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E621Keith

Lanthanum glass - what's so special about it?

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We all know what ED glass is and what it does. It's a glass with abnormal dispersion property that can reduce CA in refractors objectives. Many of us would even recognise them by their manufacturer name and have some idea of how they compare with each other. FPL53. FPL51, OK4, FCD1, FK1 ....

By contrast, we rarely hear about Lanthanum glass. Vixen says it out loud with their NLV and LVW, but if you read the small prints you'll find just about every top eyepiece manufacturer uses it, including Televue, Pentax and Zeiss.

This lead to the questions, what does it do and why only use them in eyepieces?

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I can recall reading the blurb when the original Vixen Lanthanum eyepieces came out (the older 45 degree FoV ones) and it referred to the glass containing "rare earths". I've no idea what that means to be honest :undecided:

As the Cloudynights thread says, it's a glass type that seems to be used, as appropriate, by a number of the "big names" in eyepieces.

Does anyone know if Lanthanum glass is proprietary to a certain manufacturer (eg: Schott, Ohara etc) or if it is a type / specification of glass that can be made by anyone ?

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From Wiki:

"Lanthanum(III) oxide (La2O3) improves the alkali resistance of glass, and is used in making special optical glasses, such as infrared-absorbing glass, as well as camera and telescope lenses, because of the high refractive index and low dispersion of rare-earth glasses. Lanthanum oxide is also used as a grain growth additive during the liquid phase sintering of silicon nitride and zirconium diboride".

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So it sounds like it just contributes to the whole ED effect, presumably in a similar manner to the fluorite in FPL53 etc?

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My understanding is that lanthanum glass is a tool in the designer's toolkit, and widely used, but there's nothing special per se. But it does sound quite cool for marketing purposes :)

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Marketing reason makes sense. I guess it's the same for ED glass in eyepieces.

However, there must be a reason why only high end manufacturers uses it. I can't imagine the mass market manufacturers (e.g. Synta and Kunming) would keep it quiet if they use Lanthanum elements in their eyepieces.

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However, there must be a reason why only high end manufacturers uses it.

I think it's just that it's more expensive than other glasses, so probably out of budget. At the high end it's one option if you need a high(er) refractive index element; at my rather limited level of understanding this is useful as you can bend light more for a given curvature, or use less curvature (i.e. easier to figure accurately) to achieve a particular design. However, Lanthanum is not the only option for doing this.

I don't think that Lanthanum glass is generally useful for colour correction, although it may be useful with specific other glasses. Roland Christen's comments explain it better than I can: http://www.astromart.com/forums/viewpost.asp?forum_post_id=541076

In order to achieve meaningful correction, one or both glasses need to lie significantly off the Abbe Normal, so that they have essentially the same partial dispersion value (example: OK4, FPL53 and Fluorite have nearly the same partial dispersion as BK7, so these are a good match for close to zero color error). You can see on the chart that FK5 and lanthanum glasses really do not produce any meaningful advantage in color correction because they all have very different partial dispersions, therefore the different colors will not line up at the same focus.

541076.jpg

If nothing else, that graph shows how 'different' FPL5x, OK4, and Fluorite glasses are

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Rare earth elements are those elements in the periodic table table that "sit below" the rest. Lanthanum and Cerium are the first two. I think the key to lens design, especially in terms of controlling chromatic aberrations is to have lenses of different dispersion/refractive index ratios. This gives more degrees of freedom in the design, and this can allow better correction. If the key properties of glasses plotted in the graph above are all along a straight line, this means you cannot choose dispersion and Abbe(Vd) independently of eachother. glasses with properties away from that line are needed to obtain that independence.

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For some reason, I always thought the ED element replaces the soft flint element in a doublet, but as the graph shows, EDs are crown glass as well.

Looking at that graph, OK4 and FPL53 look more crown like than BK7.

Does anyone know if Lanthanum glass is proprietary to a certain manufacturer (eg: Schott, Ohara etc) or if it is a type / specification of glass that can be made by anyone ?

They seems to be proprietary to certain manufacturers, just like EDs. Just a quick glance through RefractiveIndex.info shows Schott has 11 types of Lanthanum crown, 8 types of lanthanum flints and 11 types of Lanthanum dense flint. Hoya has 8 types of lanthanum crown and 5 types of Lanthanum flint etc...

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this is way over my head, if the views are good buy it

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Something in the depths of my brain recalls Lanthanum glass being mildly radiactive & long term stabilitly of the glass questionable.

If so, not sure i'd want prolonged close proximity to my eyes.

Can anyone confirm/ refute this?

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If thats the case then a lot of astronomers are in trouble as many premium brands use Lanthanum glass elements in their designs !

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I have read something about radioactivity in Lanthanum lenses, but they were from thorium oxide additive in older lens. Most of them were made before 1970.

Apparently thorium oxide have similar low dispersion property as fluorite. Adding them to glass will help to reduce chromatic aberration, just like adding fluoite to make FPL53. (... apart from the gentle green glow coming from the lens :D)

http://camerapedia.wikia.com/wiki/Radioactive_lenses

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I think that modern lenses no longer contain thorium. To be totally honest everything is radioactive at some level (including every living organism (which is why carbon dating works)). Alpha and beta rays are not that dangerous externally, as the dead skin cells of the outer layer of the skin are sufficient to block them. The cornea is much more sensitive (as noted in the link above). In my case any charged particles emanating from the EP will be blocked by glasses, so the cornea is safe (as long as the glasses themselves are not radioactive above background).

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If the radio-active part is true I will have to look out. Having said that cellphones are meant to be bad for you and that don't seem to have stopped anyone.

Alan

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If the radio-active part is true I will have to look out. Having said that cellphones are meant to be bad for you and that don't seem to have stopped anyone.

Alan

Weeeeellllll, if you hear what some very frequent cell-phone users natter on about (in trains, buses and other places where there is no escape), you have to worry that their brains might be going soft. On the other hand, their brains might have been in a sorry state beforehand :D

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I think that modern lenses no longer contain thorium. To be totally honest everything is radioactive at some level (including every living organism (which is why carbon dating works)). ...

My brother, who is a medic, told me the about the banana equivalent dose. Apparently the radiation dose from a chest x-ray is roughly equivalent to eating 200 bananas.

Edited by E621Keith

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Lanthanum tends to increase the dispersion properties of glass - the opposite to ED.

It means that a lens surface need to have less curvature to achieve the same focus, so less machining and a little better accuracy. This is useful on eyepieces where the focal lengths are short, not much advantage on a telescope objective lens.

The advantage will I guess be minimal as then you need to correct for the additional dispersion of the colours that it will bring to the system. The advantage in the use is I suspect a reduction in spherical aberation and better edge definition therefore, and it is what could have enabled these 82 and 100 degree eyepieces to have appeared.

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I have read something about radioactivity in Lanthanum lenses, but they were from thorium oxide additive in older lens. Most of them were made before 1970.

Apparently thorium oxide have similar low dispersion property as fluorite. Adding them to glass will help to reduce chromatic aberration, just like adding fluoite to make FPL53. (... apart from the gentle green glow coming from the lens :D)

http://camerapedia.w...ioactive_lenses

Cheers for the link, sounds like modern glass is ok then.

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