Confused--ED/APO terminology.

I'm confused as to the correct use of ED and Apo terminology, upon reading some books that give a history on the development of telescope optics it seems that the term Apochromatic applies when there are 3 lens elements that bring the red, green and blue parts of the visible spectrum to a common focus thus eliminating chromatic aberration (CA).
So it would seem to me that ED is a two-element doublet with a different glass type (than an achro.) to acheive nearly the same result but is not quite as good as a "true" Apo, looks like the terms are used interchangeably but I'm not sure which is correct

Thanks. :S

Wayne Wlocka said:

I'm confused as to the correct use of ED and Apo terminology, upon reading some books that give a history on the development of telescope optics it seems that the term Apochromatic applies when there are 3 lens elements that bring the red, green and blue parts of the visible spectrum to a common focus thus eliminating chromatic aberration (CA).
So it would seem to me that ED is a two-element doublet with a different glass type (than an achro.) to acheive nearly the same result but is not quite as good as a "true" Apo, looks like the terms are used interchangeably but I'm not sure which is correct

Thanks. :S

This topic has been discussed at length. The recent thread "True APO" should answer your question.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=669970&poll_id=&news_id=&page=

In short, ED is a description of special glasses with abnormal dispersion characteristics, APO is a term with a specific definition, ie "Three color crossings, corrected spherically at two"... This can be achieved with two or with three elements... And too, three elements does not necessarily mean a scope is an APO...

All modern "APO" scopes use ED glass.

Jon