81 replies to this topic

[Alan French]

In my experience capillary action does not have any effect in side way motions. hence this could be an issue if the glass elements sliding a tiny bit from each others.

Isn't one of the advantages of the manufacturing process of oil spaced apos that inner surface doesn't have to be so smooth polished? I've heard that statement often on this subject which again counteracts capillary effects. To have this effect the surfaces in contact with the oil does indeed have to very smooth.

A third thing.. It seems the objective has spacers between the elements! If so how can this oil spacing have this capillary actions as you mension?

According to Roger Ceragioli (Winter Star Party talk, 2009) Wolfgang Busch and Roland Christen independently came up with the idea of replacing the air space with a thin layer of oil. Busch didn't polish the adjacent surfaces. 

 

I've seen no evidence that any current manufacturer uses this approach.

 

I believe what you take to be spacers are actually between the back of the lens and the back retaining lip/ring on the cell. A triplet I have here has something similar, and quite obviously no spacers between the elements. 

 

Clear skies, Alan

[Alan French]

There must be reasons for use of oil but not overly sure what.

Cemented lens have the restriction that R2=R3 so less parameters to play with, in an air spaced the 2 radaii need not be the same. Additionally the air gap seperation is a factor tha can be used in the performance/design. But to get the oil to perform as best I can tell it is back to R2=R3 for the surface tension to hold the surfaces together, so the lens design is again limited.

 

Since oil is a fluid that means that it can leak, as it can flow. suppose warm oil flows more easily.

 

Is the use of oil a manufacturing reason? The only one I can think of it that the surface need not be produced so finely as the oil sort of "fills in the surface imperfections". Which I hope is just incorrect these days. Half a reason say 50 years ago but not now.

 

I don't think I would buy an oil filled lens, or I would have to have a good reason for doing so.

Liquids respond to the pull of gravity and, if unrestrained, will flow downhill. However, in narrow spaces, there is more going on. (Well, it's always going on, just usually overwhelmed by gravity.) 

 

Take an eyedropper and fill it half full of water. Look carefully at the top of the water inside the dropper. It is not flat, it is concave (meniscus). The glass molecules pull on the water molecules, which pull on adjacent water molecules. The net force pulls upward and is stronger close to the glass, so the water closest to the wall is pulled upward the most. As you move away from the wall the force decreases and the water isn't pulled as high. The result is that the top surface of the water is concave, not flat. (Try it with something more viscous, like mineral oil.)

 

If you have a hollow tube small enough, and place it in a cup of water, the water will be drawn up into the tube. The smaller its diameter, the higher it will go. This is capillary action. It's also at work in the microns thin layer of oil between two matched lens elements and is stronger than the gravitation pull on the oil. 

 

If you put a drop of oil between two microscope slides, or small pieces of glass, and push them together, it's hard to pull them apart. Sliding works better. (Please where gloves if you experiment!) 

 

I have a clear plastic bottle of mineral oil, and the meniscus around the top edge of the oil is obvious.

 

My guess is that actual oil leaks result from making the oil layer too thick or not having exactly matching surfaces and a varying thickness of oil. As referenced in post #45, Baader says that thermal shock can cause problems. I've also seen references to gels being used, presumably differing from oil. Some problems may be related to choice of material. 

 

It would be interesting to hear if there are any first hand reports of "oil problems" without any visible leakage. (I wonder if folks just assume a problem is due to leaking oil.)

 

This is really a classic sci.astro.amateur topic.  

 

Clear skies, Alan

[Sketcher]

OK, oiled triplets can leak; but they can also survive in mint condition for many years.  My oiled triplet is 23 years old.  I'm the original owner.  It's never shown any signs of a leak.

 

Why do some leak while others not leak?  I suspect there's no easy answer; but some potential factors might include the manufacture and assembly process, transportation treatment (out of our control until the telescope lands in our hands), and care/treatment by the end user(s).  I would be hesitant to include improper storage -- I'm not even sure what that might entail; but I would suspect sitting on a shelf for 1+ years would not be a problem unless the scope underwent some kind of trauma (or manufacturing errors) prior to ending up on the shelf.

 

The main advantages (for the end user) of an oiled-triplet:  Improved light transmission and contrast due to only two air-glass surfaces (an air-spaced triplet would have six air-glass surfaces). and better cooling properties due to the absence of two, air-filled, layers of 'insulation'.

[PETER DREW]

I have an old Ross 5" F15 triplet. 40 years ago the late Horace Dall refigured it for me and oiled at least one of the components. 20 years on, no leaks but the objective developed astigmatism. A colleague, an expert optician found that the original oil medium had "hardened" creating tension in the thin biconcave element. Re-oiling corrected the problem. I also understand that excess oil can be used initially as insufficient can cause variations, the excess oil is then allowed to leak out until the lenses have settled, after which the edges are taped.

[Alan French]

I have an old Ross 5" F15 triplet. 40 years ago the late Horace Dall refigured it for me and oiled at least one of the components. 20 years on, no leaks but the objective developed astigmatism. A colleague, an expert optician found that the original oil medium had "hardened" creating tension in the thin biconcave element. Re-oiling corrected the problem. I also understand that excess oil can be used initially as insufficient can cause variations, the excess oil is then allowed to leak out until the lenses have settled, after which the edges are taped.

 

Interesting. Thanks.

 

Clear skies, Alan

[Paul G]

>>Paul G, on 16 Nov 2018 - 06:23 AM, said:

Now you’re just being silly.<<

 

Not really.  If I read what you said as you have 6 oiled Apos on storage for 20 years untouched, then OMG...I mean OMG!!  However, if you meant that for the past 20 years you keep your scopes horizontal when not in use but that you do take them out into the field, well then that is not the scenario the OP is presenting so not relevant.

Really. You can certainly read what I clearly posted as something else entirely if you choose.

 

I disagree, it is relevant. Even a scope that gets moderate use spends the vast majority of its time in its case. Every refractor case I've ever seen was made for the scope to sit horizontally. And with the cutouts for focus knobs most go back into the case in the exact same position every time. If being stored horizontally caused oil leakage given the several thousand oil spaced objective scopes out there spending the vast majority of their time horizontal in the same rotational position we'd be seeing hundreds of reports of leakage. Not the very small number mentioned in the "oil spaced objectives leak" threads here.

 

Given the very small number of leaking lens assemblies reported it is statistically interesting that the OP has personal experience with three. Maybe being around him causes the oil leaks! 

Edited by Paul G, 16 November 2018 - 07:47 PM.

[gjanke]

A-P is not the only company making oil spaced lenses. IIRC, someone on CN reported that Baader repaired their lens after an oil leak. And while I’m on that topic, here is what Baader says about oil spaced lenses. https://www.baader-p...-spaced-lenses/

Can everyone just go to the link and call it a day? Its not exactly an empirical study but I believe they are basing it on more than just opinion, perhaps something more valuable; money. If the design cost them money they would stop manufacturing it. Doesn't appear to be the case.

[MooEy]

Well, my opinion here on air spaced. Not saying they are no good, but we don't really know how stable some of these glass types are. We barely have ED glasses for the past 20-30 years or so. 

 

An oil spaced triplet would put the ED/Fluorite glass between 2 piece of crown glass, which have been known to be rather stable. That's 4 less surface to worry about. 

[dr.who]

Oil leaking over time is always an additional risk.  There certainly are enough examples of posts here and elsewhere of leaks developing in the various oiled Apos.  What is interesting here is that it seems that storing the scope in a fixed horizontal position for multiple years might increase the risk of a leak developing at the lower end of the objective.  I know of more than one case of this happening so not a one-off issue.  As long as the scope is actively used this would of course not be an issue.  Long term storage though seems advisable to store vertically.  No reason IMO for anyone to get defensive relative to oiled optics...actually good info to know for lower risk storage of oiled designs.

 

Thanks for the post

 

Bill - This is my point exactly. 

[dr.who]

Given the very small number of leaking lens assemblies reported it is statistically interesting that the OP has personal experience with three. Maybe being around him causes the oil leaks! 

 

My God Man! You are right! Maybe *I* AM the cause!

[BillP]

Can everyone just go to the link and call it a day? Its not exactly an empirical study but I believe they are basing it on more than just opinion, perhaps something more valuable; money. If the design cost them money they would stop manufacturing it. Doesn't appear to be the case.

 

I read the post.  The very first bullet makes oiled not for me where it was talking about how they are sensitive to temp shocks of 50 deg C, but 25 deg C seems to be ok.  I routinely run my scopes with differentials of 40 deg C in the winter so I am not going to change my observing because of a design limitation.  Bottom line though is that there are pros and cons to each and up to the individual as to which set of risks they want to have.  So not a matter of air being better than oil or visa versa, but each has real issues that impact them more than the other so entirely up to the consumer as to which issue risks they want to live with.  IMO people should stop being so defensive about it as it would be better for all of us to recognize rather than sweep aside potential issues (unless someone is proposing that we should keep consumers uninformed).  Pros/Cons as I see them are below.  Both designs have their issues...no need for any angst so just choose your poison and move on.  Whether the risks are high, medium, or low for each is of course debatable since no one here is privy to OEM records on repairs.  But largely doesn't matter as the risk is still there and from the purchasers perspective it will always be 50-50 as to whether it will happen to them or not.

 

Air Spaced designs potentially have higher risks of:

• fungus due to more exposed glass surfaces
• decollimation/centering due to shocks
• longer cool down times

Oil Spaced designs potentially have higher risks of:

• oil leakage
• oil cavitation
• temperature shock issues
• needing more specialized servicing from places that know the proprietary oil/substance used in the prescription

 

"Oil leaking over time is always an additional risk. ...  No reason IMO for anyone to get defensive relative to oiled optics...actually good info to know for lower risk storage of oiled designs."

 

Bill - This is my point exactly. 

 

I think everyone got the point you made.  It's just that for some reason many oil spaced owners seem to take it personally.  The risk of these issues/events happening is always low, but the risk is still there so best to acknowledge it without taking it personally as we need to keep the community informed and let everyone make their own decisions.  I mean I'm glad I know the risk of fungus is greater in air spaced designs simply due to the extra surfaces.  I also know it is fairly rare, but I still take handling precautions.  I also know that there are oiled scopes that have leaked and have had cavitation.  I decided for myself that those risks I did not want to take because they were issues I do not want to have to mitigate by not taking my scopes into temp extremes and because I do not want to have to deal with the additional issues that can come with oil vs. air regardless of how rare it might be (i.e., both can get fungus, but air can't leak or cavitation).  Anyway, glad you posted this as even if the issue is rare, I still want to know because should always be my decision to make and to be fully informed.

Edited by BillP, 17 November 2018 - 12:29 AM.

[gjanke]

I still want to know because should always be my decision to make and to be fully informed.

I absolutely agree with you. You should be informed when purchasing a very expensive piece of equipment.

[starcanoe]

 

Oil Spaced designs potentially have higher risks of:

• oil leakage
• oil cavitation
• temperature shock issues
• needing more specialized servicing from places that know the proprietary oil/substance used in the prescription

 

 

 

I suspect that for lenses that only use the oil to mate two optical surfaces with the same curvature (as opposed to an actual "oil SPACED" lens) that any just about any old oil would work well enough that you'd be hard pressed to detect the difference between that and the magic oil the manufacturer used. Use a little common sense and some optical engineering knowledge and IMO you'd be fine.

[Daniel Mounsey]

There’s plenty of experiences on this topic but they are seldom shared. 

[starcanoe]

My God Man! You are right! Maybe *I* AM the cause!

 

That blue police box you tool around in might have something to do with it...

[junomike]

There’s plenty of experiences on this topic but they are seldom shared. 

Please elaborate!

[Sketcher]

I read the post.  The very first bullet makes oiled not for me where it was talking about how they are sensitive to temp shocks of 50 deg C, but 25 deg C seems to be ok.  I routinely run my scopes with differentials of 40 deg C in the winter so I am not going to change my observing because of a design limitation.

Yes, if that's how you interpreted the post, an oiled triplet would not be a good choice for your use.

 

Yet, many people continue to use their oiled triplets when the inside-outside temperature differential is 40 deg C or more -- with no problems.  The temperature differential doesn't cause problems -- as long as the owner knows how to deal with it.  I could add my own personal experiences; but I would likely just be accused of being "defensive".  The reality is that I've 'defended' reflectors, refractors (achros and apos), and catadioptrics whenever I felt the need and desire -- since I have experience with all of those telescope types.

 

Any telescope is susceptible to damage by users who do not know how to properly use and care for it.  This is a prime reason why I do not recommend expensive telescopes (of any type) for those new to the hobby.  It's better for people to make their mistakes using relatively inexpensive telescopes.  Then, they would be less likely to unintentionally abuse an expensive telescope later.

 

Unfortunately, many of today's newcomers want to start out with "the best"; and we often see the unfortunate results on these forums.

[Kunama]

.................. It's just that for some reason many oil spaced owners seem to take it personally.  The risk of these issues/events happening is always low, but the risk is still there so best to acknowledge it without taking it personally as we need to keep the community informed and let everyone make their own decisions.  I mean I'm glad I know the risk of fungus is greater in air spaced designs simply due to the extra surfaces.  I also know it is fairly rare, but I still take handling precautions.  I also know that there are oiled scopes that have leaked and have had cavitation.  I decided for myself that those risks I did not want to take because they were issues I do not want to have to mitigate by not taking my scopes into temp extremes and because I do not want to have to deal with the additional issues that can come with oil vs. air regardless of how rare it might be (i.e., both can get fungus, but air can't leak or cavitation).  Anyway, glad you posted this as even if the issue is rare, I still want to know because should always be my decision to make and to be fully informed.

Well said Bill,  there is no point people taking this issue personally and getting defensive about it.  Seems the issue is rare but does occur, whether due to a manufacturing fault, inappropriate storage, high temperatures or failure of the sealing mechanism.  I agree it is best to be made aware of the possibility so that one can reduce the likelihood.

 

(Fungus on the other hand in my view is a different issue and is a lack of proper care for the conditions one places these precision optics in... Fungus does not care whether your optics are oiled or aired, it just wants some moisture, food and a dark place without UV to do its thing.... All my scopes are stored with desiccant when in cases, properly dried after use, after 40 yrs with precision optics I have yet to fall victim to its tentacles..)

Edited by Kunama, 17 November 2018 - 03:37 PM.

[Daniel Mounsey]

Please elaborate!

 

Well, let me first say that I think oil spaced triplets have unquestionably yielded some of the highest contrast images obtainable. I'm not on any mission to discredit oil or gel spacing and hopefully this method continues for years to come since I enjoy the visual results it yields. I think it's obvious that even these liquids act like ketchup. If you place the bottle upside-down or on its side, the ketchup eventually makes its way down and through. Some observers make the mistake of storing these scopes in extremely hot locations such as a non insulated garage for long periods of time. Pretty soon, it's 100 def F in the summer inside the garage. This happens out this way but further inland in certain locations. Personally, I've experienced these problems four times myself working with this to solve these problems and had to get it paid for as well. I also had a dear friend who's oil spaced refractor had to be completely re-done after a number of years.

 

Companies are probably not going to want to devulge in their number of experiences dealing with this stuff. Why would they want to? But let's be realistic here. I know a good number of instances where this has occurred from experienced insiders and I've personally concluded that oil spaced refractors have a life expectancy unless they are stored properly for long periods of time. I am only saying this because this is what my own personal experiences have proven to me. There have been questions of what liquid materials are currently used. Whatever they are, they're obviously susceptible to failure and so the case is currently pretty much closed for me as to whether it's safe to suspect that these problems may occur. It's very obvious that they do occur. If they do in the future, then who's going to repair them? Will those companies still be run by competent people? Will there be a newer generation of people doing this? I really don't know. 

 

There are numerous times this has occurred, but those experiences are not always shared. Just because we seldom hear them does't mean they don't occur. I'm sure others just prefer not to share their stories. A very good comment was made earlier about a scope for a lifetime and that few observers keep their scopes and end up selling them anyway. That actually would not apply to me since I do keep the same scopes for many years. The reason? Because once I know I have something special, I practically never let it go. All my personal refractors are Takahashi fluorite doublets. A couple of them are probably about 25 years old and still perform nice to this very day without even the slightest bit of concern. With liquid spaced elements, I think I would be considering how and where I have them stored because that's what my personal experiences have proven to me. It's not really necessary for me to search for any validation as to whether this is true or not. It's true.

 

Maybe on the east coast or places where the climates are cooler more often, it's not an issue. Maybe the scopes get used and moved around enough that it's not an issue, even for many years. I think observers just need to use some common sense. Others may say these liquids don;t react to temperature, okay. I'm just going based on what my personal experiences have been so far. 

 

Something I forgot to mention is fungus and life growing inside of air spaced elements which others correctly mentioned earlier. I was told by a close friend that I would have to be very careful in Philippines with my doublets. So, it probably mostly depends on where these telescopes are used and stored. 

Edited by Daniel Mounsey, 17 November 2018 - 04:10 PM.

[gjanke]

I once owed a Nissan Altima that required very little repair and ran for 10 years and got me 210,000 miles. I traded it in and got another Nissan Altima which burned oil and got me 114,000 miles and required lots of repair. Given the chance I didn't buy another Altima yet there are hundred of thousands Altimas on the road with satisfied customers. Guess which one always comes to mind when I think of a Nissan Altima.

[Peter Natscher]

https://www.baader-p...-spaced-lenses/

 

I do not want to point fingers. I do not want this to turn into a flame war. So if you feel strongly enough on the subject to react in anger please step back from the keyboard and take a moment. I am not naming the vendor that made this scope. I am not naming the vendor who made the other scopes I have seen this problem with. It doesn't matter who made it. What matters is this can happen with oil spaced triplets. Once is a fluke, twice is a coincidence, three or more times and something is going on...

 

... This will be the third one of these I have personally seen develop this problem. So far it has been due to storage for long periods (1+ year) in a horizontal position. I am guessing the two ways to prevent this are:

 

1. To use the scope regularly and often

2. rotate the position that it is stored or perhaps store it vertically(?)

 

In each case the vendor has made things right for the owner. Unfortunately there was still a cost involved. And for larger apertures the shipping price can become prohibitive by itself regardless of the cost for the vendor to perform the repairs. And in all three cases these were not 20+ year old scopes.  

[John O'Hara]

Interesting topic.  I own a 1986 6" f/8 A-P (one of the old standard, non-ED glass ones).  So far, so good.  However, I do use this scope and it definitely doesn't sit in storage.

[dr.who]

I think everyone got the point you made.

I was hoping this would be the case but in my reading of the replies, as you said, some took it personally or read into it things I didn’t say or imply.

[dr.who]

That blue police box you tool around in might have something to do with it...

  I KNEW that could be a problem! From now on: No oil spaced triplets on the TARDIS. No Exceptions.

[MooEy]

Quite a lot of the clubs here have got refractors with fungus in between the glass due to poor storage and negligence.

Sending the scope back for servicing isn’t quite an option. If all these had been cemented or oil spaced lens, those scopes would have been quite a fair bit more usable.

Anyway, the main aim of oil spacing is to work around the issue with cementing the lens elements. Cemented lens are typically done for smaller lenses. Once the size gets larger, difference in expansion causes the lens to easily decement and haze.

I see air spaced lenses as more prone to failure than oil spaced lens. I can easily count more scopes with fungus in between the lens elements than you can count the number of oil spaced lens that have leaked.

Even if we adjust to balanced out the number of oil lens against air spaced lens, the numbers will still be in oil spaced favour.

If we are looking at longevity, oil spaced is vastly superior.

This is not even taking into account of the issue of corrosion. Oil spaced lens with exotic glass or crystal sandwiched in oil and pyrex is pretty much the best you can ever do.

I have many filters that have “aged” and rusted around the edges. Sure, those are metallic in nature and more likely to corrode.

Fluorite and ED glass types have only been used for the past 20-30 years. I’m willing to bet all those earlier FC type lens with the uncoated fluorite will eventually fail. It may not be now, but give it another 20-30 years, we will see reports of either the crystal getting stains or haze.

Edited by MooEy, 18 November 2018 - 12:55 AM.