While it's true that Roland Christen's use of the definition is correct, he was not the first to use the principle in designing lenses. So the definition isn't "his". The history of who authored the definition first is murky because optics companies in the 19th century kept a lot of data proprietary. The first practical example of an apochromatic lens used for telescopes (AFAIK) is the Cooke triplet in 1893 invented by Harold Dennis Taylor. But Zeiss's Ernst Abbe probably invented the concept earlier and applied it to microscopes in 1886, the current definition is from Abbe. So Christen is using Abbe's ideas.Bowlerhat wrote: ↑Mon May 31, 2021 2:26 am Other than scientific "APO", in practicality there are other definitions. Like "Marketing APO" - which brands just abused the term "Apo" for selling scopes which isn't..and there's also "Practically APO" which an experience of CA-free in non-APO scopes. I mean I use my classic scopes a lot and tbh I don't see CA (because of long focal length). There's also "Lens APO" where people start to define "apo" based on lens numbers..that doublets can't be APO, or the opposite. This is muddled up further by types of glass. FPL 51,53, 55, BK71, FCD 100, etc..
As a person who has used mirrors, achromats, EDs, fluorites..and eventually long achros, I'd say the term is rather flexible in the field. Personally, I'd just stick to Roland Christen's definition: "bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths."
https://en.wikipedia.org/wiki/Harold_Dennis_Taylor
https://en.wikipedia.org/wiki/Ernst_Abbe