Fluorite EP's?

[AG Hybrid (Adrian)]

Originally Posted by Hans Tucker

I have used Takahashi LE, Televue Radians, Ethos, University Optics and Pentax XW eyepieces and I prefer the Takahashi LE's which have ED lens elements but then I am biased towards Takahashi.

I put an order in for the HI-LE 2.8mm & LE 10mm to complete my LE collection and have been waiting over a month for delivery

On a side subject - A fellow IISer brought around a Nikon NAV 17.5 and we tried to use it on my Takahashi FSQ-106ED without any success, it just wouldn't come into focus. I tried the LE-18 which did come into focus and thinking it could be a 2" eyepiece problem I threw in the Ethos 13mm which also came into focus. It was a real shame about the NAV 17.5 as I have been fascinated about these eyepieces for a while.

Brian used a key word in regards to eyepieces...subjective. Eyepieces are a personal choice and I am reluctant to suggest one over the other. The issue over what material they are made from is kind of ridiculous (as is saying Fluorite is better than ED glass) because there is more that goes into an eyepiece. There is design and figuring of the elements that is more critical than what lens material was used.

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 (As avatar)]

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.

Originally Posted by 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.

[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]

Originally Posted by 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)

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.

Conrady 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.

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".

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

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]

Originally Posted by Wavytone

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.

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.