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oil spacing in fracs

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#26 Wildetelescope

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Posted 07 January 2020 - 12:40 AM

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

I believe You are correct. I used the term “seal” incorrectly to describe the intimate contact between lens and oil.  My lens ARE wrapped in Kapton tape along the edge.   I suspect that has more to do with preventing the ingress of moisture at the glass-oil interface, than keeping the oil in place. 

 

Jmd



#27 Uwe Pilz

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Posted 07 January 2020 - 12:54 AM

> 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.

 

We have an discussion about this here.

 

It seems that it was a reason in the past, but the problems with grinding crystals are overcome now.


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#28 Jeff B

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Posted 07 January 2020 - 10:46 AM

I believe You are correct. I used the term “seal” incorrectly to describe the intimate contact between lens and oil.  My lens ARE wrapped in Kapton tape along the edge.   I suspect that has more to do with preventing the ingress of moisture at the glass-oil interface, than keeping the oil in place. 

 

Jmd

There are several reasons for the Kapton tape.  One is to help keep the lens stack stable, another is to provide a bit of an additional material buffer to the cell walls and then there is sealing of the edges.

 

Regarding cooling of oiled triplets, I don't believe oiled objectives cool all that much faster that an air spaced lens, rather it's more of how the cooling process affects the overall wave front during the process.  The airspace is part of the refractive design and as the individual elements change their figure by cooling from the edge in they each interact with the air gap in a way that's "off design", typically resulting in very noticeable undercorrection during cooling.  Oiled lenses also cool from the edge in but they remain in contact via the oil which means their figure changes do not have as much of an affect on the overall wave front while cooling, though there is still some under correction involved..  And that has been my experience at the eyepiece as well.

 

Jeff



#29 Alan French

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Posted 07 January 2020 - 01:22 PM

Evaporation, I presume. Perhaps contamination as well.

I believe the oil used was designed for space applications, has a very low vapor pressure (molecules tightly held and don't tend to leave or evaporate), and is expensive.

 

Clear skies, Alan



#30 Paul G

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Posted 07 January 2020 - 03:44 PM

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

Or aspherize one surface.


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#31 donadani

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Posted 07 January 2020 - 03:57 PM

of course! waytogo.gif



#32 lionel

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Posted 08 January 2020 - 01:41 AM

Or make a hybrid air/oil triplet. The latest iteration of the AP130, the GTX, is air spaced between the front element and the rear oiled doublet.  No aspherizing is needed. The fully spherical system has excellent CA control and preserves some optical and cooling advantages of an oiled triplet.

 

Lionel


Edited by lionel, 08 January 2020 - 01:51 AM.

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#33 Tom Masterson

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Posted 08 January 2020 - 01:53 PM

My 1985 AP lens is still good as new. No issues with the oil. Had a friend who purchased one at the same time and had to sent it in to AP probably 20 years ago because of gaps showing up in the oil around the edge.


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#34 Jared

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Posted 08 January 2020 - 03:11 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?

It can happen that an oiled lens will need to be re-oiled.  It's not a maintenance item, though, like it is in a car.  You won't need to change the oil every ten years or anything like that due to either evaporation or discoloration.  Something has gone wrong if the oil needs to be replaced.  That being said, things DO go wrong, even with high-end telescopes, so I don't think the stories of TEC/AP/Zeiss telescopes needing to be re-oiled is apocryphal.  It's just very unusual.  There is an equivalent, unusual issue with air spaced lenses, though, and that is decentering of an element.  While refractors hold their collimation very well--usually for a lifetime--things can go wrong with air spaced lenses also.  An impact--even something encountered in transportation, can cause a lens to shift.  Doublets can often be fixed by a skilled owner, but triplets generally need to go to someone with a proper optical bench for realignment.  As with most things, there are trade-offs.  You'll frequently read about TeleVue scopes needing alignment and also Takahashi Sky 90 scopes needing alignment of the air spaced elements.  It can be frustrating for the owners, but as with oil-spaced refractors the issue is quite rare.  OK, maybe not so rare in the case of the Sky 90, but I suspect that is mostly because small scopes like that are more likely to be involved in airline transport and therefore occasional rough handling.

 

Basically, pick your poison.  Air spaced lenses have the potential for better optical correction, especially chromatic correction, due to the additional degrees of freedom the designer has since internal radii need not match the mating element.  Oiled designs have the potential for increased contrast due to the fewer air-to-glass surfaces, though this advantage is often overstated, especially considering the efficiency of modern AR coatings.  Oiled designs are also less susceptible to pinched optics since you don't have alignment screws holding the elements in place.  Both designs are perfectly appropriate for a lifetime of maintenance-free observing. Even cemented lenses can be good for a lifetime as long as the lenses aren't too big.  


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#35 donadani

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Posted 08 January 2020 - 03:46 PM

My Lichtenknecker R125 has an cemented optics - lens is as good as it can be... made around 1984 :)



#36 gjanke

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Posted 08 January 2020 - 10:01 PM

 and have a nice salad!

There it is. I was reading, waiting and hoping and then bam if finally came. The signature statement when you are talking oil spaced optics. "salad".

Thank you for not disappointing. 

 

Clear Skies smile.gif


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#37 starryhtx

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Posted 09 January 2020 - 08:06 AM

I wouldn't recommend it, the leaks I've seen have been pretty bad. 



#38 DrGomer

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Posted 09 January 2020 - 10:16 AM

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.

You were so close to having this right and they are great details to bring in.  Where you lost it is that ALL optics we will ever use in refractors (except for the $99 special :p) will have AR-AR coatings which substantially bring down the reflection.  We get narrow band coatings with 0.1 to 0.2% back reflection and broadband I think is like 1-0.5%. The variability isn't just the substrate but largely the type of coating (composition of the dielectric stack) and the pass band of the coating... and the quality of the coating.   One could obtain an AR coated crown glass with a lower reflection than FPL-53.

 

Oiled optics can still have AR coatings if the dielectric is stable in the oil.  



#39 peleuba

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Posted 09 January 2020 - 11:07 AM

Where you lost it is that ALL optics we will ever use in refractors (except for the $99 special tongue2.gif) will have AR-AR coatings which substantially bring down the reflection.  

 

<SNIP>

 

Oiled optics can still have AR coatings if the dielectric is stable in the oil.  

 

 

Maybe this is true, but its an academic discussion as the oiled telescopes available to the amateur market (A-P, TEC and CFF) don't coat the surfaces that are directly coupled with oil (or in some cases gel).  

 

In other words, you don't incur the added expense and added risk to coat the surfaces that are in contact with the oil.  All air-to-glass surfaces are multicoated with the latest BBAR coatings.

 

I am sure that's the assumption that Bill made, and he is correct.


Edited by peleuba, 09 January 2020 - 11:10 AM.


#40 gjanke

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Posted 09 January 2020 - 03:05 PM

Maybe this is true, but its an academic discussion as the oiled telescopes available to the amateur market (A-P, TEC and CFF) don't coat the surfaces that are directly coupled with oil (or in some cases gel).  

 

In other words, you don't incur the added expense and added risk to coat the surfaces that are in contact with the oil.  All air-to-glass surfaces are multicoated with the latest BBAR coatings.

 

I am sure that's the assumption that Bill made, and he is correct.

Okay Paul,

 

Is there anything you don't know about telescope engineering? My gosh man you are a wealth of knowledge! Hope the new year is treating you well.



#41 Jeff B

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Posted 09 January 2020 - 04:17 PM

I wouldn't recommend it, the leaks I've seen have been pretty bad. 

But quite fixable.

 

Trouble is, with over 35 years of experience in this hobby, I've seen some air spaced lenses with astigmatism and coma issues due to decentering/rotation of the elements relative to each other.  I've experienced no issues (leaking or optical really) with oil spaced lenses ( a couple dozen) , though I've had issues with older gel spaced and cemented lenses due to "aging" of the compound, but those were fixed by the OEM with oil spacing.   I suspect that issues with CFF, AP, TEC or other modern oiled lenses would be very rare indeed.  

 

Jeff 


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#42 junomike

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Posted 09 January 2020 - 06:44 PM

I have seen first hand (friends Apo) how issues with Oil-Spaced Apo's can potentially  fail.

It scared me enough to go with only air-spaced Apo's even with their disadvantages and potential issues (fungus)


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#43 salico

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Posted 10 January 2020 - 09:39 AM

so it looks like generally no problems appear, but sometimes it gets ugly...



#44 bobhen

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Posted 10 January 2020 - 10:56 AM

so it looks like generally no problems appear, but sometimes it gets ugly...

Keep in mind that that the same thing is also true with air-spaced lenses.

 

Bob



#45 salico

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Posted 10 January 2020 - 11:50 AM

Keep in mind that that the same thing is also true with air-spaced lenses.

 

Bob

Also, if they are executed to the same level as oiled lenses from the Top Branches?



#46 Jeff B

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Posted 10 January 2020 - 04:16 PM

Also, if they are executed to the same level as oiled lenses from the Top Branches?

Yes.

 

Each design has its own pluses and minuses.  

 

Personally, I've move away from airspace triplets in favor of oil space triplets, at least in apertures over 5", as I find their lens blocks to be physically more robust and thermally benign than those with air spaces.

 

Now, of course, if the scope is large, permanently mounted and not subject to rapidly changing thermal conditions, I may well prefer and choose the air spaced design.

 

Jeff


Edited by Jeff B, 10 January 2020 - 04:16 PM.


#47 junomike

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Posted 10 January 2020 - 06:27 PM

Yes.

 

Each design has its own pluses and minuses.  

 

Personally, I've move away from airspace triplets in favor of oil space triplets, at least in apertures over 5", as I find their lens blocks to be physically more robust and thermally benign than those with air spaces.

 

Now, of course, if the scope is large, permanently mounted and not subject to rapidly changing thermal conditions, I may well prefer and choose the air spaced design.

 

Jeff

Interesting as it was the changing thermal conditions (thermal shock) which caused the OTA in post #42 to fail (according to Yuri, not me).
 



#48 BillP

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Posted 10 January 2020 - 10:30 PM

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

Ask the manufacturers that seal the rims.  Off the top I would imagine to eliminate air contact with the edge which will promote some level of evaporation of the oil.  Another reason would be if it does ooze out for whatever reason, as we have seen from people posts, then at least it will stay contained as opposed to moving out into other areas.  You can ask why all you want, we know however that it does leak out or cavitation from the folks who had the problem and posted pics.  So not like no one has never seen this before - just read this thread an see the multiple people chime in about their TEC or APs that had oil leaks.

 

https://www.cloudyni...-or-maintained/

 

So perhaps more accurate if you say that when "properly" applied it is "most likely" microscopically thin with intermolecular forces between the glass and liquid being very strong.  I doubt you know the exact amount applied to all the oiled scope optics over the years by the many maker to state it as a fact always, nor can you know how many were oiled improperly as there is no such thing as zero errors when humans are involved. 

 

So bottom line IMO, safer to put a seal around the rim than not, and easy enough to do, and some manufacturers do.  Oiled optics can show leaking or cavitation issues.  That is a fact.  No one knows what the probability is as no one has that proprietary data.  So anyone's "guess" only based on their incomplete and not statistically valid anecdotal data.  I'm not saying it is uber rare or common, just the fact that it can and does happen as enough have reported it even in the small slice of those in the hobby who are here on CN.


Edited by BillP, 10 January 2020 - 10:39 PM.


#49 bobhen

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Posted 11 January 2020 - 07:42 AM

Here is a “top of mind” list of air-spaced telescopes that all had serious lens centering issues

Meade ED apos from the 1990s
Takahashi first run TOAs
Synta first run 150mm ED doublets

 

In each case, if someone had an issue the scopes had to go back and if they didn’t have an issue the odds are that they would down the road. The cells had to be redesigned or a Band-Aid fix applied.

 

 

In some cases, even the fix was problematic, as with the Meade doublets and in “some cases” never corrected to the customer’s satisfaction. This does not count mold issues or dirt between the lens elements that can happen with air-spaced lenses.

 

It is true that we do not have an exact percentage of the oil-spaced issues but even if you count the number of issues in that CN thread, when compared to the number of scopes that have been produced over the last 40-years, the percentage is low. And the fix is easy compared to an air-spaced cell that needs a cell redesign or the addition of centering screws or trying to correct a mold issue.

 

I’ve owned air and oil-spaced objectives for over 30-years and never had an issue with either. Just get the scope you want and take care of it.

 

I did have a neighbor’s cat jump into my 15” Dobsonian and scratch my primary mirror and I had to send the mirror back.

So sometimes stuff happens, even to reflectors.

 

Bob


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#50 edif300

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Posted 11 January 2020 - 08:50 AM

Here is a “top of mind” list of air-spaced telescopes that all had serious lens centering issues

Meade ED apos from the 1990s
Takahashi first run TOAs
Synta first run 150mm ED doublets

 

In each case, if someone had an issue the scopes had to go back and if they didn’t have an issue the odds are that they would down the road. The cells had to be redesigned or a Band-Aid fix applied.

 

 

In some cases, even the fix was problematic, as with the Meade doublets and in “some cases” never corrected to the customer’s satisfaction. This does not count mold issues or dirt between the lens elements that can happen with air-spaced lenses.

 

It is true that we do not have an exact percentage of the oil-spaced issues but even if you count the number of issues in that CN thread, when compared to the number of scopes that have been produced over the last 40-years, the percentage is low. And the fix is easy compared to an air-spaced cell that needs a cell redesign or the addition of centering screws or trying to correct a mold issue.

 

I’ve owned air and oil-spaced objectives for over 30-years and never had an issue with either. Just get the scope you want and take care of it.

 

I did have a neighbor’s cat jump into my 15” Dobsonian and scratch my primary mirror and I had to send the mirror back.

So sometimes stuff happens, even to reflectors.

 

Bob

 

 

Seems that serious lens centering issues with "TOAs" and its "first run" was:

 

"There is some misunderstanding about the earlier model of TOA-130. At the first stage of our introduction of TOA-130, we have received many claims for almost all TOA-130 OTA from our USA dealer that there was astigmatism on TOA-130, and according to their expectation, the astigmatism was caused by the lens cell.
After receiving this claim from USA, our technicians have checked and investigated in details spending a lot of time and finally found that the USA dealer has used inaccurate collimation tools and they have visited Japan, and have acknowledged their misjudgment. This is the real story and we confirm that there was no problem at all on the lens cell of TOA-130 from the beginning. If you heard such claim from somebody, it is a definitely rumor, and we have never changed nor modified the lens cell of TOA-130 since our first introduction till now." (Takahashi Japan)

 

https://www.cloudyni...40-tak-toa-130/




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