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APO Doublet vs Triplet

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#26 Derek Wong

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Posted 17 June 2015 - 11:25 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.

 

It is not that simple.  If you go by the strict Baker definition, almost no scopes meet that criteria, triplet or not.  However, for visual use, try a Nikon 100mm f/12ED.  If you want better color than that, get one of these:

http://www002.upp.so.../hosi1.4x8e.htm

 

Derek



#27 GShaffer

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Posted 17 June 2015 - 12:24 PM

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.

 

Definitely not........ There are a LOT of doublets that due to careful design of the optical prescription and using specific glass properties result in apochromatic visual performance.



#28 Jon Isaacs

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Posted 17 June 2015 - 01:21 PM

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.

 

I don't think it is as simple as that...  In one of his essays, Roland Christen points out that a doublet can bring three separate colors to focus. and Roland uses the term "ED Doublet apo" to describe an FPL51-KF6 doublet...

 

"I did a quick combine of some favorite glasses that can produce a semi-apo, ED doublet apo, Fluorite doublet and highly corrected triplet lens, along with their respective color error. Certainly others exist with slightly different corrections, but these are the glasses that you can actually get in reasonable quality levels."

 

Achro, Semiapo, ED, Fluorite, etc (R.C.)

 

I consider Roland to be an authority of the subject of apochromats, both in terms of all aspects of the telescopes themselves and also in terms of the generally accepted terminology.

 

Jon



#29 BillP

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Posted 17 June 2015 - 03:51 PM

Any doublet of sufficient focal ratio, even flint-crown, can reach the apochromatic definition.  And likewise it would be easy enough to take any triplet out there and shorten its focal ratio and it would no longer be apochromatic.  So the criteria for apochromatic stands apart from number of elements or implementation.



#30 doctordub

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Posted 17 June 2015 - 06:30 PM

Here is another link that will help one visualize what Bill and John are saying.

 

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

 

CS

Jonathan



#31 Scott in NC

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Posted 17 June 2015 - 06:47 PM

Thanks for sharing that link to Roland Christen's AM post, Jon. The attached chart and accompanying explanation is really helpful.



#32 TheFacelessMen

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Posted 17 June 2015 - 07:44 PM

RC's posts on APO's does not dispel the original definition at all, rather it explains the improvements in materials and production that has significantly increase the performance of both triplets and doublets.  

 

What I see being the case is that many of the manufacturers are conveniently calling refractors APO's more as a sales ploy than anything else.


Edited by TheFacelessMen, 17 June 2015 - 07:46 PM.


#33 GShaffer

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Posted 17 June 2015 - 09:22 PM

The original definition?  How about defining that? I don't see that it dispels anything but rather defines it in terms of performance. Apochromatic is a term for a performance level not a physical construction IMO......

 

I do agree that some scopes are being called APO's when they really do not meet that standard though and that's not limited to doublets necessarily.



#34 BillP

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Posted 17 June 2015 - 09:45 PM

Not one manufacturer uses a consistent definition.  Not sure anyone used the original definition.  http://geogdata.csun...definition.html



#35 Jon Isaacs

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Posted 17 June 2015 - 10:31 PM

RC's posts on APO's does not dispel the original definition at all, rather it explains the improvements in materials and production that has significantly increase the performance of both triplets and doublets.  

 

What I see being the case is that many of the manufacturers are conveniently calling refractors APO's more as a sales ploy than anything else.

 

I have to disagree.  Roland points out two things:

 

-A doublet can have three color crossings as per the original definition.

 

- Neither doublets nor triplets meet the original definition of an apochromat because they only have one spherical null.

 

- The fact that Roland who does not manufacture doublets is happy to call an ED doublet or a Fluorite doublet, an apochromat, indicates to me that it is more than just marketing, that is what he believes to be the case. 

 

Jon



#36 TheFacelessMen

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Posted 17 June 2015 - 10:41 PM

The original definition?  How about defining that? I don't see that it dispels anything but rather defines it in terms of performance. Apochromatic is a term for a performance level not a physical construction IMO......

 

I do agree that some scopes are being called APO's when they really do not meet that standard though and that's not limited to doublets necessarily.

 

See the earlier post :-)

 

Originally APO was "defined" or perhaps "classified" as both a physical construction and outcome.

 

I think the use of the word performance muddies the water and means absolutely nothing with regards to the classification/construction. 

 

As an example there are V8 engines, V6 Engines and 4Cylinder Engines for vehicles.  However performance is only loosely tied to the design and construction as some 4 Cylinder and V6 engines perform much better than some V8 engines.  This still does not make a 4 Cylinder or V6 engine a V8 though lol

 

It merely means that any particular engine can be made and tuned to much better performance levels.

 

Like Bill mentions as well the definitions were not really consistent so we may all just be arguing in circles.

 

Personally I tend to go with the baseline that an APO is at minimum a triplet construction. How well that is executed depends on the manufacturer, designs and materials used.



#37 GShaffer

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Posted 18 June 2015 - 01:16 AM

 

The original definition?  How about defining that? I don't see that it dispels anything but rather defines it in terms of performance. Apochromatic is a term for a performance level not a physical construction IMO......

 

I do agree that some scopes are being called APO's when they really do not meet that standard though and that's not limited to doublets necessarily.

 

See the earlier post :-)

 

Originally APO was "defined" or perhaps "classified" as both a physical construction and outcome.

 

I think the use of the word performance muddies the water and means absolutely nothing with regards to the classification/construction. 

 

As an example there are V8 engines, V6 Engines and 4Cylinder Engines for vehicles.  However performance is only loosely tied to the design and construction as some 4 Cylinder and V6 engines perform much better than some V8 engines.  This still does not make a 4 Cylinder or V6 engine a V8 though lol

 

It merely means that any particular engine can be made and tuned to much better performance levels.

 

Like Bill mentions as well the definitions were not really consistent so we may all just be arguing in circles.

 

Personally I tend to go with the baseline that an APO is at minimum a triplet construction. How well that is executed depends on the manufacturer, designs and materials used.

 

 

So is it your contention that one cant make a doublet that doesn't focus the 3 colors to the same point?  If it is then we disagree......There are a number of them that have been made in the last 20 years....... I happen to have one made by Vixen that has absolutely no chromatic aberration when focused regardless of what I am looking at.......and there are of course others. 

 

Since you posted that article by Thomas Back......here is one you might get better info from since it deals directly with the Abbe definition which is the original.



#38 Jon Isaacs

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Posted 18 June 2015 - 01:31 AM

 

Originally APO was "defined" or perhaps "classified" as both a physical construction and outcome.

 

That is not my understanding.  As far as I know, this is the original definition of the apochromat:

 

"bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths." 

 

It is a performance based specification, there is nothing about how you achieve that result.. If you could do it with a singlet lens, that would also qualify.. 

 

Jon



#39 TheFacelessMen

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Posted 18 June 2015 - 04:14 AM

 

 

The original definition?  How about defining that? I don't see that it dispels anything but rather defines it in terms of performance. Apochromatic is a term for a performance level not a physical construction IMO......

 

I do agree that some scopes are being called APO's when they really do not meet that standard though and that's not limited to doublets necessarily.

 

See the earlier post :-)

 

Originally APO was "defined" or perhaps "classified" as both a physical construction and outcome.

 

I think the use of the word performance muddies the water and means absolutely nothing with regards to the classification/construction. 

 

As an example there are V8 engines, V6 Engines and 4Cylinder Engines for vehicles.  However performance is only loosely tied to the design and construction as some 4 Cylinder and V6 engines perform much better than some V8 engines.  This still does not make a 4 Cylinder or V6 engine a V8 though lol

 

It merely means that any particular engine can be made and tuned to much better performance levels.

 

Like Bill mentions as well the definitions were not really consistent so we may all just be arguing in circles.

 

Personally I tend to go with the baseline that an APO is at minimum a triplet construction. How well that is executed depends on the manufacturer, designs and materials used.

 

 

So is it your contention that one cant make a doublet that doesn't focus the 3 colors to the same point?  If it is then we disagree......There are a number of them that have been made in the last 20 years....... I happen to have one made by Vixen that has absolutely no chromatic aberration when focused regardless of what I am looking at.......and there are of course others. 

 

Since you posted that article by Thomas Back......here is one you might get better info from since it deals directly with the Abbe definition which is the original.

 

 

 

 

 

Originally APO was "defined" or perhaps "classified" as both a physical construction and outcome.

 

That is not my understanding.  As far as I know, this is the original definition of the apochromat:

 

"bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths." 

 

It is a performance based specification, there is nothing about how you achieve that result.. If you could do it with a singlet lens, that would also qualify.. 

 

Jon

 

 

 

I have to admit to not paying much attention to development of doublets so I am happy to be corrected :-)

 

My understanding is that current doublets only achieve this for 2 wavelengths in the visible spectrum and third in non visible.

 

Yep as I mentioned earlier perhaps the specification should be purely performance based and applicable to all configurations.  But for all practical purposes (and still happy to be corrected on the doublet point) my understanding is that this is still only practically achievable in a triplet design.

 

But from memory the earliest definition of APO came from Triplet Designs which were the only practical way of achieving this.



#40 doctordub

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Posted 18 June 2015 - 08:01 AM

Here is another link that will help one visualize what Bill and John are saying.

 

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

 

CS

Jonathan

Looking back at this post, I realize that Eddgie has already posted this link. :blush:

CS

Jonathan



#41 Jon Isaacs

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Posted 18 June 2015 - 10:25 AM

 

 

I have to admit to not paying much attention to development of doublets so I am happy to be corrected :-)

My understanding is that current doublets only achieve this for 2 wavelengths in the visible spectrum and third in non visible.

Yep as I mentioned earlier perhaps the specification should be purely performance based and applicable to all configurations.  But for all practical purposes (and still happy to be corrected on the doublet point) my understanding is that this is still only practically achievable in a triplet design.

But from memory the earliest definition of APO came from Triplet Designs which were the only practical way of achieving this.

 

Apparently it is true that the third crossing in not in the visual but it also apparently true that it still helps bring the colors to focus in the visual.  Roland seems comfortable calling a ED or Fluorite doublet an apo, that's good enough for me.. 

 

Let me ask you this:

 

Have you ever looked though a 80mm-120mm ~F/7 FPL-53 doublet at Venus? In my experience, Venus is about the most challenging object in the sky for a refractor.  It is pretty amazing, when all is said and done, apochromat basically means that it has far better color correction than the crown-flint achromat that was the standard for 200 or so years. 

 

In my experience, looking though my various FPL-53 doublets I have owned and still own, the way I can tell that they are not reflectors (besides no Central Obstruction):  On Venus, they show some chromatic aberration out of focus, in focus, I see none. A triplet would also show out of focus color.

 

Jon

 

 



#42 sg6

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Posted 18 June 2015 - 11:11 AM

 

RC's posts on APO's does not dispel the original definition at all, rather it explains the improvements in materials and production that has significantly increase the performance of both triplets and doublets.  

 

What I see being the case is that many of the manufacturers are conveniently calling refractors APO's more as a sales ploy than anything else.

 

I have to disagree.  Roland points out two things:

 

-A doublet can have three color crossings as per the original definition.

 

- Neither doublets nor triplets meet the original definition of an apochromat because they only have one spherical null.

 

- The fact that Roland who does not manufacture doublets is happy to call an ED doublet or a Fluorite doublet, an apochromat, indicates to me that it is more than just marketing, that is what he believes to be the case. 

 

Jon

 

So why if Roland says a doublet is as good an apo as a triplet does he produce triplets?

From a manufacturing aspect it is a lot less cost to produce a doublet and supposidely as good.



#43 Starman81

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Posted 18 June 2015 - 11:16 AM

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

 

My experience was similar. I owned the EON 110 for a short time, having sold my EON 80 which is also a relatively-fast doublet but with FPL-53 glass. I was let down by the color I was seeing in the 110 and ran back to the EON 80. The build quality of the EON 110 was quite nice though, but what matters most is the optics. I think the EON 110 would fall squarely into the 'semi-apo' category. 

 

About the price for one used, I bought mine for $950 which was on the high side, I have seen them now for $800-$900. I think the one sale for $500 was an outlier. The seller is new and seemingly inexperienced in selling used gear or determining their market value. 



#44 GShaffer

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Posted 18 June 2015 - 11:27 AM

 

 

RC's posts on APO's does not dispel the original definition at all, rather it explains the improvements in materials and production that has significantly increase the performance of both triplets and doublets.  

 

What I see being the case is that many of the manufacturers are conveniently calling refractors APO's more as a sales ploy than anything else.

 

I have to disagree.  Roland points out two things:

 

-A doublet can have three color crossings as per the original definition.

 

- Neither doublets nor triplets meet the original definition of an apochromat because they only have one spherical null.

 

- The fact that Roland who does not manufacture doublets is happy to call an ED doublet or a Fluorite doublet, an apochromat, indicates to me that it is more than just marketing, that is what he believes to be the case. 

 

Jon

 

So why if Roland says a doublet is as good an apo as a triplet does he produce triplets?

From a manufacturing aspect it is a lot less cost to produce a doublet and supposidely as good.

 

 

Where does Roland say a doublet is or even can be as good an apo as a triplet?  Also keep in mind this discussion has (at least from my perspective) been about -visual- performance.  Imaging changes the game drastically when it comes to color correction, my understanding is that it isn't possible to provide the level of performance from an imaging perspective with a doublet that can be had with a well made and well designed triplet. It is however quite possible to make doublets that perform very close to indistinguishably as well as a triplet -visually-.



#45 BillP

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Posted 18 June 2015 - 04:26 PM

The original definition?  How about defining that?

 

1. Three widely spaced wavelengths are brought to the same focus.

 

2. Two of the wavelengths are corrected for SA and Coma.

 

3. One of the crossing points for color correction coincides with one of the crossing points for SA and Coma and this crossing point is as close as possible to the Fraunhofer e-line.

 

FYI, I did the underlining to show that there is wiggle room even in Abbe's original definition.  And you see this in practice because for those manufacturers that publish their color crossing data, the wavelengths are always changing :lol:

 

But as Thomas Back pointed out, what is the purpose of this definition in visual astronomy?  Of course it is to achieve a visually CA free image.  So that being the case, there are other definitions that meet this outcome for visual observers, such as:

 

1. Any lens, be it a doublet, triplet, quad, air-spaced or Petzval

 

2. Has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better

 

3. Coma corrected

 

4. Diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better

 

5. Has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit

 

IMO, the use of Apochromat has evolved for astronomy to simply mean color free visually.  Whether one likes it or not, that is what the popular usage is evolving into.  Language evolves.  Would be interesting to see someone place on an objective "Abbe Apochromat".  That would certainly convey that it meets the original definition. 


Edited by BillP, 18 June 2015 - 04:37 PM.


#46 GShaffer

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Posted 18 June 2015 - 04:38 PM

 

The original definition?  How about defining that?

 

1. Three widely spaced wavelengths are brought to the same focus.

 

2. Two of the wavelengths are corrected for SA and Coma.

 

3. One of the crossing points for color correction coincides with one of the crossing points for SA and Coma and this crossing point is as close as possible to the Fraunhofer e-line.

 

FYI, I did the underlining to show that there is wiggle room even in Abbe's original definition.  And you see this in practice because for those manufacturers that publish their color crossing data, the wavelengths are always changing :lol:

 

But as Thomas Back pointed out, what is the purpose of this definition in visual astronomy?  Of course it is to achieve a visually CA free image.  So that being the case, there are other definitions that meet this outcome for visual observers, such as:

 

1. Any lens, be it a doublet, triplet, quad, air-spaced or Petzval

 

2. Has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better

 

3. Coma corrected

 

4. Diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better

 

5. Has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit

 

 

With Abbe's original definition I would say it says nothing about having to use 3 lenses to achieve it which was my point in the 1st place. Visual apo performance can be, has been and will continue to be achieved using two lenses just as RC says.



#47 junomike

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Posted 18 June 2015 - 05:46 PM

Having owned several Refractors (Achro, ED, Apo Triplet) I feel that there is a certain "feel" the each of these specific designs.  I rarely refer to my Meade 152ED as an "Apo"  preferring to call it an ED Apo or just ED. 

 

As far as definition, I leave that to my eye which tells me if a specific telescope is "Apo" enough for me or not.

 

Mike



#48 Cometeer

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Posted 18 June 2015 - 06:33 PM

From what I gathered, I'll probably purchase a triplet or a doublet with fluorite glass sometime in the near future. Thanks for the help guys. I appreciate it!



#49 doctordub

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Posted 18 June 2015 - 07:00 PM

AP=Triplet, Petzvel or quad

Visual=ED doublet, Fluorite Doublet, light weight, less scatter, and faster cooling.

I am a visual observer.

CS

Jonathan


Edited by doctordub, 18 June 2015 - 07:00 PM.


#50 junomike

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Posted 18 June 2015 - 07:45 PM

AP=Triplet, Petzvel or quad

Visual=ED doublet, Fluorite Doublet, light weight, less scatter, and faster cooling.

I am a visual observer.

CS

Jonathan

This is how I see It also!

 

Mike




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