Resolving Power of a Telescope

a quantity characterizing the ability of a telescope to produce separate images of two stars close to each other on the celestial sphere; in Soviet usage, it is a quantity that is the inverse of the minimum angular distance between two stars that are distinguishable as separate in the telescope. The theoretical resolving power of a telescope is due solely to light diffraction at the edge of the objective. For radiation with wavelength λ (mm), a telescope with an objective of diameter D (mm) is capable of resolving two stars of equal brightness separated by a distance ∊_d = 251,600λ/D) sec of arc. For the visible region of the spectrum, λ = 0.000555 mm and ∊_d = 140/D sec of arc. This can be achieved only by telescopes of the highest quality used in space. Because of residual aberrations of the objective, manufacturing errors, temperature and weight deformations, and, above all, atmospheric interference, telescopes on earth seldom achieve a resolving power better than 1″. The resolving power of a telescope is characterized more completely by the frequency-contrast curve and the related spread function.