Measuring the focal lengths of long-focus optical systems

A method of measuring the focal lengths of optical systems having a focal l ength greater than 10 m is proposed and experimentally implemented. The met hod is based on the use of an optical wedge with a small vertex angle (less than 10'). Two readings of the position of a luminous slit with the optica l wedge introduced into the beam path and without it are taken in the focal plane of the optical system. It is shown experimentally that a focal lengt h of 25 m can be measured with an error of 0.1%. (C) 1999 The Optical Socie ty of America. [S1070-9762(99)01905-3].