52 replies to this topic

[Darren Drake]

I've read a number of reports regarding lanthanum elements in eyepieces resulting in better performance. Just what type of benefits does this glass type offer over more traditional glass types and is it a real benefit that is noticeable in real world conditions? Thanks.

[rmollise]

I've read a number of reports regarding lanthanum elements in eyepieces resulting in better performance. Just what type of benefits does this glass type offer over more traditional glass types and is it a real benefit that is noticeable in real world conditions? Thanks.

Quite a few manufacturers--Pentax, for example--use Lanthanum glass elements. The reason? To minimize chromatic abberation in eyepieces. Does it work? Yep.

[Darren Drake]

Thanks Rod. Is it more for off axis color correction or on axis also? Are non-lanthanum eps necessarily more prone to ca than any lanthanum based ep?

[BillP]

I understand that both the Panoptics and Radians use Lanthanum elements as well.

[Lawrence Sayre]

The generally higher level of index of refraction for glass that contains lanthanum oxide (whereby the index of refraction can range from about 1.8 on into the 1.9's) vs. glass that does not would mean that less surface curvature as well as less lens thickness would be required to obtain a given 'lens power' for lanthanum oxide containing lens components.

And simply because it is easier to precision figure and polish a lens with less surface curvature, figure and polish related aberrations "MAY" be less likely to occur. For the same reason a given lenses "POTENTIAL" of achieving a superior scratch/dig surface rating may also be improved by lowering its surface curvature.

So while the chromatic aberration benefit is real (primarily for instances where the flint component of a doublet grouping contains lanthanum oxide), it might also be possible that there are other "POTENTIAL" benefits such as those I have listed above for lanthanum oxide containing lens elements.

Also, since a lens with a high RI bends light to a higher degree with less surface curvature (by definition), this should directly (not just potentially) reduce spherical aberration.

The "power" of one surface of a lens is given by the general formula:

P = 1/FL = C * (N - 1)

Where:
FL = focal length
C = curvature
N = refractive index

From this it can easily be seen that in order to achieve the same power 'P' (and thereby perform the same function) as a lanthanum oxide containing lens element of RI = 1.8 (while maintaining the same focal length), a less expensive lens element of perhaps RI = 1.6 or so would be required to have a greater 'C' (or surface curvature).

[Doug76]

Quite a few manufacturers--Pentax, for example--use Lanthanum glass elements. The reason? To minimize chromatic abberation in eyepieces. Does it work? Yep.

Kinda makes you wonder why it ain't used in objective lenses.

[Lawrence Sayre]

I understand that both the Panoptics and Radians use Lanthanum elements as well.

I believe that it is highly likely that the T5 and T6 Nagler's do likewise, whereas the first generation Nagler's (at least) most probably did not. Lanthanum may have played a large part in TeleVue's being able to downsize the Nagler's, and/or permit FL's like 26 mm and 31 mm which were impossible to earlier generations.

[Lawrence Sayre]

Kinda makes you wonder why it ain't used in objective lenses.

Perhaps where the term 'ED' is used in regard to objective lenses, lanthanum is one of the several potential 'ED's' that are available to be chosen as a representative of the 'crown' component of a doublet?

But since ED means 'extra low dispersion', and 'flint' glass is where the high refractive index of lanthanum is most likely to be found of benefit, and on top of this flint glass has the general characteristic of high dispersion, my thinking is quite likely all wet here.

There do exist however both LaK (lanthanum Crown) and LaF (lanthanum flint) types of glasses, but I can't imagine where the benefit of lanthanum would be of much use in a low dispersion application where so many lesser cost glasses would seem to suffice.

[Teal'c]

Quite a few manufacturers--Pentax, for example--use Lanthanum glass elements. The reason? To minimize chromatic abberation in eyepieces. Does it work? Yep.

Kinda makes you wonder why it ain't used in objective lenses.

It was used in the Stellarvue 102ABV doublet...and the result, was quite nice. the problem was manufacturing cost. It cost as much as a triplet to produce.

[Mike Hosea]

There's a very good reason why Pentax and Tele Vue don't talk about lanthanum glasses and just use them whenever they want. It's because what the designer wants, ideally, is for each element the (probably different) glass that has the properties needed to deliver optimal results with respect to the particular design being implemented. It's this relation of "best" and "design" that is always lacking when lanthanum comes up. What the designer would really like is the ability to dial in dispersion and index of refraction without offsetting drawbacks such as compromising transmission in any relevant wavelengths, having too brittle or too soft a glass, an inhomogeneous glass, prohibitive cost, etc. Lanthanum glasses provide more good choices, and that's a good thing for designers. I don't see how it's even relevant to consumers. It's easy to produce a bad design with expensive glasses. What consumers should be concerned with is how well a design performs in the field. That's where you find out whether the designer has taken advantage of what the glass can do or whether he has squandered its potential. For my part, lateral color in Vixen "lanthanum" eyepiece completely cured me of interest in glass types in eyepieces as a predictor of performance.

[Lawrence Sayre]

It [lanthanum] was used in the Stellarvue 102ABV doublet...and the result, was quite nice. the problem was manufacturing cost. It cost as much as a triplet to produce.

I'm assuming it was used in the flint (or high dispersion) component of the doublet, and not the Crown (low dispersion, possibly even 'ED') component. Do you know if this was the case?

[Scott in NC]

I'm assuming it was used in the flint component of the doublet. Do you know if this was the case?

Yes, Larry, you are correct. The SV102ABV is a 102mm f/7.75 doublet apo with OK4 glass for the low-dispersion element, and lanthanum glass for the mating element.

[BillP]

... I don't see how it's even relevant to consumers ...

It's relevant, IMO, because there is a deep desire in human nature for there to be an element of mystery or magic in everything. It keeps life interesting and imaginative. So whether it be "FPL-53" or "Lanthanum" or "TV" or "AP" or "Spacewalk" or "Majesty Factor" or even the somewhat clinical "Well executed design", each of these qualities is really nothing more than the "magic element" for the consumer to latch onto. Different folks grab onto different elements as containing the "magic" for them, just depends what sparks one's imagination.

[Lawrence Sayre]

There is nothing magical about lanthanum. It is in fact only a substitute for either lead or thorium in achieving a high index of refraction flint type glass.

Lead is considered highly environmentally unfriendly, and thorium (actually used from roughly 1932 through at least WWII in various military optics) has a radioactive component of decay which makes it undesirable, as well as yellowing the lens with time. Some day lanthanum will probably be found to either be environmentally a disaster to dispose of, or to have some unsafe level of radioactive decay (there are naturally present radioactive isotopes of lanthanum), and another substitute will have to be found which duplicates the same optical properties.

[Paul G]

I understand that both the Panoptics and Radians use Lanthanum elements as well.

The Zeiss Abbe orthos use it as well.

[Edwin Quiroga]

... I don't see how it's even relevant to consumers ...

It's relevant, IMO, because there is a deep desire in human nature for there to be an element of mystery or magic in everything. It keeps life interesting and imaginative. So whether it be "FPL-53" or "Lanthanum" or "TV" or "AP" or "Spacewalk" or "Majesty Factor" or even the somewhat clinical "Well executed design", each of these qualities is really nothing more than the "magic element" for the consumer to latch onto. Different folks grab onto different elements as containing the "magic" for them, just depends what sparks one's imagination.

Well stated, sir. For that reason people trends to think that the best eyepieces or scope are the hyper-expensives or the discontinued ones.

[Edwin Quiroga]

I understand that both the Panoptics and Radians use Lanthanum elements as well.

The Zeiss Abbe orthos use it as well.

...and Pentax XW...

[Darren Drake]

So what DOESN'T use lanthunum? How about the Stratus line?

[Sgt]

ethos? doesn't that use radioactive plastic or some such?

[geoffcin]

So what DOESN'T use lanthunum? How about the Stratus line?

I think that the Orion Stratus DOES use Lanthanum glass.

[Mike Hosea]

It's relevant, IMO, because there is a deep desire in human nature for there to be an element of mystery or magic in everything.

I never thought of it that way. I thought it was just an attempt to overlay some objective data on the subjective matter of eyepiece performance. As I said, I don't think this particular datum (does it use lanthanum glass types or not) is helpful for that. Generically, high index glasses are of some interest, however, for the reason I think it was Lawrence who pointed out. However, lanthanum glass is of particular interest in this regard why? Only reason I've ever heard or know about is that the marketing geniuses at Vixen managed to make this rather obscure thing a household name.

[BillP]

I kind of think of it as a level of "distinctiveness" and one can say with some pride when they hand their eyepiece to show another that there's Lanthanum in there! They also use the word "exotic" with it usually. So those terms make it more special. Same thing with all the multicoating hoo-haa when it went from fully to words like broadband and phantom. Like everything, sometimes these special items add to performance, sometimes not. I know many people scoff at marketing and when people latch on to some unique named element of an eyepiece or objective, but it's an important motivator and important psychologically to an experience. There's plenty of room in the universe for both the realm of hard science and the realm of mystique/imagination to coexist very happily

[Lawrence Sayre]

Speaking of buzz words in optics, I wonder who will be the first telescope eyepiece manufacturer to announce that their eyepieces crown elements are made with Fluorite (which isn't technically glass at all, being a ceramic material). Fluorite Crown (which would most assuredly qualify as 'Super ED') combined with Lanthanum Flint would make for a powerful doublet combo with the potential for apochromatic performance. Lots of buzz and marketing words for the eyepiece marketers and advertisers to consider here!

Canon was the first (to my knowledge) to introduce fluorite into their photographic lenses. Who will be the first to do so for telescope eyepieces (and at what 'astronomical' price level)? Will Fluorite eyepieces make the Ethos line appear to be in the 'economy' class?

[Tamiji Homma]

Canon was the first (to my knowledge) to introduce fluorite into their photographic lenses. Who will be the first to do so for telescope eyepieces (and at what 'astronomical' price level)? Will Fluorite eyepieces make the Ethos line appear to be in the 'economy' class?

It isn't eyepiece but Baader FFC (FlatField Converter) uses CaF2 element.

It is as good as 4x Powermate if it isn't better. Visually, I can not see difference between FFC and 4x Powermate.
It is excellent barlow lens.

Tammy

[kroum]

This thread has been very educational!

Thank you guys.