The Clave' plossl are considered the only true plossl because the original optical design of a plossl was a couple of different doublet (asymmetric doublet). A more economical design of plossl is the common symmetrical design where the two doublet are the same, but obviously nothing to do with asymmetrical design. The Clave' eyepiece remain the best of best with some other eyepiece like Zeiss Abbe II, without distinction of planetary or deep sky eyepiece: this is a non-sense. If an eyepiece has a superior optical correction with a very small number of optical surfaces like Clave' or Zeiss it means it transmits much more light then a five or eight elements eyepiece despite of "fantasy" of some observer: number of surfaces x % of light absorption, it is very easy to understand. If you have a very large field eyepiece you have to correct the field with a lot of optics: you will lose light! A like this eyepiece is like a big TV but it will eat light and optical correction. Simple design and high quality glasses and coating is the Clave' secret.
I have a complete set of Clave' eyepieces ( 1"1/4: 30mm, 20mm, 16mm, 8mm, 2": 65mm, 55mm, 35mm) one 2" barlow, one 1"1/4 barlow, and many other optics and two Clave' refractors 130mm F1908 and a 80mm F600mm. I have had many other eyepieces from other producers, I consider the Clave' optics unbeated.
If an eyepiece has a superior optical correction with a very small number of optical surfaces like Clave' or Zeiss it means it transmits much more light then a five or eight elements eyepiece
How much more light? Can you give us specific numbers measured in magnitudes... It shouldn't be hard to estimate and/or measure.
As far as optical correction goes... superior to what exactly?
For example, which of the 5 Seidel aberrations does a Clave plossl correct better than an Ethos, Nagler or one of their derivatives?
All the five aberrations are better corrected if you are not using an "extreme" optical design. Extreme wide field means correction with more lenses to correct the field, so this will increase the optical aberration. I want to be optimistic: the light transmission of each optical surface can be 95% so for each single surface it means a light loss of 5%, extreme optimistic: 2%??? it means an eight elements eyepiece=10%-40% of light dispersion, more: each surface gives another problem, the internal reflection=lost of definition on bright images (planets for example), this is why refractors or some reflectors are best for planetary images (not catadipotrics). In terms of magnitude it is easy to say: your main optics-(10%-40% of light grasp)= true main optics aperture. I have some "TV eyepiece" very nice and comfortable, but I belive it is an optical non sense! The maximum Clave' field is 68° on the 2" series, with extreme contrast, definition, optical correction, brightness. Zeiss abbe II has the same optical concept: minimum possible glass. Why is so difficult to find a Clave' or Zeiss abbe II? Why these optics are so expensive? Simply, these are the best!
I want to be optimistic: the light transmission of each optical surface can be 95% so for each single surface it means a light loss of 5%, extreme optimistic: 2%??? it means an eight elements eyepiece=10%-40% of light dispersion, more: each surface gives another problem, the internal reflection=lost of definition on bright images (planets for example), this is why refractors or some reflectors are best for planetary images (not catadipotrics). In terms of magnitude it is easy to say: your main optics-(10%-40% of light grasp)= true main optics aperture.
Modern "multi element" eyepieces with newer glass types and multilayer coatings can do notably better than this. The Pentax XW's, which use multi layer partial phase coatings, have a transmission > 90% across the entire visible spectrum; and in the critical range from 500 nm to 620 nm they achieve almost 95% light transmission (96% @ 550 nm [green light]).
Whilst the Clave Plossls are very good, they are also very warm in their colour tone (coffee colour) and are the complete opposite to the Zeiss orthos and Brandons in this regard. Although the warm tone does help with some planetary observations, notably on Jupiter; it is something I really don't like much, particularly on the moon.
about ausastronomer: "The Pentax XW's, which use multi layer partial phase coatings, have a transmission > 90% across the entire visible spectrum; and in the critical range from 500 nm to 620 nm they achieve almost 95% light transmission (96% @ 550 nm [green light])."
But....this is the transmission for each single optical surface, correct? It means a light loss of 4-5% x each optical surface, I suppose. I cannot belive the light comes out from the eyepiece with a total of 5% light loss. (if you say YES I will buy some XW just tomorrow!!!).
about the warm colors of Clave', I agree,
about mplanet62: very interesting link! I hope this new coating will work fine for astronomical optics as well, because, it is not exactly the same thing of a terrestrial optic. But if it works, I will buy some other "TV eyepiece" with this coating!
Cheers
Fabrizio
PS I love Brandon eyepieces as well as Clave' and Zeiss eyepieces
PS2 can you help me?: what is the light transmission of the best diagonal mirror?
Fabrizio, the 95% figure is for the entire lens.
Coatings are getting so good these days that even complex designs (such as the ethos) have high light throughputs and no noticeable internal reflections.
As for field aberrations, the picture below shows how plossls compare to a variety of other eyepieces:
As you can see, the field aberrations of a plossl are of a similar magnitude to those of an erfle.
This one shows the field correction of an old Nagler design compared to an erfle.
Basically at f/5, the edge correction of an erfle (and implicitly a plossl) is 10x worse than that of the old nagler design.
The modern day ultra wide fields are a step beyond the original naglers in field correction and light throughput.
There is something to be said for extra degrees of optical freedom.
Also, you might find the following article interesting. It gives a fair bit of history on the plossl and includes a comparison of a few commercial offerings (GSO versus Televue versus Brandon).
The results are quite interesting.
fwiw) You can no longer buy them new, but another excellent eyepiece to consider is the old Masuyama. I believe Celestron copied it and called it the 'Ultima'. Whether Celestron bothered to apply the same degree of polish to their glass is something I cannot say. What I can tell you though is that the 42mm Ultima is a noticeably superior eyepiece compared to a Meade 40mm SWA. (albeit with a smaller field of view)
Anyway, if you were thinking of trying a TeleVue, I would suggest that their plossl range isn't the best eyepiece that they produce by a long shot. I'm actually surprised that they still make them. Perhaps it is because they still sell well enough to keep the line going.
~c
Last edited by clive milne; 18-02-2012 at 02:15 PM.
about ausastronomer: "The Pentax XW's, which use multi layer partial phase coatings, have a transmission > 90% across the entire visible spectrum; and in the critical range from 500 nm to 620 nm they achieve almost 95% light transmission (96% @ 550 nm [green light])."
But....this is the transmission for each single optical surface, correct? It means a light loss of 4-5% x each optical surface, I suppose. I cannot belive the light comes out from the eyepiece with a total of 5% light loss. (if you say YES I will buy some XW just tomorrow!!!).
Cheers
Fabrizio
Hi Fabrizio,
The figues I have quoted are the "net" transmission figures for the entire eyepiece. The top quality multi element widefields with modern glass types and coating technology will all achieve 92% to 96% "net" transmission for the entire eyepiece. These include the new Televue ETHOS and DELOS, The 12.5mm Docter (possibly the best eyepiece I have ever used) and the Nikon NAV HW's. Despite what you might read on Cloudy Nights, I don't think the older Televue eyepieces (earlier Naglers and Panoptics) are quite up to the newer offferings in terms of ligth transmission and contrast.
about Clive Milne post: I have seen the aberration magnitude of the book shown, it is for symmetric plossl not for asymmetric plossl (Clave') that was infact much more expensive to produce, it is a completly different thing!
about ausastronomer post: I am sure technology has improved light transmission, but can I ask you where have you found the graph attached to your post about Pentax, is it an official Pentax graph? Have you a link? Because if you take a look of the post of mplanet62 he has added this link:
It would seem to bear out the prior assertion that not even an asymmetric lens arrangement saves the plossl from strong field astigmatism when they are used with fast focal ratio telescopes.
I'd be really interested to see a zemax plot for symmetric versus asymmetric plossl's at different primary speeds.
I will buy some XW just tomorrow!!!).
