Reduction of diffraction errors in radiometry by means of toothed apertures

Purcell and Koomen have shown [J. D. Purcell and M. J. Koomen, Coronagraph with Improved Scattered-Light Properties, Report of NRL Progress, 1962 (U.S. GPO, Washington, D.C., 1962)] that a toothed occulting disk greatly reduces diffraction problems in coronagraphy. In the present paper, the use of toothed apertures to reduce diffraction errors in radiometry is studied in detail. Simple formulas are derived which allow the design of toothed apertures for given applications. Experimental measurements are presented which demonstrate the usefulness of toothed apertures to reduce diffraction errors by at least an order of magnitude. This technique is shown to be particularly useful for radiometric measurements involving a series of apertures; in such cases, the diffraction errors of the individual apertures can simply add up, resulting in important over-all errors if ordinary circular apertures are used.