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oil spacing in fracs

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#1 salico

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Posted 06 January 2020 - 12:04 PM

I read now and than about severe problems with oil leaking, even in high end APOs. What is your experience with that issue, and if it happens regularely, why do Top companies like AP, TEC, CFF and APQ Jena use oil spacing often?



#2 Eric H

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Posted 06 January 2020 - 12:46 PM

It can happen but its not a regular thing. TEC has a lifetime warranty so no worries there. I'm sure the other companies can also fix and refurbish any lens they made.


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#3 Eric H

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Posted 06 January 2020 - 12:48 PM

Oil spacing allows for no air to glass surfaces, a triplet lens would only have 2 air to glass surfaces as opposed to an air spaced which would have 6 surfaces, each of which could reflect light.

This equals better light transmission and quicker cool down times.


Edited by Eric H, 06 January 2020 - 12:49 PM.

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#4 Stellar1

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Posted 06 January 2020 - 12:58 PM

I've always wondered about oil spaced lenses, does the oil degrade in some way over time? does it change colour and, need replacing after years of use?.



#5 BillP

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Posted 06 January 2020 - 01:11 PM

Oil spacing allows for no air to glass surfaces, a triplet lens would only have 2 air to glass surfaces as opposed to an air spaced which would have 6 surfaces, each of which could reflect light.

This equals better light transmission and quicker cool down times.

There is light loss any time there is a change in the refractive index.  So as example, the light loss from uncoated crown glass to air is 4.25%, but from uncoated FPL-53 to air is 3.2%.  So to really determine the exact light loss you need to know the refractive index of the glass types used and the oils used in these lenses.  There would be no light loss only when the refractive index of all the glasses and oils were the same.  Doubt that is the case for oiled optics.  FYI, the refractive index of air is 1.0003.

 

Here is a calculator to figure out the light loss between different refractive indexes - http://hyperphysics....opt/reflco.html

 

Bottom line is that even if we assume the ideal and the only losses for the oiled optic are at the air interfaces, with 99.5% multicoating efficiencies the point becomes moot as the additional light loss from the greater air surfaces in the air spaced optic are less than negligible.  E.g., to equalize the light gathering of a "more" efficient 5.000" oiled triplet, the air spaced one would have to be 5.0514".  So not anything in the visual threshold -- could you imagine someone touting as an advantage how their scope was 5/100ths of an inch larger in diameter than yours?  Well, maybe here on CN lol.gif

 

So from the operational standpoint, the only advantage worth mentioning would be thermal behavior and less surfaces to get potential fungus growth.  And of course the potential disadvantages would be professional servicing most likely needed if a leak/cavitation or some breakdown of the oil over time (rare but the former does happen as we have seen here in CN postings from both AP and TEC if not others also).


Edited by BillP, 06 January 2020 - 01:50 PM.

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#6 Richard O'Neill

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Posted 06 January 2020 - 01:42 PM

 Good question. I'm hoping the oil in my refractors lasts longer than I do. If it doesn't then I hope the maker is still in business. Otherwise, punt!

 

 In the meantime I never subject oiled lenses to severe thermal stress. When not in use my scopes are stored vertically, firmly secured from falling in the hope that this orientation may prevent the oil from creeping out or the elements from separating.

 

 However, storing vertically could be unnecessarily risky, plus it's contrary to AP's advice. Other than a risk of falling I have no idea why AP suggests storing horizontally. Perhaps it makes no difference to the oil's stability. Time will tell I suppose.



#7 Wildetelescope

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Posted 06 January 2020 - 01:54 PM

 Good question. I'm hoping the oil in my refractors lasts longer than I do. If it doesn't then I hope the maker is still in business. Otherwise, punt!

 

 In the meantime I never subject oiled lenses to severe thermal stress. When not in use my scopes are stored vertically, firmly secured from falling in the hope that this orientation may prevent the oil from creeping out or the elements from separating.

 

 However, storing vertically could be unnecessarily risky, plus it's contrary to AP's advice. Other than a risk of falling I have no idea why AP suggests storing horizontally. Perhaps it makes no difference to the oil's stability. Time will tell I suppose.

I have two AP ones going over 30 years and they are right as rain.  Take two panes of glass. Spread a thin layer of oil on one in the center and press the second pane on top so there are no air bubbles.   See how long it takes for more oil to drip out. Glass has a very high surface energy.  I would guess that the oils used like to wet it preferentially.   It is not like these gaps are huge.  Surface tension/Capillary action likely keeps the oil in pretty well under most circumstances. Again, I would guess that in cases where the oil has leaked out, something else has happened to "break" the seal, allowing air to get in.  

 

Just thinking out loud.

 

JMD


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#8 Steve C.

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Posted 06 January 2020 - 01:55 PM

The hard part is finding a dipstick to check the oil with.


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#9 Wildetelescope

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Posted 06 January 2020 - 01:57 PM

The hard part is finding a dipstick to check the oil with.

In my case, the Dipstick and Owner are one and the same:-)

 

JMD
 


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#10 BillP

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Posted 06 January 2020 - 01:57 PM

However, storing vertically could be unnecessarily risky, plus it's contrary to AP's advice. Other than a risk of falling I have no idea why AP suggests storing horizontally. Perhaps it makes no difference to the oil's stability. Time will tell I suppose.

I would guess that the reason for that recommendation is that 1) there is no scientific study on impacts to oiled optics stored horizontally, and 2) the retractable dew shield is probably not engineered structurally specifically for that type of storage where it would be bearing all the weight of the scope.



#11 BillP

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Posted 06 January 2020 - 02:04 PM

Again, I would guess that in cases where the oil has leaked out, something else has happened to "break" the seal, allowing air to get in. 

The seal is probably the culprit in the vast majority if not all of the cases.  I would think it would have to be.  But what could be causing the seal to lose integrity could be any one of a multitude of reasons, including seal material fault, improper seal application, impacts of constant thermal contraction and expansion forces on the seal from the glasses as they acclimated, if the seal is in contact with the lens cell, then adverse impacts on the seal material over time from those contact points, inadvertent user damage, etc.


Edited by BillP, 06 January 2020 - 02:05 PM.


#12 mikerepp

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Posted 06 January 2020 - 02:07 PM

Use synthetic and you can go 10,000miles between changes. lol.gif


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#13 BillP

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Posted 06 January 2020 - 02:24 PM

Use synthetic and you can go 10,000miles between changes. lol.gif

I feel that plant-based oils would be better.  At least then, during the zombie apocalypse when food becomes scarce, one could disassemble the oiled objective of their scope, put the oil on some freshly gathered Dandelion greens, and have a nice salad!


Edited by BillP, 06 January 2020 - 02:25 PM.

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#14 Eric H

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Posted 06 January 2020 - 02:40 PM

I feel that plant-based oils would be better.  At least then, during the zombie apocalypse when food becomes scarce, one could disassemble the oiled objective of their scope, put the oil on some freshly gathered Dandelion greens, and have a nice salad!

APO vinaigrette?


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#15 Orogeneza

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Posted 06 January 2020 - 03:52 PM

The seal is probably the culprit in the vast majority if not all of the cases.

My APQ100/1000 developed issue with astigmatic (triangular) stars last winter. It is one of the first APQ objectives from before 1990, cell is simple design with 3 point support similar to Zeiss AS and other earlier lenses (later APQ cells are temperature compensated with centering bolts). There is no "seal" anywhere, surface tension is the only thing that holds oil in place.

 

There is an article* describing rigorous testing of APQ lens assemblies and specially oil bond between lenses in vacuum chamber at 1/10 of atmospheric pressure without any problems. Optical properties were tested in hot and cold chambers with many cycles ranging from -25 to +45 deg C. Even higher and lower temperatures were tested for limits of transport and storing. Objectives were also tested for mechanical shock and impact at 50G in several different directions.

 

Back to my beloved APQ100/1000.. I was troubled with those triangular stars, tried to back up the retaining ring to small effect, then I noticed alignment marks on elements skewed a little, I disassembled the cell, realigned the rotated upper element and noticed that the bottom element is firmly logged with center CaF2, unlike upper element which was literally swimming on oil bed and rotated freely. Time for specialists with special oils  confused1.gif

 

I was explained at Baader that: "the Lens looses oil due to thermal changes in very low amounts over years and the lenses at some time stick to each other rather then "swimming" on each other. This can causes one lens (usually the fluorite) to be stressed and finally to get astigmatism. So adding a new layer of oil makes them swim again. Astigmatism is caused by the pressure and should go away completely after the re oiling and some settling time."

 

End of story: The lens was sent for servicing, oil was changed with new one after 30 years, astigmatism went completely away.

 

* Dr. J. Pudenz, Dipl.-ing. A. Karnapp, Apochromatische Objektive und Falschinformationen, Information Klarstellung, Carl Zeiss Jena GmbH, Jena, Juni 1992
 


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#16 Chris Cook

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Posted 06 January 2020 - 03:57 PM

My A-P 130EDT lens was serviced by Rolando in 2012 using Wesson synthetic.


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#17 Codbear

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Posted 06 January 2020 - 04:01 PM

Use synthetic and you can go 10,000miles between changes. lol.gif

10,000 miles or 10,000 light years?!? grin.gif


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#18 bobhen

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Posted 06 January 2020 - 04:10 PM

I read now and than about severe problems with oil leaking, even in high end APOs. What is your experience with that issue, and if it happens regularely, why do Top companies like AP, TEC, CFF and APQ Jena use oil spacing often?

They use it because it DOESN’T happen regularly. If people were "regularly" returning scopes no one would use the oil method.

 

Over time, there is about the same chance for dust and mold to get into an air-spaced lens than there is for an oil issue.

 

Air-spaced lenses (especially triplets) also need more rigorous cell designs to keep the individual elements aligned.

 

Had 2 AP 6" refractors for 22-years with no oil issues. Both scopes were used for solar in summer and in sub-freezing temps in winter. 

 

Bob


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#19 Wildetelescope

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Posted 06 January 2020 - 06:05 PM

My A-P 130EDT lens was serviced by Rolando in 2012 using Wesson synthetic.


I have heard the original ap triplets referred to as salad oil triplets. Always wondered if that were true? In any event, it works!

Jmd

#20 Sketcher

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Posted 06 January 2020 - 07:02 PM

I've had an AP oiled triplet for 25 years and have never had any problems with it.  OTOH, not all owners provide the same level of care for their refractors, and not all telescopes have been subjected to the same treatment/care during shipping.

 

If the telescope has been manufactured with sufficient care, and the shipper(s) have done their job(s) responsibly, and the owner(s) haven't abused/misused their telescope(s); then there shouldn't be much to worry about.

 

On the other side of the coin:  I've far too often read about people (right here on CN) who have abused their precious telescopes -- usually through ignorance.  Too many people think there's no need to take the time to read what the manufacturer has to say about product care.  IIRC, I expressed one of my concerns prior to placing my order, so I knew about precautions that pertained to using an oiled triplet when the outside temperature was 90 degrees colder than the inside temperature.

 

I would be very hesitant to purchase a used oiled triplet -- to the point of avoiding it entirely, but that's just me and my fears.  Nowadays some people purchase an oiled refractor as their first telescope, before they've learned some of those dos and don'ts that many of us have to learn through experience.  So I try to avoid recommending an oiled refractor (or any "expensive" telescope) to anyone as a first telescope.  There are just too many sad tales floating around about unintentional telescope abuse and the sad results.

 

P.S.  I've always stored my refractors horizontally.  Common sense (at least for someone with a background in physics) says horizontal storage will present less (not more) opportunity for problems -- even for an oiled refractor.


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#21 Nippon

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Posted 06 January 2020 - 07:30 PM

Doesn't oil spacing lessen the need for the lens surfaces that are in contact with the oil to be as perfect a surface as the surfaces contacting air? I don't know if I said that right.



#22 Alan French

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Posted 06 January 2020 - 08:34 PM

The seal is probably the culprit in the vast majority if not all of the cases.  I would think it would have to be.  But what could be causing the seal to lose integrity could be any one of a multitude of reasons, including seal material fault, improper seal application, impacts of constant thermal contraction and expansion forces on the seal from the glasses as they acclimated, if the seal is in contact with the lens cell, then adverse impacts on the seal material over time from those contact points, inadvertent user damage, etc.

Why do you need a seal? The layer is microscopically thin and the intermolecular forces between the glass and liquid are very strong.

 

Clear skies, Alan


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#23 Richard O'Neill

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Posted 06 January 2020 - 09:12 PM

Evaporation, I presume. Perhaps contamination as well.



#24 Jeff B

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Posted 06 January 2020 - 09:26 PM

Having owned a couple of dozen oiled lenses over the last 35 years, I've never had an issue with any of them concerning the oil spacing.

 

I have had some issues with lenses that were spaced with glue or gels due to "aging' of the compound but,again, zero issues with those that use/used oil.

 

Jeff



#25 donadani

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Posted 07 January 2020 - 12:26 AM

Had oilspaced apos from AP, TEC, CFF and AOM and never problems with leaking at any of them. Know one AP-130EDF from a starfriend for sure with slight oil leaking but it was repaired by Baader for free and fast.

 

My oldest one is an AP-130EDT from the first run around ´89/´90 - it´s as good as on first day without any oil-change - so needed service-intervals are very driver-friendly wink.gif

 

Store all of them horizontal and if I ever would store them vertical it would stand on the focuser to avoid anything falling on the lens inside.

 

Oil filled optics offer extrem contrasty views and cool faster then air spaced triplets. At the same f-ratio they´re not as well corrected for CA like an air-triplet so a slower design or fluorite as center element is welcome but not a must have. Especially TEC and CFF are unbeatable when it comes to compactness/apterture.

 

Love them! smile.gif

 

cs

Chris


Edited by donadani, 07 January 2020 - 12:38 AM.



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