ON THE REDUCTION OF DIFFRACTION IN STELLAR IMAGES

Abstract

In securing high-quality photographic images from a reflecting telescope, one is interested in reducing the diffraction spikes accompanying the images of bright stars caused by the supporting members of the upper mirror or plateholder. Such diffraction spikes can interfere seriously with the resolution of close faint companions or with iris photometry of the images unless they can be rotated or reduced, as the case may require. Some years ago, Couder devised an ingenious scheme to suppress this kind of diffraction by introducing a curved shape to the outline of the supporting vanes.1 Regrettably, he did not provide a description of the necessary curvatures and one is left to guess the required shape. In attempting to exploit this scheme to improve the image appearance on photographs taken with the Seyfert 24-inch telescope, it was found that Couder's scheme provided squareshaped images, with diffuse light scattered in the form of a Prussian cross, which were not well suited to iris photometry. It is quite possible that our attempt to apply Couder's technique was not entirely correct and could be improved. Another method, suggested to the writer by an article describing a coronograph occulting disk,2 was attempted; here the vanes were fitted with saw-tooth edges. However, in this application, no curvature is present as in the case of the periphery of an occulting disk, and consequently evenly spaced saw teeth such as obtained by pinking shears cannot be used owing to a residual grating effect. Instead, random-spaced saw teeth were cut into edges of thin sheet aluminum using a cornernotcher and a table of random numbers. Subsequently many