TheSkySearchers

It seems that we do not have a comprehensive definition of apochromation. So then it becomes an approximation. I am very fond of Wolfgang Rohr's approach that measures how far apart the various focal points of the colors are. (so only the colors anyway) That means that many contemporary refractors fall within the limits of apochromatism. Even some doublets.

When I get this spun I like to look back at Galileo's Telescope,and what he did with it,it’s not the scope but the person behind it

APO

The apochromasie has been defined by Ernst Abbe in 1886 (I have the reprint).

When APO,
a) Then 3 different wavelength come to a common focus
b) And for two different wavelengths the Strehl achieves its maximum value

You simply can't meet these requirements with just 2 lenses even if made of any exotic extraordinary dispersion glass materials, how much you ever might try.
(It is not as in the fun video on "Make The Right Angle Right").

This is the reason, why Zeiss and Lichtenknecker have switched from the Abbe's strict apochromasie requirement to the softer RC figure of merit, relying on the depth of focus overlaps, in reference to the Fraunhofer C-e-F secondary spectrum achromasie.
And the ZEISS APOs have been actually designed as the RC APOs with the depth of focus overlapping for the three spectral lines.

Best,
JG

What JG said!

@j.gardavsky This is a very clear explanation, thanks!

This is a great thread. Will probably need to read it through another 4-5 times before much of it makes sense to my caveman brain :-) I mean that as a sincere compliment to each and every person who has given a response!

I like the Stradivarius analogy given above. It seems like a lot of the key optical differences, especially between decent quality doublets and triplets will be transparent to the average enthusiast.

Mike