Four Element Apos
A well designed triplet Apo, as we have seen, closely approaches optical perfection. Images snap to a focus, have no false color, and have very little in the way of spherical aberration, astigmatism, and coma. Many triplets show traces of spherochromatism, as evidenced by some out-of-focus color, but not enough to seriously affect image quality. Only one Seidel aberration remains in triplets, and, in the scheme of things, it’s rarely serious enough to dwell upon if you’re a purely visual observer. We’re talking, of course, about field curvature. There are two ways of eliminating this effect. Either you can use an add-on field flattener or you can introduce another optical element into the design. In general, using a dedicated field flattener negates the telescope’s use for visual applications. But there are telescopes – all four element designs – that can be enjoyed visually as well as photographically.