The APO Revolution
If you’ve faithfully read some or all of the material on achromatic refractors presented thus far, you’ll have noted I’ve used the terms “chromatic aberration,” “secondary spectrum,” and “false color” an awful lot. As we have seen, all achromatic refractors show it to a greater or lesser degree, and even the finest long-focus achromatic refractors cannot completely eliminate this optical defect. Harold Suiter, in his book Star Testing Astronomical Telescopes, provides an excellent analogy to describe the essence of a classical achromat: “Achromatism can be compared to tying the spectrum in a knot. The brightest parts of the visible spectrum are deliberately folded into the tightest bundle, with the deep red and violet ends hanging out like shoelaces.” You’ll recognize Suiter’s “shoelaces” as the origin of the purple fringes seen around high-contrast objects by day and by night. But such color fringing, however slight, takes information away from an image. During daylight use, color fringing robs the viewer of seeing high-contrast detail at the boundary between dark and light zones. Just have a look at some green leaves through an achromat at high power against a bright background sky to see what is meant. Now, recall the image of a star at high power again with its central bright spot, the Airy disc, surrounded by a luminous halo of unfocused purple. Light that doesn’t end up inside the Airy disc cannot add information to the in-focus image. The only way to reduce false color beyond that of a long-focus achromat is to bring more than two colors of light to a common focus, while retaining a sharp image. Such an instrument is called an apochromat (Apo), and the first models were put together over a century ago by the hands of a brilliant Briton.