The Imaging Properties of the Rotation Collimator

Properties of the rotation collimator as a method of detecting point sources at infinity, such as cosmic X-ray sources, are discussed. Gratings of various types are considered, and it is shown that the image formed has a central and satellite peaks, each surrounded by circular fringes. Varying the mark-space ratio affects the amplitude of the satellite peaks; by superposing gratings with ratios of 1 : 1 and 1 : 2 a maximum number of them are suppressed. The angular size of the central image is of the order of a/l where a is the ruling period and l the separation of the masks. Next, possible two-dimensional patterns are analysed, and it is shown that both a chequerboard pattern and a grid of circular holes have suitable properties. The angular resolution with these is higher by a factor of $$\sqrt{2}$$ than for the grating, but the sensitivity in the presence of isotropic X-radiation background is several times lower. The effect of sliding one grid alone or both together in their own planes is considered and it is shown that the effect of the first is easily taken into account, whilst the second has no effect whatever. This eases considerably the problems of constructing collimators of high angular resolution since the only critical adjustment required is to align the patterns accurately parallel. Finally, several different processes of reconstruction are discussed and it is shown that it is possible by a suitable choice to reduce the number and amplitude of the spurious fringes surrounding the bright central image and that to obtain the necessary flexibility in this process image reconstruction in a computer rather than by an analogue method is to be preferred.