Abstract
The ability to measure the absolute effective focal length (EFL) of high resolution tele-scopic systems is limited by errors in the alignment of the test equipment used for the measurement. The types of errors are usually dependent upon the specific test equipment used and the way in which it is set up. While these time independent (D.C.) misalignments are of serious concern, other environmentally-dependent misalignments can be equally dis-turbing; the periods of these dynamic misalignments range from 30 minutes or more to a millisecond. In an effort to measure EFL to better than one part in 104, an extensive study of all known sources of misalignment has been carried out in addition to simply evalu-ating the statistical error associated with repeated EFL determinations. This investiga-tion utilized strain gauges, precision knife edges, assorted lasers, mechanical fixtures, accelerometers, seismometers, position sensing detectors, interferometers, and a PDP 11/70 computer. Reported here are The real constraints to be observed in setting up a laser interferometer How support fixtures should be manufactured The effects of test-table bending in thermally unstable environments The ways to minimize beam steering effects over long optical paths The inadequacies of "vibration-isolated" tables How precisely one can expect to measure EFL absolutely
