Thomas Back wrote an essay titled '
defining apochromatism' that you might find interesting.
In it he states
"The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma. "
That essay specifically mentions Al Naglers petzvel telescopes.