Fluorite EP's?

[LewisM]

Did/does any manufacturer make fluorite EP's? I know Baader makes SOME fluorite accessories, but any EP's at all?

And who is making FPL53 EP's? I guess a second best option :)

[AG Hybrid]

Your obsessed with this fluorite stuff aren't you? You just can't get enough of it, can you? I bet you know which brand of tooth paste and which flavor variants have the highest content of fluorite in parts per million off by heart.

Traditionally, eyepiece manufacturers are "hush-hush" about what the content and makeup of the glass they use in their eyepieces. We know Pentax and Vixen market Lanthanum as a major component of their eyepieces. But, manufacturers like Televue as far as I know have never published that information.

I'm surprised you don't know the answer to your own question. If I was going to ask this question you would have been the first person I would have asked. :P

[Larryp]

No fluorite in toothpaste, I'm afraid! Either sodium or stannous fluoride
Fluorite is calcium fluoride.

[AG Hybrid]

We'll. I assume he knew that was the case too.

[Hans Tucker]

The only eyepiece manufacturer I have ever come across that gave a name to the glass they used in their eyepieces was Vixen with their LV series which uses Lanthanum glass which itself is a low-dispersion. But being a Vixen fan Lewis you would have already known this bit.

Takahashi uses ED Extra-low-dispersion glass in their LE & UW series which is also used in the TMB, Televue and Saxon brands. Now FLP-53 is considered an Extra-low-dispersion glass so by association FLP-53 could be used to manufacture one or more elements in these multi element designed eyepieces. Cheaper eyepieces just use flint and crown glass.

I have not come across any manufacturer that manufactures fluorite eyepieces possible due to cost and difficulty working with the material particularly since is is a fragile material.

I do have a Baader Flat Field Converter (FFC) that has elements manufactured from fluorite and that was a very expensive item to buy.

[alocky]

You could always brush with dioxygen diflouride for that extra clean feeling...

[Allan]

TeleVue use Lanthanum, they even publish it on their website somewhere. Fluorite would be a problematic material to construct eyepieces from. Just stick to the drug in your objectives. :)

[Regulus]

I hesitate to suggest Lewis this given the things I have read when his name is mentioned, but Cris (Astro_Melb) seems to have a deep knowledge of manufactures and decades of experience in trading in quality optics.
Perhaps a message to him might help.
Just a thought.

Trev

[Profiler]

I would love to know where this appears in print

[RB]

Careful you don't get Fluorite in your eye from the eyepiece, it'll make you stupid just like town water does.

:P :lol:

[Hans Tucker]

I had to go looking and found a suggestion of Televues uses of Lanthanum here:
http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=78#.VJqRRA8Y

And I found other websites that state that Lanthanum was used in the Panoptics and Radian eyepieces.

[ariefm71]

The link also mentioned TV uses "fluorite type" glasses.

[AG Hybrid]

Well how 'bout that? "The finest performance requires the finest materials such as high index lanthanum and fluorite type glasses, in some cases costing up to 15x more than ordinary glass types."
I suppose FPL designated glass would be considered Fluorite type glass.

[Hans Tucker]

Not quite all FPL, only FPL-53 (artifical fluorite) has been regarded as being close to calcium fluorite type glass because its abbe value (94.93) is close to that of genuine fluorite (94.99). FPL-51 has an abbe number of 81.54 and FPL-52 has an abbe number of 90.29.

Apparently FPL stands for:

F - Fluorite
P - Lead
L - Low refractive index
51, 52 or 53 - The 51st, 52nd or 53rd glass in its family

[Allan]

There you have it

[AG Hybrid]

Those numbers would suggest a major difference between the common glasses used in telescopes, FPL-51 and FPL-53. But, how does that translate at the eyepiece or at the CCD chip? IS the color correction 15-20% better or does it scale differently?

[Hans Tucker]

I would think the use of abbe numbers would apply to any lens elements used in optical design be it telescope, camera lens or an eyepiece. It is all about matching lens elements to achieve the focus of the incoming light to a single point. With eyepieces you want to achieve magnification whilst maintaining control over contrast, sharpness and color definition.

When designing eyepieces you still have issues like chromatic aberration and spherical aberration to correct for hence the need to match as close as possible the lens elements whilst trying to get and maintain the maximum light transmission.

[LewisM]

Thanks for the serious and sarcastic answers - all much appreciated.

Yes, fluorite is the wonderful elixir of life. ED is just a cheap(er) substitute :) :P

[Camelopardalis]

I'm seeing a difference in the cheaper glass with my new camera I tried out last weekend, the blue halos are more prevalent than with my other cam, although it's probably mostly because my little scope is only a doublet ;)

[brian nordstrom]

:) Great discussion going on here , thanks Lewis .

Hans , a few years ago I had a full set of the beautiful Vixwn LV's , beautiful eyepieces in every way , but I grabbed an 18mm Radian from the classifieds here and now have a full set of the TV's , they are a better eyepiece IMOO , so sold the LVs to a lucky IIS'r .

The Radians do impart a more natural tinge to the Moon and Planets in my eyes and these are my favourite night sky objects , in my refractors more natural hues , but especially in my 9 1/4 inch Cat the Radians really shine in this scope .
They are , the ( LV's) and Radians special eyepieces for luna/planetary viewing but I do find my Panoptic's better for deep sky, cleaner ? perhaps :shrug: , very subjective subject .

Great discussion this one and a merry Christmas all .

Brian.

[AG Hybrid]

I was under the impression that the FSQ line were astrographs and not optimized for eyepiece use. I wouldn't be surprised if it lacked the necessary in-focus for some eyepieces.

[Stefan Buda]

To go back to the original question:
I have designed a few eyepieces over the years and found that the most difficult aberration to correct was field curvature - where ED glass would not have helped.
Eyepieces contain strong lens elements that call for high refractive index glasses, to minimize surface curvatures. Lanthanum glasses are high index and ED glasses are low index.
My guess is that it's not worth the trouble and expense including Fluorite or ED glass into an eyepiece design.

[brian nordstrom]

;) field curvature ? Panoptics in some scopes , Pin cushion ? its not a problem for most but you are right on this Stefan .
Good design and quality of manufacture like the TV's allow the best to be made at a reasonable price for us average 'Joe Blog;s; ,and thanks for your imput .
Brian.

[ralph1]

It may be a somewhat old thread but I was under the impression that the maximum effective aperture of an eyepiece was 7mm(the largest exit pupil) and the effective F/ratio is the F/ratio of the telescope. The Conrody standard says that the F/ratio should be five times the aperture in inches using standard glasses to produce acceptably low amounts of chromatic aberration. For an aperture of 7/25 inch, the F/ratio should be at most F/1.4. How many people observe visually using F/1.4 telescopes? The fastest visual telescope I've heard of is F/2.5. That is well more than the minimum and is an extreme.
Ralph

[Wavytone]

This is governed by your eye's pupil diameter. For youngsters perhaps 7mm, for those older 5 or 6mm. Secondly in light-polluted skies 5mm may be optimistic.



As one who studied optical design and has actually applied Conrady's methods to design lenses analytically, Conrady's rule was only an approximation and it was intended to be used for designing achromatic doublets for refractors in the common sizes of his day - 2" to 5" aperture. It was never intended to be applied to an aperture so small as 7/25".




Simple: you can't - and you would be wasting your time trying - for a very simple reason - your eye focal length internally is about 17mm so its focal ratio is about f/3.5 at a maximum iris aperture of 5mm. If the telescope you are using is faster (say f/2.5 or f/1.4) all that means is there's a lot of light blocked by your iris, not entering the eye - which means the iris is the aperture stop, limiting the complete optical train to f/3.5.

Anyone bragging about fast optics also is unaware of another inherent problem: the lens in nature's Mark I eyeball is far from diffraction-limited. Most people's eyes have errors amounting to several wavelengths once the pupil exceeds 3-4mm and it gets much worse if attempting make full use of your fully dark-adapted aperture of 5-6mm.

The result is that if you are using a very fast newtonian (I had an f/3.7 many years ago) do not expect to see very sharp images - even if the telescope has exquisite optics (which is unlikely at f/3.7). You're much better off using a scope at f/5 or f/7 if you want sharp images.

Nature has another clue: Your eyeballs lens isn't particularly well corrected for monochromatic aberrations, nor chromatically. But in daylight your eye stops down to 1-2mm aperture - an f/ratio around f/10 to f/15 and is using the best (central) part of the lens. Hence things are pretty sharp in daylight. It's just as well we don't see colour at night because if we did most of the scene would be a bit of a blur.

[ralph1]

Or maybe it's the other way around; We don't see colour at night because if we did it would be too blurry.

[Don Pensack]

TeleVue is moving away from Lanthanum glass due to the reduced availability of Lanthanum for glass made in Japan and Taiwan. The PRC controls the world-wide distribution of many rare-earth elements, though this may change. Currently, lanthanum for glass from Japan comes from the PRC to the US and then gets sold to Japan, since the PRC refuses to sell it directly to Japan due to some trade disputes. That has resulted in many companies finding other ways to produce glass with low dispersion and high refractive index (or simply having the products made in the PRC). It's possible to achieve the same thing with more elements or sharper curves, or both. The current Ethos and Delos eyepieces are examples.
Ideally, glass with high refractive index and low dispersion is desirable--the first because it reduces the extremity of needed curves on the surfaces, and the second because chromaticism is desirable to control.
Note that lanthanum in glass is not the only way to achieve that.

There are many kinds of ED (extra-low dispersion) glass in the market, with a wide variety of refractive indices. Which is most appropriate is a function of the glass used in other elements.
I also note that some optical gurus say that ONLY FPL and OK glass is truly ED, while all the others are not, but the market seems to call anything with low dispersion ED, which would include the lanthanum-containing glasses.
For more info, see:
http://en.wikipedia.org/wiki/Low_dispersion_glass and the links therein.