Aerial Refractor Telescopes and the Development of Reflectors

Abstract

Before the advent of the innovative achromatic lens, it was recognised that it should be possible to obviate the problems of spherical and chromatic aberrations by having a long focal length objective lens. This stemmed from an empirical observation that the image quality of a lens—remembering they were all of spherical section and made of crown glass—was improved by a decrease in curvature. Taken to the extreme limit, a flat piece of glass has neither spherical nor chromatic errors, but also no capacity to focus closer than infinity. By having reduced curvature and therefore a long focal length, the aperture ratio (f) decreases. The f value is created by the simple expedient of dividing the focal length by the aperture. Thus, the best images at the time were generated by long focal length lenses with a resulting very small aperture ratio (confusingly designated by a large number). Aperture ratios of 1:150 were not unknown at this time and were an inevitable result of only being able to produce long focal length lenses of relatively small diameter.