The alignment of the optical elements of a telescope at the correct angles to the light path. If not properly collimated, a telescope will deliver distorted images ( lopsided or elongated stars, hazy planetary images, an inability to split close binary stars, etc.) Atmospheric turbulence ( seeing ), thermal currents within the telescope, and dirty or defective eyepieces can often mimic poor collimation. The illustrations below simulate the Airy disks and diffraction rings of a properly collimated scope as well as those of a mis-aligned scope.

. . . our 38th year