Image Degradation Caused By Direct Scatter From Optical Components Into The Image Plane

Abstract

Surface irregularities range in lateral dimensions from those usually associated with optical figure error through values associated with zonal errors to those usually described as microroughness and extending to submicron dimensions. Typically, the irregularities are a small fraction of a wavelength A in height so that physical, not geometrical, optics must be used to calculate their contribution to optical performance. When the ratio of the root mean square irregularity height to the wavelength is small, first-order perturbation methods can be used to predict the scattering characteristics of such surfaces. No upper limiting value on the lateral dimensions of the surface irregularities appears in such a calculation, although, for normal incidence, much of the scattering becomes virtual when the lateral dimension 2. of the irregularities becomes less than X. Lateral dimensions associated with figure and zonal errors yield scattering in the near-specular region. These errors reduce image contrast and cause the optical system to be sensitive to stray light from extraneous near-axis sources. Lateral dimensions associated with microroughness yield scattering at large angles from the specular direction. Large-angle scattering may also be important but becomes of particular concern for an imaging system such as a telescope where light may enter the optical system from large off-axis angles, strike the optical component, and be scattered into the focal plane. A calculation of the effect of surface errors having a range of autocovariance lengths on the performance of a typical mirror telescope will be given to demonstrate the possible degra-dation effects of both near- and large-angle scattering. Vignetting effects that occur when the mirror is illuminated at off-axis angles are also considered. It should be pointed out that we are discussing scattering of light into the optical path by the optical components themselves and that no arrangement of baffles following the component itself will have any effect on this type of scattering intensity.