Is it an apo?

Re: the debate over what constitutes an apo....
I don't have a lot of experience with refractors (various 60 mm's, Astroview 90, C-102, ST-80, TV-102), but a thought occurred to me--
Since astronomical objects are varyingly listed by their visual magnitude and their photographic magnitude, couldn't apos be listed this way as well? A visual apo would be a scope that corrected for false color to a satisfactory level visually, and a photographic apo would correct so well as to show no false color photgraphically.
Just a thought.

The problem here is that there is no accepted standard that you can get everyone to agree on and much is still open to interpretation.
Buck

I would suggest that a lens is apochromatic if the Strehl ratio is above a certain level for a certain wavelength range.

For a visual scope I would call it a visual apochromat if the Strehl were above 90% from C to F (wavelength range 656nm to 486nm). Furthermore, the theoretical Strehl should be above 99% for a mid-wavelength somewhere between d and e (588nm to 546nm).

For an imaging scope I would call it apochromatic if the Strehl were above 80% for a wavelength range from r to g wavelengths (706nm to 436nm).

These are both extremely strict limits which are not easily reached with compromised designs. What these limits do is to define the theoretical maximum sharpness of the lens over a particular wavelength range, rather than the absolute color correction.

It may surprise some that an optimized aplanic 12" F8 SCT may appear totally colorfree, but it does not meet the definition for an apochromatic imaging scope since the Strehl ratio is below 50% at 436nm in the blue-violet. Being a mirror scope with zero-power corrector, it does not have any primary or secondary chromatic aberration, but it does have sphero-chromatism, which introduces a certain amount of softness into the image at the ends of the spectrum.

In summary, the color correction is composed of many individual components, but the Strehl ratio for a given wavelength range will determine the sharpness of the final image and the snap at focus, the contrast and definition of detail on a planetary disc.

Rolando