What does "ED" mean?

I'm new (obviously). I see "ED" a lot. What does it mean?

ED refers to a family of glass types that have extra-low dispersion when compared to the normal glass types. Use of these glasses can help to reduce secondary spectrum (color) in lenses. Naturally, these glasses cost more than the usual glass types.

You hear it a lot because some manufacturers use the ED term to differentiate their scopes from the run of the mill achromats. Typically, the ED scopes do have less color but it is not guaranteed. There are a lot of other factors that contribute to the lens performance.

hope this helps,
dan

Bill, good question. "ED" is a type of glass used in refractors. Specifically, "ED" stands for "extra-low dispersion" glass which produces images that do not suffer (as much??) from chromatic aberration. Chromatic aberration is the inability for light of different colors to focus at the same focal point. Pete

As others have said, "ED" stands for "Extra-low Dispersion." (Although the common usage of "ED" in television commercials may imply otherwise. )

It is important to keep in mind that it is not the extra-low dispersion of the glass that allows it to be used for improved color correction. If you want to design a lens that can be made in a reasonable focal length, is free of aberrations, and is agreeable to a production environment, you need to pick two glasses that have markedly different dispersions. For an achromat, this usually means a Crown like BK7 and a flint like F2.

A flint glass spreads out or disperses light more than a crown glass. If that was the only difference, we'd be in good shape. Unfortunately, the difference between the dispersions varies with color. In other words, there is no single number that would describe the difference between the two across the entire spectrum.

With ED material, the variation between the dispersion and the dispersion of some crowns as you move across the spectrum is much smaller. You could almost say at every point of the spectrum the crown spreads out the light X percent more. When a proper crown is matched with an ED material (fluor-crown) you can get a large reduction in secondary spectrum.

I hope that was understandable.

Clear skies, Alan



Clear skies, Alan

Bill Wheaton said:

I'm new (obviously). I see "ED" a lot. What does it mean?

You have gotten some good answers, here's my two cents...

In the beginning, in Galileo's time, telescopes used only a single element objective. This magnified things but it also acted like a prism, it spread out the colors. The cure for this is to increase the focal ratio so the light is not bent as much. In the 17th and 18th centuries, this led to telescopes that were over 100 feet in length that were relatively small, maybe 4-6 inches.

People had figured out that you could use two lenses made different glasses with different properties and different curves to refocus the light but it was a rather complicated problem. This was called the Achromat and there is some dispute as to who built the first achromats but it is clear that the Dolland family patented the design and built many telescopes at the end of the 18th century. These were a huge improvement and over the years, the design and manufacture of the Achromatic objective has been perfected. An achromat allows the colors to come to focus to about 1 part in 2000 over the visual spectrum, pretty good.

However, in larger telescopes and in faster telescopes, the unfocused red and blue light combine to make a visible purple haze which can be objectionable visually and cause problems photographically.

Enter the Apochromat which achieves even better color correction than the Achromat. The Achromat has two colors that are in focus, the Apochromat or APO, has three. This allows virtually perfect color correction.

In order to achieve this much improved color correction, glasses that are better matched in their properties are required and for the best results, three elements are best. In order to provide the good color correction one of those elements needs to be "extra low dispersion" glass or ED glass. These are glasses made with Fluoride's rather than oxides, some are even made with Fluorite, Calcium Fluorite but there are synthetic materials such as "FPL-53" which have virtually the same properties.

There are a variety of definitions of an Apochromat, Roland Christen who often posts on Astromart has some good articles on the subject. Over the past 30 years, he has been instrumental in bringing the Apochromat to the amateur community and his venture, I consider him the father of the Modern APO. Astro-Physics is still at the forefront of providing the highest quality equipment, both optical and mechanical.

http://geogdata.csun.edu/~voltaire/roland/

It's worth a good read and there are some great photos there as well.

Anyway, if one uses an Extra-low dispersion glass, it does not guaranty that the image will be free from chromatic aberration. There are a variety of glasses to choose from, some more costly, this adds to the complexity. And too, larger objectives and faster focal ratios both make controlling the false color more difficult.

So, somewhere between what many would call the APO and the Achromat, there telescopes made with good quality ED's glasses which show much better color correction than a similar size and focal ratio achromat but do not quite meet the requirements to be a true APO. These are most often doublets.

One group of ED scopes is the recently introduced 102mm F/7's doublets sold by a variety of companies including Astro-Tech, Orion, StellarVue and William Optics. Compared to a 102mm F/7 Achromat, these scopes show much less false color, but if pushed, one can see a faint violet tinge at high magnifications around bright stars like Rigel and there is definitely out of focus color.

There are other ED scopes which use glasses which are ED in name only and do not offer significantly improved color correction over a similar Achromat. The TeleVue Pronto was one. In the past this was somewhat common but today, the cost of ED glasses has been reduced and APO's such as the Orion ED-80/100 and the very enjoyable scopes like the 102mm F/7 ED's are very affordable.

Hope this is a help.

Jon Isaacs

On a refractor group, ED means "warning! Extensive Discussion ahead!"

or "Extra Discourse"!

or "Everyone Disgruntled"!

or "Evangelical Discussions" ahead!

regards
Greg N