Updated 6-29-03

With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. 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.

Here is a more detailed analysis for those that are interested. The term "Apochromat" is loosely used by many manufacturers and amateurs astronomers. Let's look at the history of the definition, and maybe a more modern one. Ernst Abbe, in 1875, met and worked for Carl Zeiss, a small microscope, magnifier and optical accessory company. They realized that they needed to find improved glass types, if they were going to make progress with the optical microscope. In 1879, Abbe met Otto Schott. Together they introduce the first abnormal dispersion glasses under the name of Schott and Sons. Abbe discovered that by using optically clear, polished natural fluorite, in a microscope objective, that apochromatism could be achieved. These first true apochromatic microscope objectives were so superior to the competition, that Zeiss gained nearly the entire high end market. So secret was the use of fluorite, that Abbe marked an "X" on the data sheet for the fluorite element, so as to keep it secret from the other optical companies.

Abbe's definition of apochromatism was the following. Apochromat: an objective corrected parfocally for three widely spaced wavelengths and corrected for spherical aberration and coma for two widely separated wavelengths. This definition is not as simple as it sounds. I have designed thousands of lenses: simple achromats, complex achromats, semi-apos, apochromats, super-achromats, hyper-achromats, and Baker super-apochromats. Abbe's definition, to put it in clearer terms (I hope) is that a true apochromat is an objective that has three color crossings that are spaced far apart in the visual spectrum (~4000A, deep violet to ~7000A, deep red). However, just because a lens has three color crossings, doesn't mean that it is well corrected. Let's say that a 4" lens has three color crossings at the F, e and C wavelengths (4861A, 5461A and 6563A). Fine, this objective is now considered an apochromat by most amateurs and even optical designers because it has three color crossings in the blue, green and red -- the common definition of an apochromat. But what about the levels of spherical aberration at each of these wavelengths? If the lens is 2 waves overcorrected at 4861A, and 1.5 waves undercorrected at 6563A, is it still an apochromat? No. It is no better than an achromat, as the OPD wavefront error is worse than a 4" f/15 achromat.