The modulation collimator in x-ray astronomy

Modulation collimators have been used in recently reported work to determine the angular sizes and celestial positions of the X-ray sources Sco X-1 and Taurus XR-1 (Crab Nebula) with precisions of 15 to 30. The measurements were made by means of four-grid collimators, star photography and optical imaging of the collimators. In the present paper we discuss (1) the principles and uses of various forms of the modulation collimators as they pertain to X-ray astronomy, (2) several methods for determining the celestial positions of X-ray sources with these collimators, (3) the techniques for the alignment and calibration of these detection systems, (4) an image-forming collimator, and finally, (5) some of the optical properties of these grid systems. The modulation collimator is quite versatile and is particularly suited for measurements from spacecraft with relatively poor pointing capability. Thus it should be a useful tool in X-ray astronomy for some years to come.This work was supported in part by the National Aeronautics and Space Administration under contracts NASw-1284 and NASw-1535 and grant NSG-386 and in part by the United States Atomic Energy Commission under contract AT (30-1)2098. In addition, certain portions were carried out at California Institute of Technology under National Aeronautics and Space Administration grant NSG-426 and at the Institute of Space and Aeronautical Sciences, Tokyo.