X-ray imaging techniques— modulation collimator and coded mask

Imaging over the hard X-ray energy band may be achieved by masking the flux with proper obstacles. The imaging modulation collimator has been developed and has been applied thus far, e.g., to produce hard X-ray pictures of the solar flare and to construct the X-ray image of the Crab Nebula up to the angular resolution of ～ 10 arcsec. Variations of the concept such as the Fourier Transform Telescope are discussed. Virtue of the modulation collimator is that high angular resolution may be achieved with a relatively simple detector system and that a wide field of view may be accomodated. Among several proposed coded masks, the techniques of Hadamard transform are discussed in some details. The coded mask is provided with a better total transmission, but its angular resolution is limited due to presently limited spatial resolution of the detector. Developments of the technique of the position sensitive detector are awaited for further improvements of the resolution.