Note 14 — Chapter IV · THE PROPERTIES OF OBJECTIVES AND MIRRORS
Reading edition
A very useful treatment of the aberrations of parabolic mirrors by Poor is in Ap. J. 7, 114. In this is given a table of the maximum dimension of a star disc off the axis in reflectors of various apertures. This table condenses to the closely approximate formula
a = lld/f²
where a is the aberrational diameter of the star disc, in seconds of arc, d the distance from the axis in minutes of arc, f the denominator of the F ratio (F/8 &c.) and 11, a constant. Obviously the separating power of a telescope (see Chap. X) being substantially 4.″56/D where D is the diameter of objective or mirror in inches, the separating power will be impaired when a > 4.″56/D. In the photographic case the critical quantity is not 4.″56/D, but the maximum image diameter tolerable for the purpose in hand.