Triplet Apos

Abstract

We’ve covered a lot of ground so far, so this might be a good time to take stock on the story to date. Let’s talk lenses. First off, a simple lens, like the magnifying glass you fiddle with from time to time, has two curved (spherical) surfaces. No matter how well you figure and polish these surfaces, the lens will never focus red light and blue light at the same point. A doublet lens – such as our classical crown-flint ­achromat – adds a second element that whips the red and the blue into line, as it were, so they come to a common focus. When we add another element, so creating a triplet, it’s possible to bring more than two colors into perfect focus. This naturally reduces the amount of spurious color observed. But you can also harness the refractive muscle of the triplet to bring light entering at the edge of the lens into sharper focus with light entering at its center. That cuts down on spherical aberration. Do you get the idea? Basically, each element you add gives you more ways, more degrees of freedom, to perfect the image by carefully choosing its material properties and shape. Of course, the cost and complexity increases with each new element you add. So you’d expect a triplet to exhibit sharper, more color-free images compared to a doublet ED or fluorite Apo of similar aperture and focal length. So how do the current line of triplet Apos square up to these expectations? Arguably the finest triplet Apos ever produced came from the factories of the Swiss optical giant Carl Zeiss, in the form of the legendary APQ line. Sadly Zeiss has ceased producing refractors for the amateur market, but other innovators have since reached comparable heights of optical artistry. Roland Christen is, for example, telescope maker extraordinaire and founder of the Illinois-based company Astro-Physics. In the early 1980s, the company introduced the first high-performance triplet apochromats to the amateur market. These early instruments, though not as entirely color free as their current line, were nonetheless quite revolutionary, being a major ­influence in the rebirth of refractors in the modern era.