The modulation collimator in X-ray astronomy

Abstract

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.