Kutno
scholastic sledgehammer
Reged: 08/17/09
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Re: Fluorite eyepieces
[Re: Scott in NC]
#5329158 - 07/22/12 12:52 AM
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But you certainly are correct that at least two EP lines that use lanthanum elements also happen to have long ER, the Vixen LV and Vixen LVW. Are there others?
Radians
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Scott in NC
Mad Hatter
Reged: 03/05/05
Loc: NC
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Re: Fluorite eyepieces
[Re: Jon Isaacs]
#5329164 - 07/22/12 01:00 AM
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Lanthanum seems to have the highest index of refraction of any of the optical glasses. The high index allows for shallower curves and I suspect that is an advantage in the design of long eye relief eyepiece... Just a guess.
Jon Isaacs
Very good point, Jon.
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brianb11213
Postmaster
Reged: 02/25/09
Loc: 55.215N 6.554W
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Re: Fluorite eyepieces
[Re: Jon Isaacs]
#5329277 - 07/22/12 04:05 AM
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Lanthanum seems to have the highest index of refraction of any of the optical glasses. The high index allows for shallower curves and I suspect that is an advantage in the design of long eye relief eyepiece... Just a guess.
I think that's hit the nail pretty squarely on the head. The "right" choice of glass means that you can make the design simpler and/or cheaper to produce for any given set of design parameters. The choice will only change if there is a big change in the availability of a particular type of glass that you've chosen.
The restricted supply & high price of lanthanum glass is having an effect on the market at the moment. Also supply of some other types of premium optical glasses made in Japan was badly affected by last year's earthquake/tsunami disaster.
Fashions also affect the market. Around the 1970s there was a trend to using optical glasses containing uranium & thorium salts; these had properties which were very useful but fell out of favour when the elfin safety "experts" criticised them for being mildly radioactive; the fact that the radioactivity was low energy alpha particles which don't travel even 1cm through air being deemed irrelevant, together with the fact that household smoke detectors contain much larger amounts of a much more potent alpha emitter.
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Astrojensen
Post Laureate
Reged: 10/05/08
Loc: Bornholm, Denmark
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Re: Fluorite eyepieces
[Re: SeattleScott]
#5329294 - 07/22/12 04:35 AM
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ES 100* eyepieces also use lanthanum, and I think Ethos and Delos as well. ES100 and Ethos are not long ER eyepieces.
The Radians do use Lanthanum glass and that is exactly the reason they're going the way of the Dinosaur, as the cost of Lanthanum is skyrocketing. The Delos use NO Lanthanum glass, just like the Ethos, in order to make them immune to the high prices of Lanthanum. This was a very specific design criteria in the design of the Delos eyepieces. I have this from Al Nagler personally.
Clear skies!
Thomas, Denmark
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dcoyle
professor emeritus
Reged: 10/11/05
Loc: Turbulent but dark skies, N.M.
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Re: Fluorite eyepieces
[Re: Astrojensen]
#5329504 - 07/22/12 09:45 AM
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I think the link between high index of refraction glasses, like Lanthanum, and long eye relief is a little odd. High index glasses bend light more, as we saw in the ball eyepieces, where materials with higher indexes than fused silica had shorter eye relief.
As pointed out, if the glass bends the light more, one doesn't need as much curve on the lens, which leads to less problems for off axis rays.
Dan
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Paul G
Post Laureate
Reged: 05/08/03
Loc: Freedonia
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Re: Fluorite eyepieces
[Re: Scott in NC]
#5329568 - 07/22/12 10:29 AM
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But you certainly are correct that at least two EP lines that use lanthanum elements also happen to have long ER, the Vixen LV and Vixen LVW. Are there others?
Zeiss Abbe Orthos Company 7 ZAO
Radians
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Sarkikos
Postmaster
Reged: 12/18/07
Loc: Suburban Maryland, USA
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Re: Fluorite eyepieces
[Re: Scott in NC]
#5329863 - 07/22/12 02:19 PM
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Scott,
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But you certainly are correct that at least two EP lines that use lanthanum elements also happen to have long ER, the Vixen LV and Vixen LVW. Are there others?
Here is a list of eyepieces that I own which my notes say have "ED" glass (I assume this means Lanthanum), along with their eye relief:
Pentax XO 2.5, 3.9mm er
Pentax XW 3.5, 20mm er
Pentax XO 5.1, 3.6mm er
AT Paradigm Dual ED 8, 13mm er
Celetron X-Cel ED 8, 20mm er
Vixen LVW 8, 20mm er
ES 100deg 9, 14.5mm er
AT Paradigm Dual ED 12, 13mm er
ES 82deg 14, 15mm er
ES 100deg 14, 14.5mm er
Orion Epic ED-2 22, 20mm er
The eye relief is all over the place.
Mike
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Scott in NC
Mad Hatter
Reged: 03/05/05
Loc: NC
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Re: Fluorite eyepieces
[Re: Sarkikos]
#5329879 - 07/22/12 02:30 PM
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Interesting info, Mike--thanks for looking all that up and sharing it. I have no idea whether "ED" in this case really is synonymous with "lanthanum" or not, but you may very well be right. 
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Sarkikos
Postmaster
Reged: 12/18/07
Loc: Suburban Maryland, USA
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Re: Fluorite eyepieces
[Re: Scott in NC]
#5329908 - 07/22/12 02:53 PM
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The specs and descriptions I've seen for some of these eyepieces specifically say "Lanthanum" (e.g., Vixen LVW), while others only say something like "special ED glass" (e.g., Orion ED-2).
Mike
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slack
sage
Reged: 05/02/12
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Re: Fluorite eyepieces
[Re: Sarkikos]
#5330019 - 07/22/12 04:06 PM
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Here is a list of eyepieces that I own which my notes say have "ED" glass (I assume this means Lanthanum)...
I wouldn't make that assumption. I do not believe that the marketing term "ED" has a material correlation with Lanthanum glass.
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BillP
Postmaster
Reged: 11/26/06
Loc: Vienna, VA
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Re: Fluorite eyepieces
[Re: Astrojensen]
#5330373 - 07/22/12 08:29 PM
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The Radians do use Lanthanum glass and that is exactly the reason they're going the way of the Dinosaur, as the cost of Lanthanum is skyrocketing. The Delos use NO Lanthanum glass, just like the Ethos, in order to make them immune to the high prices of Lanthanum. This was a very specific design criteria in the design of the Delos eyepieces. I have this from Al Nagler personally.
FWIW, Pentax XWs use Lanthanum and their prices are quite stable and their performance remains top tier.
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Sarkikos
Postmaster
Reged: 12/18/07
Loc: Suburban Maryland, USA
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Re: Fluorite eyepieces
[Re: slack]
#5330529 - 07/22/12 10:27 PM
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Slack,
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I wouldn't make that assumption. I do not believe that the marketing term "ED" has a material correlation with Lanthanum glass.
Neither should I make that assumption, I suppose. Yes, I agree, it is not safe to assume that "ED" necessarily means Lanthanum glass. There are probably other types of material utilized as ED glass in eyepieces. Apparently - according to R. C.  - virtually all modern eyepieces incorporate what could be termed "ED glass." Perhaps just some companies choose to mention it in their ad copy?
But IME, it's difficult or impossible to find out exactly what material the ED glass is in many of these eyepieces. When the material's name is given, it is Lanthanum. Otherwise ... ED!
Mike
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Sarkikos
Postmaster
Reged: 12/18/07
Loc: Suburban Maryland, USA
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Re: Fluorite eyepieces
[Re: Sarkikos]
#5330542 - 07/22/12 10:36 PM
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FWIW, all of the eyepieces I own which incorporate Lanthanum or are advertised as having "ED" glass perform very well ... except for the Celestron X-Cel 8mm. These are the original version of the X-Cel. I do not think I need to elaborate. Others have collaborated my experience.
Well, the ES 82deg 14mm do show obvious field curvature, but I've mentioned that once or twice before.
Mike
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gmazza
sage
Reged: 03/10/09
Loc: RS, Brazil, 29S 51W
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Re: Fluorite eyepieces
[Re: Sarkikos]
#5330580 - 07/22/12 10:53 PM
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It shouldn't be a size problem, as there are microscope objectives with fluorite elements.
Is possible most eyepieces (even the cheaper ones) aren't the piece in the optical path that introduce most of the CA.
Color correction is good, in all aspects of a optical assembly, but more critical at objective side.
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EdZ
Professor EdZ
Reged: 02/15/02
Loc: Cumberland, R I , USA42N71.4W
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Re: Fluorite eyepieces
[Re: gmazza]
#5331609 - 07/23/12 04:07 PM
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Are there any fluorite eyepieces?
And if not, why not?
...........................
If fluorite is the highest index lens element (it is crystal not glass) and so important in objectives, why are we not seeing fluorite eyepieces?
johnnyha
Well I won't speculate as to which ones are ED, or if it provides er or anythig else like that. Really, that dances all around the issue without addressing the WHY.
The amount of false color in a system is contributed in the ratio of the focal lengths of the objective and the eyepiece. So for instance, a 1000mm telescope with a 25mm eyepiece has 1000/25 = 40:1 contributed by the objective. The ratio of false color contributed by the Obj and the ep in this system is 40x more due to the obj. lens. That's less than 2.5% of the false color is due to the eyepiece.
When the 1000m objective has a 5mm eyepiece for 200x, the ratio contributed by the objective lens is 1000/5 = 200:1, or less than 1/2 of 1% due to the eyepiece.
As you can see from these examples, the ratio of false color in the system contributed by the objective vs the eyepiece is equal to the magnification. In a low power system at 25x, the eyepiece contributes only about 4% to the CA error, but false color is less noticed at low powers, so this becomes insignificant. In a high power system at 100x, the eyepiece is responsible for only 1% of the false color error.
Frankly, it would be a waste of money to design eyepeces with fluorite lenses.
edz
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saemark30
scholastic sledgehammer
Reged: 02/21/12
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Re: Fluorite eyepieces
[Re: EdZ]
#5333065 - 07/24/12 12:56 PM
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If this math is correct EdZ, we should be able to use Huygens and Ramsden eyepieces at high power.
Having looked through such eyepieces, this would not be a good idea.
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csrlice12
Post Laureate
Reged: 05/22/12
Loc: Denver, CO
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Re: Fluorite eyepieces
[Re: saemark30]
#5333145 - 07/24/12 01:47 PM
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Hmmm, makes me wonder how an eypiece made with diamond lenses would peform (not to mention cost).
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azure1961p
Postmaster
Reged: 01/17/09
Loc: USA
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Re: Fluorite eyepieces
[Re: Jon Isaacs]
#5333271 - 07/24/12 03:16 PM
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I dont think eyepieces typically refract light to the degree an objective does to warrant a flourite element. Ive a feeling the angles are too small and deviations to fine to begin spreading the light apart like a large refractor objectove 60mm and up. Probably what refraction is occuring with so little color deviation, the costs cant justify the reasons.
Pete
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Jon Isaacs
Postmaster
Reged: 06/16/04
Loc: San Diego and Boulevard, CA
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Re: Fluorite eyepieces
[Re: csrlice12]
#5333400 - 07/24/12 04:25 PM
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Hmmm, makes me wonder how an eypiece made with diamond lenses would peform (not to mention cost).
The index of refraction of diamond is about 2.41 so it would probably be a very good choice for eyepiece glass.
Jon
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csrlice12
Post Laureate
Reged: 05/22/12
Loc: Denver, CO
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Re: Fluorite eyepieces
[Re: Jon Isaacs]
#5333417 - 07/24/12 04:34 PM
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Hmmm, makes me wonder how an eypiece made with diamond lenses would peform (not to mention cost).
The index of refraction of diamond is about 2.41 so it would probably be a very good choice for eyepiece glass.
Jon
*$&^()#)* Jon, You think I'm made of money? No doubt the expense is the reason why there aren't any. I guess one could use industrial diamonds, or maybe they're not pure enough. Is an interesting concept though. Wonder how it would perform against a Nagler all other things being equal.
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