Long Focus Achromats
Ask a general member of the public to think of a “telescope” and chances are he or she will describe a device made from a long tube with a lens at one end. The iconography embodied in long tube refractors is not a new thing. They’ve been around since the first generation of telescopic adventurers turned their humble spy glasses turned the heavens. By making the focal length of the telescope longer with respect to its aperture, ever better performance can be coaxed from it because, as we have seen, it minimizes all of the aberrations that can plague an image. Even today, many discerning observers return to these instruments again and again as they rediscover their sharp, high-contrast views of the Moon, planets, and double stars with little in the way of false color. Yet, as we shall see, these instruments – relics from the halcyon days of the refracting telescope – have attributes that have largely been forgotten by a generation whose observing experiences have been shaped by using instruments with shorter focal lengths.