Resolving power (or resolution) of an optical telescope is the ability to see small details in the image. It is an angular distance between two stars that are barely visible as two separate images – it is usually expressed in units of arc-seconds.
Rayleigh limit is as measure defining the limit at which two stars can be seen as two separate components and is equal to angular resolution. It describes the distance between the center of two Airy disks where the second star is placed on the valley between the first star's central disk and the first diffraction ring.
Rayleigh limit
Angular radius of the Airy disk is equivalent to Rayleigh limit:
The human eye is the most sensitive to yellow light. Therefore, the value of 550 nano-meters is used.
To get the Rayleigh resolution in terms of arc-seconds, the equation for angular distance needs to be converted:
