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Air Spaced Doublet - What is the benefit?

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#26 M57Guy

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Posted 30 March 2021 - 11:05 PM

good topic. I found some additional discussion on air-spaced vs. oil-spaced refractors from an earlier thread here: https://www.cloudyni...ced-refractors/



#27 Jon Isaacs

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Posted 31 March 2021 - 05:48 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.

 

Cemented doublets can fail, the cement starts to debond so a feather-like pattern begins to grow.  Repairing this is possible but not easy.  What I read is that if you have managed to separate the two lenses, then oiling them rather the recementing them is a good way to go.  

 

The larger the objective, the more likely it is the cement will fail as the thermal changes between the two elements creates more stress in the cement.  Small objectives are more often cemented.

 

Another difficulty with a cemented objective is that if something goes wrong with the cementing process, you are pretty much stuck.  I have had scopes with cemented objectives that were not even decent performers on-axis, it seemed like something wasn't right with the cementing. 

 

In general, I much prefer airspaced objectives to cemented.  

 

Jon


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#28 aeajr

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Posted 31 March 2021 - 07:58 AM

Wow!

So much I didn't know I didn't know.
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#29 jimhoward999

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Posted 31 March 2021 - 08:31 AM

Cemented doublets can fail, the cement starts to debond so a feather-like pattern begins to grow.  Repairing this is possible but not easy.  What I read is that if you have managed to separate the two lenses, then oiling them rather the recementing them is a good way to go.  

 

The larger the objective, the more likely it is the cement will fail as the thermal changes between the two elements creates more stress in the cement.  Small objectives are more often cemented.

 

Another difficulty with a cemented objective is that if something goes wrong with the cementing process, you are pretty much stuck.  I have had scopes with cemented objectives that were not even decent performers on-axis, it seemed like something wasn't right with the cementing. 

 

In general, I much prefer airspaced objectives to cemented.  

 

Jon

Believe it or not, stress is actually independent of doublet lens diameter.  A pretty good approximation I have recently learned  is: Stress (PSI) = 0.75 x delta CTE x delta temperature x (average Youngs modulus).   If you keep the stress under 1000 PSI the cemented doublet shouldn't fail.

 

Now common sense says that bigger lenses should be more prone to failure because there is more "strain", which is just delta T x Delta CTE; and I think that is also everyone's experience.  But in theory you shouldn't cement small lenses either unless the CTE mismatch is small....< 1x10-6  in/in/C.   



#30 Mitrovarr

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Posted 31 March 2021 - 09:39 AM

Does one often encounter scopes with cemented lenses? As far as I know the only time I see cemented lenses is with finderscopes, binoculars, and eyepieces.


Edited by Mitrovarr, 31 March 2021 - 09:40 AM.

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#31 bobhen

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Posted 31 March 2021 - 10:37 AM

Does one often encounter scopes with cemented lenses? As far as I know the only time I see cemented lenses is with finderscopes, binoculars, and eyepieces.

I believe both the 76mm Tele Vue Oracle and the Brandon 94mm (with a lens by Roland Christen) used cemented objectives.

 

Bob


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#32 starcanoe

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Posted 31 March 2021 - 11:49 AM

As an aside....IMO you really should try to avoid rather fast and or large temperature changes in cemented doublets. And those ungodly expensive eyepieces you have....ummmm some elements in those are probably cemented...just another thing to worry about :)

 

And don't get me started on demented doublets.



#33 teashea

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Posted 31 March 2021 - 07:51 PM

The air gap can actually act as a lens


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#34 peleuba

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Posted 31 March 2021 - 09:08 PM

The air gap can actually act as a lens

indeed.  Not only it “can”, it does.   See post #11 earlier in the thread.



#35 Jon Isaacs

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Posted 31 March 2021 - 09:34 PM

Believe it or not, stress is actually independent of doublet lens diameter.  A pretty good approximation I have recently learned  is: Stress (PSI) = 0.75 x delta CTE x delta temperature x (average Youngs modulus).   If you keep the stress under 1000 PSI the cemented doublet shouldn't fail.

 

Now common sense says that bigger lenses should be more prone to failure because there is more "strain", which is just delta T x Delta CTE; and I think that is also everyone's experience.  But in theory you shouldn't cement small lenses either unless the CTE mismatch is small....< 1x10-6  in/in/C.   

 

I'm wondering about this.  It seems to me the stress increases with the radius and at the outer diameter, there can be a large difference in displacement.  Certainly if the cement had no sheer strength, the one lens would have a larger diameter.

 

Since this is an elastic system, stress is proportional to strain..

 

:question:

 

Jon



#36 jimhoward999

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Posted 01 April 2021 - 08:43 AM

I'm wondering about this.  It seems to me the stress increases with the radius and at the outer diameter, there can be a large difference in displacement.  Certainly if the cement had no sheer strength, the one lens would have a larger diameter.

 

Since this is an elastic system, stress is proportional to strain..

 

question.gif

 

Jon

 

Here is a pretty easy-to-understand paper that I use in deciding whether or not it is okay to cement.  But I don't think one should cement anything over 3" regardless.

 

 

 

 

 

 

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#37 photoracer18

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Posted 01 April 2021 - 05:51 PM

I believe both the 76mm Tele Vue Oracle and the Brandon 94mm (with a lens by Roland Christen) used cemented objectives.

 

Bob

The TMB 80 SS is a cemented triplet.



#38 Jon Isaacs

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Posted 02 April 2021 - 08:05 AM

Here is a pretty easy-to-understand paper that I use in deciding whether or not it is okay to cement.  But I don't think one should cement anything over 3" regardless.

 

I'm thinking about the factors involved, thermal stress as a function of radius and thermal stress as a function of radius. If I have look something up I'll look at Roarke or Timohenko when I'm back home.

 

I just don't see that the stress is independent of diameter. 

 

Jon



#39 jimhoward999

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Posted 02 April 2021 - 09:44 AM

I'm thinking about the factors involved, thermal stress as a function of radius and thermal stress as a function of radius. If I have look something up I'll look at Roarke or Timohenko when I'm back home.

 

I just don't see that the stress is independent of diameter. 

 

Jon

Sounds good.

 

I recommend also looking at the paper I referenced and also the older classic paper on the topic by Nelson and Chen as these address the optics problem directly.

 

To be a little more precise, since you will be checking on me;  the maximum shear stress for a given doublet  is highest on the edge and near zero in the center.  But doublets of different diameters in theory will have similar maximum shear stress.  That is the sense in which it is independent of diameter.

 

The fact that lenses have fairly high aspect ratios (diameter over thickness) matters.  There is a factor in the first-order one-dimensional formula related to the Tanh of a complicated argument related to the aspect ratio and this becomes 1....in the usual optics geometry (reference Nelson and Chen). 



#40 van_isle_skies

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Posted 02 April 2021 - 10:27 AM

Wow!

So much I didn't know I didn't know.

You know, I thought I was relatively well-informed about many things, including astronomy and its equipment.  I stop by Cloudnights, pick a random topic to catch up on and quickly realize I know very little.

 

Sorry for the minor thread hijack.



#41 ji4m

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Posted 03 April 2021 - 08:41 AM

I believe both the 76mm Tele Vue Oracle and the Brandon 94mm (with a lens by Roland Christen) used cemented objectives.

 

Bob

Hmmm.  Didn't know that.  I always thought the 94 Brandon used an oiled AP triplet.


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#42 Jon Isaacs

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Posted 04 April 2021 - 12:20 PM

Hmmm.  Didn't know that.  I always thought the 94 Brandon used an oiled AP triplet.

 

I know some early Prontos have cemented lenses, id be surpised if the Brandons did.

 

Jon



#43 Jon Isaacs

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Posted 04 April 2021 - 12:24 PM

Sounds good.

 

I recommend also looking at the paper I referenced and also the older classic paper on the topic by Nelson and Chen as these address the optics problem directly.

 

To be a little more precise, since you will be checking on me;  the maximum shear stress for a given doublet  is highest on the edge and near zero in the center.  But doublets of different diameters in theory will have similar maximum shear stress.  That is the sense in which it is independent of diameter.

 

The fact that lenses have fairly high aspect ratios (diameter over thickness) matters.  There is a factor in the first-order one-dimensional formula related to the Tanh of a complicated argument related to the aspect ratio and this becomes 1....in the usual optics geometry (reference Nelson and Chen). 

 

I will not be checking up on you.  Still. Consider a 1 inch doublet and a 10 inch doublet, for a given delta T, I have a hard time believe the sheer stresses in the cement will be similar as long as the cement layers are of equal thickness. 

 

Jon



#44 25585

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Posted 04 April 2021 - 04:30 PM

Cemented doublets can fail, the cement starts to debond so a feather-like pattern begins to grow.  Repairing this is possible but not easy.  What I read is that if you have managed to separate the two lenses, then oiling them rather the recementing them is a good way to go.  

 

The larger the objective, the more likely it is the cement will fail as the thermal changes between the two elements creates more stress in the cement.  Small objectives are more often cemented.

 

Another difficulty with a cemented objective is that if something goes wrong with the cementing process, you are pretty much stuck.  I have had scopes with cemented objectives that were not even decent performers on-axis, it seemed like something wasn't right with the cementing. 

 

In general, I much prefer airspaced objectives to cemented.  

 

Jon

I had to have a Pronto professionally cleaned of its old glue. The lenses were re-cemented with UV glue, not sure what was originally used. 

 

My TV 85s are air spaced. 



#45 Jared

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Posted 04 April 2021 - 05:00 PM

In general, there are three different ways of mating glass elements to each other:

1) Air spacing, where the elements are held apart by spacers and (usually) supported separately from each other in an exact location within the cell

2) Cemented together with a flexible, optically transparent cement, then placed in the lens cell as a unit, often with no need to support the elements around the outside since they can’t easily become “decentered” with respect to each other when they are acting as a single unit with the same radius of curvature where they mate

3) Oiled lenses where a transparent oil is used in place of cement to, kept in place through surface tension

 

All three designs are capable of equally good results, especially since the advent of high quality multi coatings where reflections at air-to-glass surfaces are minimized. Some designs, by the way, combine more than one method. I believe the current AP-130, for example, is a so-called “broken triplet” meaning one air gap and one oil space.

 

Cement is rarely used above 50mm or 60mm since it can delaminate with temperature changes and with age. Once the elements get too large, the stress of having different thermal properties in different glass materials damages the cement over time.

 

Proponents of air and oil spacing have been fighting about “which is better” for a decade or more on this forum. You won’t find a consensus. Theoretically, you can get better correction out of an air spaced design since there are more degrees of freedom for the designer. In practice? Other factors dominate. Theoretically, an oiled design will provide better contrast since there are fewer air to glass surfaces. In practice? Very similar results these days. In theory, an air spaced design will allow the scope to be faster—more degrees of freedom for the designer again. Theoretically, an oiled scope cools faster since there is no insulating pocket of air.in practice? Diameter of the objective and number of elements are a better predictor of cooling time than air vs oil.

 

Ultimately, just buy from a trusted brand. The designers sort this all out so you don’t have to. The differences in performance between oiled and air spaced are much like the debate over how many angels can dance on the head of a pin—strong proponents on each side with few, if any, practical ramifications.


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#46 SandyHouTex

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Posted 04 April 2021 - 05:01 PM

I will not be checking up on you.  Still. Consider a 1 inch doublet and a 10 inch doublet, for a given delta T, I have a hard time believe the sheer stresses in the cement will be similar as long as the cement layers are of equal thickness. 

 

Jon

Since thermal stress is a function of length, I am as doubtful as Jon on this.


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#47 vahe

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Posted 04 April 2021 - 06:22 PM

 

Cement is rarely used above 50mm or 60mm since it can delaminate with temperature changes and with age. Once the elements get too large, the stress of having different thermal properties in different glass materials damages the cement over time.

 

 

Do you happen to know if 130mm F/12 ApoMax was a cemented triplet?

.

Vahe



#48 jimhoward999

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Posted 04 April 2021 - 08:24 PM

Since thermal stress is a function of length, I am as doubtful as Jon on this.

Believe me, I get it.   It is counter-intuitive; and perhaps as you say....doubtful.     And I am a lens designer not an ME, so I can’t speak with authority.   
 

But all the journal papers on the topic seem to say it is.   As best I can tell, the equations say that reasonably thin  lenses bend and relieve the stress.   Big lenses bend more than little ones.


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#49 vahe

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Posted 04 April 2021 - 09:32 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.

 

 

Roland offers some clarification on the subject in a post on tec-scopes@groups.io;

.

"One of the misconceptions that people have of oiled lenses is that the internal surface quality and polish can be low. This is not true. If the lens is to have high contrast, the internal polish must remove all pits, same as the outer surfaces. The quality of the surface must be high in one respect (power tolerance), but can be quite low in another (surface regularity). Power tolerance will tend to drift widely with slow pitch polishing, but surface regularity will be very high. Happily, one can polish with a rapid polyurithane polisher and achieve extremely tight power tolerances with some surface irregularities. Since these surface irregularities go away when the surfaces are oiled, you have the best of all worlds in an all-spherical oiled triplet. You can polish the inner surfaces very quickly with poly pads (1/10 the time it takes with pitch), and only need to do pitch on two of the outer surfaces.

.

So why does everyone NOT use oiled lenses if they are so easy to make? Again, one cannot make an oiled all-spherical lens with glass available today AND achieve correction all the way down to 400nm unless one uses Calcium Fluorite."

.

Vahe


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#50 Jon Isaacs

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Posted 05 April 2021 - 12:45 AM

Believe me, I get it.   It is counter-intuitive; and perhaps as you say....doubtful.     And I am a lens designer not an ME, so I can’t speak with authority.   
 

But all the journal papers on the topic seem to say it is.   As best I can tell, the equations say that reasonably thin  lenses bend and relieve the stress.   Big lenses bend more than little ones.

What is the largest cemented lens you know of?

 

Jon




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