ED glass and the optical train.

[Chaz2b]

Having plenty of time for reading during these unfortunate times, I’ve been studying up on glass manufacturing for optics, the processes involved and choice of materials.

However, I’ve not found a lot on the effects in the whole optical train when these ED glasses are used.

When you buy a telescope that advertises ED glass, it doesn’t always explain which glass has been used, I.e fl53 or fl61. Do you know the difference? I won’t explain, it gives you something to research.

But I do have a question that only experienced viewers can answer. That is, if the objective is of ED glass and I place a “standard” eyepiece in the scope, does the eyepiece undo the work of the ED glass?

Should you therefore use ED eyepieces to get the best from ED objectives? The work of producing such glass is quite a subject to read, and it is good reading! But if you state “ I’ve not really seen any difference” then has the ED eyepiece been overplayed? Of course there are also eyepiece design to take into account, plossl, erfle etc, I’m more interested over the glass itself.

what do you think?

chaz

 

Edited April 27, 2020 by Chaz2b

[John]

The use of an ED glass element (you only need one element to be ED) in the objective, if done well, corrects much of the CA and that correction is then in the optical system and will benefit optical elements further down the optical chain eg: eyepieces, whether they use ED glass or not. There are circumstances where an eyepiece will add a little CA of it's own eg: at the edges of a very wide field of view or if a non-achromatic eyepiece such as a Huygens is used in a reasonably fast scope.

It is the combination of an ED glass with other glass types in the objective of the scope, rather than the ED glass itself which is important. The optical figure, coatings, objective cell, lens spacing all make a difference to the final result as well.

The different types of ED glass have different Abbe numbers and refractive indexes which indicate the potential they have, when combined with suitable mating elements (very important that last bit) to produce a corrected image for the eyepiece optics to handle. Some brands now are choosing not to disclose the glass types used saying that their products should be judged by their actual performance.

Many quality eyepieces have used ED glass types for many years without promoting the fact. Some brands have chosen to make this a selling point though.

This whole topic is the subject of long and sometimes heated discussion on other forums !

 

 

[Louis D]

My understanding is that ED glass was more common in eyepieces 15 to 25 years ago.  Then China cornered the market on rare earths and starting jacking up the prices on them except to Chinese customers.  As a result, most newer non-Chinese eyepiece designs dropped using ED glasses because they had become cost prohibitive.  To get the same level of correction, eyepieces tended to get longer or fatter to make up for the loss of ED glass properties.