Since its invention 400 years ago, the astronomical
telescope has evolved from a small, manually pointed device for visual
observations to a large and sophisticated computer-controlled instrument
with full digital output. Throughout this development, two parameters have
been particularly important: the light-collecting power or diameter of
the telescope (allowing the detection
of fainter and more distant objects) and the angular
resolution (or image sharpness). For a perfect telescope used in a vacuum,
resolution is directly proportional to the inverse of the telescope diameter.
A plane wavefront from distant star (effectively at infinity) would be
converted by the telescope into a perfectly spherical wavefront, forming
the image, with an angular resolution
only limited by light diffraction aptly called
the diffraction limit.
Under ideal circumstances, the resolution of an optical system is limited by the diffraction of light waves. This so-called "diffraction limit" is generally described by the following angle (in radians) calculated using the light's wavelength and optical
system's pupil diameter:
where the angle is given in radians. Thus, the fully-dilated human eye should be able to separate objects as close as 0.3 arcmin in visible light, and the Keck Telescope (10-m) should be able to resolve objects as close as 0.013 arcsec.
