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Oil Spaced and Air Spaced Refractors

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#1 UFO2513

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Posted 27 October 2011 - 06:43 AM

What are the disadvantage and advantage of the oil spaced and air spaced refractors??? Let see someone who could answer them. :confused:

#2 gillmj24

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Posted 27 October 2011 - 07:17 AM

Some fear oil will leak out after many years but supposedly they cool down faster. Air spaced surfaces need to be coated too but they give the designer an extra surface curve to design (and they must also execute that design well).

Who makes the lens is more of an indicator of quality than whether it is air spaced or oil spaced.

#3 ken svp120

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Posted 27 October 2011 - 07:21 AM

Welcome to CN and congrats on post #1!

This is a topic that has been discussed a number of times in the past so rather than re-hashing it, I'll point out to you that CN has a Search feature which is very useful. I would suggest giving that a shot...maybe put in Oil Spaced as you keywords, limit it to the refractor forum, and see what you get...should be plenty of available reading :grin:

#4 jmiele

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Posted 27 October 2011 - 07:54 AM

Some fear oil will leak out after many years but supposedly they cool down faster. Air spaced surfaces need to be coated too but they give the designer an extra surface curve to design (and they must also execute that design well).

Who makes the lens is more of an indicator of quality than whether it is air spaced or oil spaced.


I agree Joe. Also, the spacing plays into design and how easy/difficult it will be to correct color. Tak uses a wider air spacing that slows cooling but is easier to correct. AP (Roland) uses a tighter air spacing that cools quicker but is harder to correct. Both designs a perfectly executed IME.

I've not owned an oil spaced AP, however there's still hope as Joe's got an AP140 available. :)

Joe

#5 DaveJ

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Posted 27 October 2011 - 09:11 AM

...supposedly they cool down faster.


No "supposedly" about it :grin:, they definitely cool down faster than air-spaced. With air-spaced, you have two trapped volumes of air between the first and third lenses, essentially thermally isolating the middle element, exactly like a triple-glass thermopane window. With oil-spaced, there are no trapped air volumes, the three lens elements behave as one. My 140mm oil-spaced triplet cools down *much* faster than my 80mm air-spaced triplet.

#6 LLEEGE

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Posted 27 October 2011 - 09:12 AM

To my knowledge there are currently only two makers who use oil spacing in their design. Astro-Physics and TEC.
Of those currently being produced are:
AP 130GT If you haven't been on the list for at least 7 years, you wont get a new one.
TEC TEC140 The 160FL and 180FL are not currently being made. Possibly in the near future. The 160ED, 200ED and 200FL are out of production. The 110FL is available on a limited basis. Yuri just announced another small run.

There are more important things to worry about then what the spacing is. Given the current oil space manufacturers, you have nothing to worry about. TEC guarantees their scopes for life. Regardless of whether you bought it new or used. I think AP does as well.

#7 Mirzam

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Posted 27 October 2011 - 09:27 AM

I will chime in to say that because the oil spaced surfaces must essentially match in radii, oil spaced lenses are less sensitive to decentering mis-collimation than some air spaced designs.

Personally, I strongly prefer oil spaced for triplets. Doublets in my experience are less problematic.

JimC

#8 Starhawk

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Posted 27 October 2011 - 09:27 AM

The oil spacing is with thin films of oil. The other huge benefit of this is having two more refracting elements in the cell. The surfaces have less relative change in index of refraction, so there aren't any internal reflections. The front cell of an AP130 is difficult to see as a result.

Tucson nights have temperatures which go slamming down at dusk. The AP130GT just doesn't show obvious thermal lag. I'm sure there is some, but the air over me takes longer to settle down.

AP does warn against subjecting the scope to rapid temperature swings, such as the heated indoors to outdoors. They suggest pre cooling by putting the scope in a garage. While observing, take the case outside and open it up so it cools, then put the scope in to gradually warm over the next day.

-Rich

#9 Eddgie

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Posted 27 October 2011 - 10:04 AM

While this topic has been covered before, I want to interject a VERY important point that seldom is mentioned in this ongoing dialog.

Telescope making is a business, and to stay in business, the manufacturer must make a profit.

At smaller apertures and medium or slow focal ratios, BOTH designs can work more or less equally well, so the MAIN difference is in the designer's approach.

This often entails much more than simply raw performance.

It is possible to get similar performance from either approach, but from a design and manufacturing perspective, these are VERY different designes to PRODUCE, and often, these dialogs do not consider this MOST important aspect (if you are making telesecopes to make a profit, which of course is the goal of the designers in this case, both of whom produce their own scopes, or have them made to their specifications.

Oil spacing does two very important things from a design and production standpoint.

The first is that it oiled lenses do not require multi-coatings on the inner surfaces. Only the front of the front lens and the rear of the real lens need to be coated. The oil acts as a coupler and eliminates internal reflections between the lenses.

By comparison, the air spaced lenses have to have excellent coatings on ALL air/glass surfaces. As lenses get larger and larger, getting coatings perfectly applied is much more difficult. The oil spaced triplet can have 96% transmission with a single MgF2 coating on the front lens face and rear lens face. To get similar transmission on the air spaced design reqires coatings on all lenses, and to BEAT this requires multi-coatings on many of the lens surfaces.

The oiled design also means that the finish on the inner surfaces does not have to be perfect. The oil itself acts as a smoothing agent, meaning that the fininshing of the surfaces on the inner faces is less critical. This can make it much less demanding to produce a high quality final lens assembly. By contrast, EVERY face of EVER lens must be worked to incredible precision in the air spaced design.

Next is the lens assembly. An oil spaced triplet is usually simply oiled and cleaned, then sealed with some tape around the outside edge. This is inexpensive and very low tech. The lens cell is very simple with basically two parts. the cell assembly and the retainng ring (and maybe a simple cork spacer at the front and rear).

By contrast, the air spaced lens requires a very precise lens cell, with very critical design to ensure that the lenses stay spaced properly and axial aligned. The production cost for this cell, and the labor required to assemble and align it are far more intensive than with an oil spaced design.

As for oil leakage, I have a 6' f/8 APO that is 26 years old, and there is no sign of leakage.

Now for the performace issues.

Oil spacing a triplet means that you will have coma. If the design is slow, the coma won't be so bad, and visually, it will be impossible to see.

When you start to image though, the mild coma of the oil spaced triplet means that you have a limit on how fast you can go with the focal ratio. You could use exotic glasses to keep the chromatic abberation at bay (just like with the air spaced design), but as you get faster and faster, you get more and more coma at the edge of the field of view.

The air spaced design on the other hand can completly elininate coma, and if the designer is willing to aspherize the lenses, the field can be kept very flat and coma free even in a fairly fast telescope. This CANNOT be done with an oil spaced design because of the need to have the inner lenses have the same radius of curvature. The air spaced lens can have a different curve on EVER face, for a total of 6 different curves, giving the designer MUCH more freedom to correct OFF AXIS abberations, which is where MOST important differences in telescope performance are to be found.


I have said this many many times on this forum.. The VAST majority of differences in performance between most different telescope designs is to be found in how they behave OFF AXIS. Just about ANY design can be made to perform extremly well at the center of the field.

So, which is better? Well, from a performance perspective, that depends.

For visual use, it isn't as critical because the coma of the oil spaced design is low enough that you can't see it. Even at f/8, the coma is low enough that it is not an issue for imaging, but once you get faster than this, the coma can start to show up.

For imaging, if you want a fast design, you almost have to go to air spacing. The air spacing is what allows the designer to correct coma, and for today's advanced imaging, the goal is to produce as wide and as flat a field as possible, with no residual abberations like coma or astigmatism.

So, there is no real answer to your question.

Part of the anwswer depends on the economics of production and the profit level the desinger needs. Oil spacing is quite simply much cheaper to make.

Part of the answer depends on the intended role of the telescope. If the role will include critical imaging, and a fast focal ratio is important to the imager, at some point, the air spaced design will start to become more important.

So, like soooooo many things, the answer here is not as simple as many make it to be. The performance issues are only relative to the intended application. At medium to long focal lenghts, air spacing has no real advantage, but at larger apertures and faster focal ratios needed for imaging (and being demanded by today's advanced imagers), air spacing becomes more and more un-avoidable.

And while it is much more expensive to produce, there is a huge line of people waiting to get large aperture, fast, air spaced triplets.

#10 CounterWeight

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Posted 27 October 2011 - 10:53 AM

Using my 8300M and/on oil spaced 160mm triplet (TEC) at F/8 and reduced to near f/6 (It's a TEC oil spaced objective with an A-P focal reducer ;) )I've had no issues with coma. So for me no coma in either case. Just a non theory real world data point. Or even several of them. (please don't hold my lack of processing skills against me but I don't reshape or resize my stars, they are 'as is')I believe George usues a TEC 140 with same reducer hope he will post in. His images I've not noticed any coma in either.

My impression is when recucing a scope -if the the reducer can effectively handle the native radius of curvature that is the main importance in being useable or not or to what degree there is deterioration in center to edge of a well performing f/8 design, and what your reliable image circle ends up at.

This is just a data point with real world gear, not meant to be a general proof, or proof of anything other than what it is. (My SSPv2 chip was a bit physically larger than the 8300M but I never used a focal reducer on it so cant report on anything there.)

Just another comment. As for imaging critical focus is everthing - and my scope I DO need to check critical focus and correct in direct proportion to the temp change, this at f/6 and f/8.

#11 Starhawk

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Posted 27 October 2011 - 11:29 AM

Post deleted by hfjacinto

#12 kevint1

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Posted 27 October 2011 - 11:31 AM

Just curious - what type of oil is used?

#13 Mike Clemens

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Posted 27 October 2011 - 11:56 AM

Just remember in the post-banking apocalyptic world, when we're all 80 and our refractors need service, I'll be popping my triplet out with one move, putting a drop of oil between the lenses and taping it back together. You air spaced guys will be ungluing elements from spacers, removing element by element, affixing migrated spacers, worried about how tight to hold each element in the cell, having centering issues, and then redoing the whole thing when you get a huge dust mote in your lens.

#14 Mirzam

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Posted 27 October 2011 - 11:57 AM

My sentiments exactly. I have a feeling that Yuri et al., will also be survivors.

JimC

#15 idealistic

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Posted 27 October 2011 - 12:00 PM

Im also curious about the oil used. Not 5w30 I assume. How much oil is actually in there?

#16 jmiele

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Posted 27 October 2011 - 12:35 PM

I don't worry about any of it. I will buy what I want regardless of it's production status. :) As to spacing, wider air spacing = easier color correction and more cooling lag. Enter Tak 150 AP160. The Tak and AP both cool equally at the start, however the Tak lags during big temp drops throughout the evening. This is because without oil (AP160 and TAK150) the wider spacing of the Tak slows things down.

Joe

#17 astroneil

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Posted 27 October 2011 - 12:50 PM

Just remember in the post-banking apocalyptic world, when we're all 80 and our refractors need service, I'll be popping my triplet out with one move, putting a drop of oil between the lenses and taping it back together. You air spaced guys will be ungluing elements from spacers, removing element by element, affixing migrated spacers, worried about how tight to hold each element in the cell, having centering issues, and then redoing the whole thing when you get a huge dust mote in your lens.


Yeah,

Here's a pristine, air spaced crown/flint objective that saw first light over 150 years ago. :p

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#18 Mirzam

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Posted 27 October 2011 - 01:11 PM

Yeah, but that's a doublet.

JimC

#19 astroneil

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Posted 27 October 2011 - 01:16 PM

Yeah, but that's a doublet.

JimC


I have looked through an air spaced 4" Cooke photovisual triplet c 1896. It's never been disassembled for cleaning either.

I'll get a photo to you as soon as I can if you wish :grin:

#20 Mirzam

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Posted 27 October 2011 - 01:56 PM

Not necessary. I certainly have no reason to disbelieve you.

My point is simply that air spaced triplets have more degrees of freedom and can be more difficult to execute properly--particularly with respect to their mounting in a cell that maintains collimation. I certainly agree that many instruments have been designed and mounted successfully. Not all however.

A specific example is a TMB 7" triplet in my basement that has a poorly designed cell and a lens design that demands ultra tight mounting tolerances. Using this telescope is an exercise in frustration, which may explain my mini-tirade.

JimC

#21 John Boudreau

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Posted 27 October 2011 - 01:56 PM

Im also curious about the oil used. Not 5w30 I assume. How much oil is actually in there?


Only a small drop at each lens interface as the layer only has to be a few microns thick. A few years ago Roland Christen mentioned that AP uses an oil rated for use in space systems for disc drives, and has virtually no evaporation under any circumstances we'd put it through here on Earth. I do not know if this is what AP used 10 or more years ago though. Yuri at TEC has claimed they make their own oil formulation. There are also production based optical oils and gels for lenses and immersion oils for microscope slides that can be used. While it's true that even simple cooking oils will work, today manufacturers have a choice of some pretty stable specialty oils that can be reliable for decades.

#22 Eddgie

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Posted 27 October 2011 - 02:19 PM

DO you have the book "Telescope Optics?"

In it, the authors ray trace a 200mm, f/10 oil spaced triplet.

At 20mm off axis (40mm image circle), the spot size is about .1 millimeter. And this is at f/10. .1mm is about 4 times the size that would be required to keep the blur below detection of film, and modern CCD chips can resolve smaller than this at the focal plane.

If anyone else will be so kind as to confirm this, it is covered on page 138.

The limitiation of the oiled design (and again, this is covered in the book Telescope Optics) is that the four interrior surfaces all have to have the same radius of curvature. This means that the designer can't use different curves to correct coma.

But don't take my word for it.

Perhaps someone else on the forum that has the book Telescope Optics can confirm what I am saying.

Also, if you use a focal reducer to make the system f/6, you are in effect reducing the magnification and the angular blur size of the abberation is beeing reduced a corresponding amount.

So the coma is still there (unless it is a coma corrector as well as reducer), but the effects are mitigated.

So, this is why there are no large, fast, oil spaced refractors on the market. It is hard to make them large and fast without pushing into off axis coma.

I mean think about it.. It is so much easier to make an oil spaced refractor, one would think there would be a competitor to the large, fast, air spaced scopes.

So, why don't they???


But hey, we're all expertes here and no one is ever right about anything. Why should this time be different.

And if you have a resource available that contradicts the book Telescope Optics, please do share it.

#23 Eddgie

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Posted 27 October 2011 - 02:23 PM

Skip my posts if you like.. I don't mind.

I try to explain, and ofthen this takes a lot of words.

Some people read my posts, and most skip them.

The people that take the time to fully read my posts I think often come to enjoy them.

Or, they decide I am long winded and choose to skip them.

The choice is yours and yours alone.

#24 Starhawk

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Posted 27 October 2011 - 02:53 PM

Uh, Eddgie, doesn't f/6.3 or f/5.6 count as fast? These are made, both with oil spacing. The reducers take them under f/5. Since these show up at 110mm and larger, I'm going to go ahead and say large ones exist.

The telescope optics website is pretty good, and explains why the vendors won't tell you what the radius of curvature is on their Petzval surfaces (it tells you a lot about the prescription).
-Rich

#25 idealistic

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Posted 27 October 2011 - 03:12 PM

I like to read them, carefully.


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