It is an amazing glass that greatly diminishes chromatic aberration significantly - but it is expensive
The biggest problem with refractor telescopes is chromatic aberration. That is the property of a lens bending different colors of light at different angles. When you look through a telescope at objects and you see colored halos around them, particularly bright objects like Saturn or Jupiter, you are seeing chromatic aberration.
This was a serious problem in the early period of telescopes. And it could be minimized by making lenses with very long focal lengths. This is why you see pictures of telescopes from a century ago that are so long! Like this Keplerian telescope.
It wasn't perfect but it did improve the problem of chromatic aberration.
But it is an awkward solution that requires an awkward telescope. Over the decades better solutions have been found.
Here in this article I will cover 5 major types of lenses in refractor telescopes and I will arrange them from worst to best in terms of how they fix chromatic aberration.
First a quick look at chromatic aberration.
When light passes through the lens of a glass it is partially separated into it's individual colors. This is because the various frequencies of light are refracted at different angles.
So, how do we fix it?
This chromatic aberration can be corrected to a degree with a pairing of lenses, made of two different types of glass. This pairing is called a doublet. And you can see that the focus point of the three colors is closer but not perfect.
This is where ED glass steps in. It is specially designed glass that further reduces chromatic aberration. A doublet is tuned to be efficient with the red green and blue. An ED doublet works well in more of the spectrum, not just red green and blue.
Another improvement in the handling of chromatic aberration is the triplet.
Typically a triplet, composed of three lenses sandwiched together performs better than an ED doublet but this isn't always the case. Depending on quality the two go back and forth in terms of quality handling of chromatic aberration.
Then we get to the best assembly for chromatic aberration. It is the triplet where one or more of the lenses are made of ED Glass.
An ED APO Chromatic triplet makes for an excellent refractor telescope because it has almost negligible chromatic aberration. But there are trade-offs.
Some of the negative properties of ED glass
1. It is expensive
2. It is less stable to temperature change
3. It has a low index of refraction so it needs deeper curves to achieve the same focal length of non ED glass.
Standard Refractor telescopes have APO (triplets) primary optics. But if you want the absolute best then you should get ED APO refractors. They are the best and offer the absolute best chromatic correction. But they are quite a bit more expensive than regular APO refractors.
Here is a look at some: