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Air Spaced Doublet - What is the benefit?

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#1 aeajr

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Posted 29 March 2021 - 10:44 AM

I was just window shopping when I came across this ES102 Air spaced Refractor

https://www.highpoin...ta-dar102065-01

 

I have no idea why air spaced would be a good thing or a bad thing.   Can someone enlighten me?

 

 


Edited by aeajr, 29 March 2021 - 10:45 AM.


#2 SeattleScott

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Posted 29 March 2021 - 10:55 AM

Well the lenses need to be spaced somehow. They can be air spaced or oil spaced. Pros and cons to both. With air spaced you don’t have to worry as much about the cell leaking. Air spaced is cheaper. Air is cheaper than oil and doesn’t need to be sealed as well.

Scott
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#3 Simoes Pedro

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Posted 29 March 2021 - 10:58 AM

Two options:

 

  • Cemented --> 3 different radii.
  • Air-spaced --> 4 different radii.

 

With 4 radii the optics can be Aplanatic, i.e., corrected for spherical and coma. With 3 radii, it cannot. 

 

However it should be noted that the level of coma is far, far lower than in a reflector.


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#4 Simoes Pedro

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Posted 29 March 2021 - 10:58 AM

Well the lenses need to be spaced somehow. They can be air spaced or oil spaced. Pros and cons to both. With air spaced you don’t have to worry as much about the cell leaking. Air spaced is cheaper. Air is cheaper than oil and doesn’t need to be sealed as well.

Scott

On the contrary, if oil spaced, then only 3 different radii. Inferior design in doublet form.


Edited by Simoes Pedro, 29 March 2021 - 11:00 AM.

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#5 Joe1950

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Posted 29 March 2021 - 11:01 AM

I believe the only advantage of an air-spaced achromat vs. a cemented doublet is that the optical designer of the lens has more freedom to reduce aberrations in the final design. It’s the same reasoning found in using an oil spaced triplet lens vs. an air-spaced. The optician has a greater freedom of design. 

 

Of course, with an oil spaced objective, you have to change the oil every 3000 observations! grin.gif

 

I think thats the reason.

 

Ahhh. Some replies beat me to it. Very good.


Edited by Joe1950, 29 March 2021 - 11:03 AM.

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#6 SeattleScott

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Posted 29 March 2021 - 11:05 AM

On the contrary, if oil spaced, then only 3 different radii. Inferior design in doublet form.

Yes, and oil spaced cools faster. Not saying air spaced is better. He just asked about the pros of air spaced.

Scott

#7 Mitrovarr

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Posted 29 March 2021 - 11:23 AM

Isn't pretty much any refractor that isn't very small (cemented) or extremely high end (possibly oil spaced) air spaced?
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#8 jimhoward999

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Posted 29 March 2021 - 11:24 AM

You typically can't cement fluorites (CaF2, FK61, FK51, FCD100 etc) because of the high CTE and CTE mismatch with other glasses....so that knocks out one of the choices.  If you could probably no one would deal with oil.



#9 bobzeq25

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Posted 29 March 2021 - 11:38 AM

I'll attempt to enlighten.

 

The very best lenses are oil spaced triplets.  It's an expensive technology.  Can't see why anyone would do it with an inexpensive design like a doublet, an oil spaced doublet is pretty much a contradiction.


Edited by bobzeq25, 29 March 2021 - 11:39 AM.


#10 Bomber Bob

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Posted 29 March 2021 - 12:05 PM

I think ES is using it in a marketing sense:  Rather than cheap cement, we air-space our achromatic doublets.  ALL of the ES lenses I've tested (ie, ALL of the China-made 4" achros) use a thin metal band to create the air gap:

 

ES-FLAR102 S09 - First Setup.jpg

 

IME testing hundreds of achromats, the better quality lenses were air-spaced, even below 60mm aperture -- EXCEPT for the Zeiss Telementor.

 

Also:  Cement can break down, yellow, etc. over the years - see it often with 40+ yr old achros.

 

$550 for a complete 4" F6.5 "fast" achro ain't bad:   https://www.highpoin...a-dar102065-01 

 

OTA is a couple of pounds lightly than the vintage Jaegers 4" F5 RFT that I built.


Edited by Bomber Bob, 29 March 2021 - 12:13 PM.

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#11 peleuba

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Posted 29 March 2021 - 12:11 PM

I'll attempt to enlighten.

 

The very best lenses are oil spaced triplets.  It's an expensive technology.  Can't see why anyone would do it with an inexpensive design like a doublet, an oil spaced doublet is pretty much a contradiction.

 

 

What?   

 

I am not sure what you mean by expensive.  Oiled triplets are expensive because they are triplets (more glass/more labor etc.)  I think the prices for high-end oiled triplets (AP and TEC) and high-end air spaced triplets (TAK and LZOS) are similar.   And, I think TAK TOA owners would argue with your statement in which you say that:  the very best lenses are oiled triplets.  You cannot make a blanket statement like this and pass off as fact.  Respectfully, this is an opinion.

 

Oiled lenses, in many respects, are easier to make:   (1)  You do not need as complicated of a lens cell compared to air spaced designs.  (2) You do not need to fully polish out the surfaces that will be immersed in oil, but most professional opticians do this as a matter of practice.

 

The argument against oil-coupled objectives is that air spacing gives the designer more degrees of freedom.  Specifically, the air space acts like another lens element.  And, the air space allows for R2 and R3 (in a doublet) to have different radii.

 

In an air spaced design all surfaces must be fully polished and coated for highest transmission and performance.  

 

The oil (or gell) is not terribly expensive - very little is used per lens and its amortized over a lot of telescopes.  Cargill makes some optical oils. 

 

I believe the overarching reason for not having any oiled doublets is there are not enough degrees of freedom to design a lens with a high degree of correction at a focal length that make sense for the buying public.


Edited by peleuba, 29 March 2021 - 12:51 PM.

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#12 Bomber Bob

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Posted 29 March 2021 - 12:17 PM

The argument against oil-coupled of objectives is that air spacing gives the designer more degrees of freedom.  Specifically, the air space acts like another lens element.  And, the air space allows for R2 and R3 (in a doublet) to have different radii.

 

Yeah, I've restored vintage doublets where the lens maker used that air gap to maximize performance -- and marks the lens edges at the best element alignment.


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#13 peleuba

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Posted 29 March 2021 - 12:28 PM

Yeah, I've restored vintage doublets where the lens maker used that air gap to maximize performance -- and marks the lens edges at the best element alignment.

 

Yes, exactly.  You can use the air gap to improve spherical correction, but there is an unintended consequence in that this change in spacing will also affect color correction.    



#14 Nippon

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Posted 29 March 2021 - 04:15 PM

I had a Edmund Scientific 60mm f/8 refractor back in the late 70s and it had a cemented doublet. The thing was really good for an f/8 achro but it did show delamination after a few years. Perhaps that was a quality control issue. I believe the large majority of binoculars have cemented doublets and I've never seen delamination in any of the 7x50 binoculars I've owned. But the objective of the 6x30 finder on my 40 year old C8 has spots of delamination. 


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#15 SeattleScott

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Posted 29 March 2021 - 04:16 PM

Isn't pretty much any refractor that isn't very small (cemented) or extremely high end (possibly oil spaced) air spaced?

Pretty much. All of ES peers, like Skywatcher, Orion, Meade and Celestron use air spaced. Some entry level models excluded.

Scott

#16 aeajr

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Posted 29 March 2021 - 05:17 PM

snip...

 

IME testing hundreds of achromats, the better quality lenses were air-spaced, even below 60mm aperture -- EXCEPT for the Zeiss Telementor.

 

Also:  Cement can break down, yellow, etc. over the years - see it often with 40+ yr old achros.

 

snip

I have a Sears 60 mm refractor that I got in Jr. High.  It is over 50 years old.  When I pulled it out about 5 years ago as I was preparing to get back into the hobby the image was all cloudy.   I bet the cement degraded over time.


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#17 Mitrovarr

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Posted 29 March 2021 - 06:34 PM

I have a Sears 60 mm refractor that I got in Jr. High.  It is over 50 years old.  When I pulled it out about 5 years ago as I was preparing to get back into the hobby the image was all cloudy.   I bet the cement degraded over time.

Could be fungused. My 76mm Sears scope had lens fungus when I got it. I was able to clean it off.



#18 aeajr

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Posted 29 March 2021 - 07:13 PM

Thanks everyone for taking the time to share your knowledge and experience.



#19 Jon Isaacs

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Posted 29 March 2021 - 08:09 PM

Yes, exactly.  You can use the air gap to improve spherical correction, but there is an unintended consequence in that this change in spacing will also affect color correction.    

 

In terms of oil spaced versus air spaced: I believe there are sufficient degrees of freedom in an oil spaced triplet, insufficient in an oil spaced doublet..

 

I know of no oil spaced doublets.

 

Jon


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#20 Jon Isaacs

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Posted 29 March 2021 - 08:14 PM

$550 for a complete 4" F6.5 "fast" achro ain't bad:   https://www.highpoin...a-dar102065-01

 

 

A few years ago they were selling for $350 and went on sale for $300.  That was a deal, you got a dielectric diagonal and an pretty much unusable straight through finder.

 

At $550, the AT-102ED is right there at $600, no diagonal but its a much better scope in every way.. I've owned both..

 

Jon


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#21 Mitrovarr

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Posted 29 March 2021 - 10:18 PM

A few years ago they were selling for $350 and went on sale for $300.  That was a deal, you got a dielectric diagonal and an pretty much unusable straight through finder.

You know, I actually like straight through finders on my refractors. You can't sight down the tube with a right angle.

 

I like right angle finders in theory but in practice I find I can't aim the scope effectively at all without also having a zero power finder as well.

 

But yeah, I'd agree, the AT100ED is barely more expensive. And in the long term, you'd want a better diagonal than the achromat comes with anyway.



#22 SandyHouTex

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Posted 30 March 2021 - 06:16 PM

I'll attempt to enlighten.

 

The very best lenses are oil spaced triplets.  It's an expensive technology.  Can't see why anyone would do it with an inexpensive design like a doublet, an oil spaced doublet is pretty much a contradiction.

Oil spacing is typically chosen because you don't have to worry about the figure and polish of the surface next to the oil.

 

It has nothing to do with "the best lenses".  It's a design choice, nothing more.

 

Air spacing also allows another degree of freedom, and can be used to correct certain aberrations.


Edited by SandyHouTex, 30 March 2021 - 06:17 PM.

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#23 daquad

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Posted 30 March 2021 - 07:00 PM

Oil spacing is typically chosen because you don't have to worry about the figure and polish of the surface next to the oil.

 

It has nothing to do with "the best lenses".  It's a design choice, nothing more.

 

Air spacing also allows another degree of freedom, and can be used to correct certain aberrations.

I think oil spacing is used so that it is not necessary to make different radii for the R2 and R 3 surfaces, i.e. one less surface to calculate and make accommodation for in production.   There is some loss in performance relative to an air spaced design at the same focal ratio. For an oil space design the focal ratio is increased to compensate for reduced control.

 

For fast scopes air spacing provides more degrees of freedom for the design to address aberrations at the lower f/ratios required by photographers.  I don't believe oil is used to reduce the requirement on polish for R2 And R3, even though that could be an advantage for lower production costs. I'm not sure what the savings would be to give the R2 and R3 surfaces an inferior polish other than less time on the polishing machine.

 

Dom Q.


Edited by daquad, 30 March 2021 - 07:06 PM.


#24 peleuba

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Posted 30 March 2021 - 08:18 PM

Oil spacing is typically chosen because you don't have to worry about the figure and polish of the surface next to the oil.

No.    

 

By figure you are referring to the curves in the glass.   These must be very precisely matched as they nest against each other with only a very thin layer of oil separating them.  It’s true one does not have to fully polish the glass surfaces that mate against each other but everyone does this as a matter of practice.   You do not have coat these surfaces.


Edited by peleuba, 31 March 2021 - 07:18 AM.

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#25 peleuba

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Posted 30 March 2021 - 08:21 PM

Air spacing also allows another degree of freedom, and can be used to correct certain aberrations.

Yes.   Mainly spherochomatism.




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