Limits to which Double Lines, Double Stars, and Disks can be Resolved and Measured*

Equations are derived for the limits of resolution and measurement, through a circular diffraction-limited aperture, (I) of the angular separation of double-line and double-star sources, and, (II) the angular diameter of a disk source. Included are the effects of the total light detected and the intensity ratio of the components of the double sources. The random fluctuation of detected photons is assumed to be the only source of noise.