in comparison it is mighty cheap!!


Edited by BillP, 16 June 2015 - 03:55 PM.


#20 Eddgie

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Posted 16 June 2015 - 04:20 PM

 

 

On the example of the $750 Skywatcher 100f/9, well it is almost as colorless as a top line TSA and that goes for $3300 new or a StellarVue goes for $2400, so in comparison it is mighty cheap!!

 

I agree.   I sold a Televue Apo that I hardly ever used and replaced it with a 100ED.   I would say that I gave up zero in terms of performance (though of course a narrower true field).

 

At $750 (new... I paid $400 for mine used. One was an Orion, one was a Vixen, both had superb optics) I think they are a good value.

 

I never used either one of them very much though..  A bit to small.  Sold them.  

 

But they impressed me mightily in terms of optical performance as compared to refractors that I spend far more money on. 



#21 Bill Barlow

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Posted 16 June 2015 - 07:13 PM

I would have been more interested in the SW 100ED if it had a sliding dew shield and a different paint job.  But just for optical quality, it is an excellent choice.

 

Bill



#22 Peter Besenbruch

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Posted 16 June 2015 - 08:17 PM

I would have been more interested in the SW 100ED if it had a sliding dew shield and a different paint job.  But just for optical quality, it is an excellent choice.

 

At least it lacks that ghastly green toothpaste trim coupled with blue lettering that SOME companies provide. ;)


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#23 Jon Isaacs

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Posted 17 June 2015 - 02:22 AM

 

 

Cheap is obviously relative to one's situation. Cheap to me is a $50 ST80 or 80f11 OTA.A $750 OTA is something warranting care and consideration.

It may be that a $750 OTA is an excellent price for that size and type.It would have to be only a couple hundred before I'd call it cheap.$500 would be a great deal.

 

Now if one could have a CG4/100EDf9 delivered to the door for $500 I would call it cheap and expect a new Golden Age of backyard stargazing.

 

 

Well don't know if what you are saying is a reasonable qualification for "cheap" since it is sliding too much.  So not cheap when it is not consistent IMO.  ST80 new is about $120.  An 80/F11 new is about $240.  To make $50 the price point for two widely varying price class instruments is not a criteria anyone could get a handle on.  If you want an ST80 cheap for $50, you are saying 50% off street price.  So the 80 f/11 in a "cheap" categorization would be $120 and not $50.

 

On the example of the $750 Skywatcher 100f/9, well it is almost as colorless as a top line TSA and that goes for $3300 new or a StellarVue goes for $2400, so in comparison it is mighty cheap!!

 

 

Bill:

 

Synta 80mm F/11s are available with a mount for $160, an ST-80 with a mount is actually more than that.. 

 

I avoid the use of "cheap" because it has too many possible interpretations, it can mean poor quality, shabbily made, or it can mean "inexpensive."   A Skywatcher 100 F/9 is of good quality but in the lofty heights of the 4 inch apo, it might be considered inexpensive.  

 

As far as the Orion 110mm F/6 with the FPL-51 based objective.. I don't think those ever sold for $2000, the other day, I saw one on Astromart for $500.. As far as the color correction goes, by ratios, one might expect it to be comparable to a 150mm F/8.2 with a FPL-51 objective. 

 

Jon


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#24 BigC

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Posted 17 June 2015 - 06:57 AM

 

 

Cheap is obviously relative to one's situation. Cheap to me is a $50 ST80 or 80f11 OTA.A $750 OTA is something warranting care and consideration.

It may be that a $750 OTA is an excellent price for that size and type.It would have to be only a couple hundred before I'd call it cheap.$500 would be a great deal.

 

Now if one could have a CG4/100EDf9 delivered to the door for $500 I would call it cheap and expect a new Golden Age of backyard stargazing.

 

 

Well don't know if what you are saying is a reasonable qualification for "cheap" since it is sliding too much.  So not cheap when it is not consistent IMO.  ST80 new is about $120.  An 80/F11 new is about $240.  To make $50 the price point for two widely varying price class instruments is not a criteria anyone could get a handle on.  If you want an ST80 cheap for $50, you are saying 50% off street price.  So the 80 f/11 in a "cheap" categorization would be $120 and not $50.

 

On the example of the $750 Skywatcher 100f/9, well it is almost as colorless as a top line TSA and that goes for $3300 new or a StellarVue goes for $2400, so in comparison it is mighty cheap!!

 

Cheap PRICE to me is when the quality is great and the money little for the type of item. There is also cheap QUALITY meaning not much money AND not expected to perform well.For the former  class I  look hard for "pre-owned",clearance , and other special deals.that makes me cheap in some eyes.

 

And part of my point was that telling one's spouse a $500 telescope without a mount is cheap will be met with disbelief and worse for a great number of would-be stargazers.

 

"A great deal" or "an excellent value" might be better ways to describe one's (planned) purchase.Telling people you bought a cheap scope (or whatever) automatically brings negatives into most minds.

 

Knowing what I do now about telescopes I would call a  $500 100ED f9 apo a great value;as an average consumer I would have thought that a lot of money for "something to look at the Moon".

 

By the way, I did purchase a SW120ED last year;of course at a discount!A fine scope it is,yet mentioning its cost to my co-workers and rural neighbors would bring expressions of incredulity and worse. Never mind that some of them spent the price of the 120ED   on cigarettes and beer during one year.

 

Think positive and keep looking up!!!!


Edited by BigC, 17 June 2015 - 07:00 AM.


#25 TheFacelessMen

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Posted 17 June 2015 - 07:27 AM

Firstly regardless of quality and type of the optical elements the traditional classification is:

 

Achromat brings 2 wavelengths to focus (i.e. a Doublet) 

 

Apochromat brings 3 wavelengths to focus (i.e. a Triplet or more elements )

 

So strictly speaking a refractor that is only a doublet is not an APO and never can be regardless of how good the glass is.




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