[ausastronomer (John Bambury)]

Hi Profiler,

You are getting overly hung up on the construction of one or two elements, of eyepieces having 6 or more elements.

Here is a quote from Roland Christen on Astromart in 2007. Roland Christen designs and builds some of the worlds best Apochromatic refractors (Astro Physics). Roland is one of the worlds most respected opticians. Rolands' post was in response to a question about the Vixen FL102S Flourite doublet APO, which was also sold as the Celestron C102F. It is in fact a very high quality flourite doublet APO.

Your Vixen Fluorite was originally made by Optron, same company that made the Tak fluorite doublets. The design used KzF2 as the mating element. This combination of KzF2 and Fluorite gives good color correction along with null spherical correction in an airpspaced doublet when all surfaces are spherical.

The lack of aspherization, plus the relatively long focal ratio made it an easy lens to make. When an optic is easy to make, it is easy for the manufacturer to make it well.

Now, it may sound like this is the best way to make a visual doublet lens, so why don't they make it this way any more? The answer is that the mating element, KzFS2, is not available, and has not been for many years. There is no good substitute for this glass, and all other mating glasses have either more color error, or they present other manufacturing problems.

I should again remind you all that the mating element is as much or maybe more important than the ED material. So why doesn't anyone ask about this, and yet they get all excited when FPL53 or Fluorite is mentioned?"

and...

"Mag-fluoride coatings are ultra-thin and have the ability to lower the reflection losses at an air/glass surface. Without it, you lose approximately 7% on the two surfaces of the calcium fluorite element.

Roland Christen"

I want to emphasize this part of Roland's post:-

I should again remind you all that the mating element is as much or maybe more important than the ED material. So why doesn't anyone ask about this, and yet they get all excited when FPL53 or Fluorite is mentioned

This brings me back to the question, "why are you so worried about whether or not an eyepiece has ONE or TWO Lanthanum glass elements when we have no clue what the other elements are made of. The material used in the construction of the other elements is infinitely more important than whether or not one or two elements are Lanthanum glass. This is purely the manufacturers' advertising hype to help them sell eyepieces. An eyepiece constructed from 4 elements of model aircraft grade perspex and two lanthanum glass elements isn't going to provide very good optical performance.

It all comes back to the fact that whatever glass Pentax uses in the construction of the XF series of eyepieces, it works very well.

Cheers,
John B