Optical Telescopes

Abstract

Ever since Galileo first aimed a telescope upward there has been a continual quest to examine the sky in ever-increasing detail. The optical quality of objective lenses largely restricted the performance of those early telescopes. In the latter half of the 17th century, the Huygens brothers recognized that long focal lengths were less susceptible to aberrations. That recognition inspired telescopes with 30- to 60-meter focal lengths, but such extremes proved so unwieldy that more modest sizes in the range 6- to 10-meter focal lengths were more effective in practice. Quite later, after mastery of the worst aberrations, diffraction was noticed as an aperture dependent restriction on resolving power. The subsequent advent of photography further complicated the interplay of focal length and aperture as they relate to telescope performance, because photographic emulsions, with their thresholds and integrating character, behave quite differently than visual observation. Much more recently, the interplay has been modified for infrared and charge-coupled-device (CCD) sensors. In any case, it is only over the last century that the two most notorious factors that limit telescope performance have become conspicuous. The first is atmospheric turbulence, and the second is the quantized nature of light.