An Instrument for Measuring the Modulation Transfer Functions of Low Power Telescopes and Telemicroscopes

An instrument using an electro‐optical Fourier method for measuring the modulation transfer function (MTF) of low power telescopes and telemicroscopes is described. Because these devices are afocal, or nearly so, relay optics are needed to form real images at the detection section of the apparatus. The system is capable of measuring the MTF in monochromatic and white light, at any target azimuth, across the field of view, and through focus. The target system contains 14 square‐wave gratings with spatial frequencies that range from 2.5 to 156 cpd. Images of these gratings are scanned across a slit. The output data are fed to a first‐order recursive digital Butter‐worth bandpass filter for MTF analysis. The apparatus is diffraction limited at f/31.4. Therefore, it negligibly affects the measurement of the MTF of telescopes and telemicroscopes tested with exit pupils of up to 6.4 mm.