The overall focal length of the instrument is designated as F t. This factor tells how much the secondary mirror magnifies the focal length of the primary by. Because the secondary is convex, it has a "magnification factor" that is referred to as m. The purpose of a Cassegrain secondary mirror is to slow the convergence of the light cone from the primary by increasing the system focal length, direct it to the focal plane and correct as best possible the aberrations in the image caused by the primary. a change of a from 1.4 inches to 1.5 inches is a da of 0.1 inches.