Studies And Applications Of Direct Demodulate Method

In the analyses of low signal-noise-ratio, low statistical and low resolution data in high energy astrophysics, the Direct Demodulation Method (DDM) has been developed into a powerful method for image and spectrum recoveries. The main idea of DDM is resolving the modulation equation iteratively with physical constraints. DDM can use the information contained in the modulation equation sufficiently and derive images of higher resolution than the intrinsic resolution of instruments, which limits the resolution of conventional image reconstruction methods. However, DDM costs much memories and computation time and converges slowly. This restricts its applications. This thesis makes an in-depth study on DDM, optimizes it by three methods: Sparse matrices are used for the store of response matrices and corresponding computation to reduce the requirements for memories and computation time; Ordered-Subsets method is used to make iterations converge faster; The formula of iterations is transformed into convolution form for shift-invariant systems. The convolution form of DDM makes the storage of response matrix not a problem anymore and the convolutions can be computed by fast Fourier transform rapidly. These methods can be combined under certain conditions to achieve better performance. Based on these methods,DDM is applied to the data process of the coded mask system of IBIS/INTEGRAL and a collimated modulation telescope HE/HXMT. Some problems are solved, which arise when DDM and conventional cross-correlation method are used to deal with the real data from these two instruments. These problems include the problem brought by the unevenness of observations, the problem that the shift-invariant property is broken when low-sampling-rate data of shift-invariant system are used to derive high resolution images, the problem that the convolution form of DDM cannot be directly used to deal with the data of multidetector- collimated-modulation telescope like HE/HXMT, etc. These problems also often arise in data processing of other instruments. Based on the solutions to these problems, the angular resolution of coded mask systems is improved by about an order of magnitude, and the main performances of the all-sky survey of HE/HXMT, such as sensitivity, resolution, locating accuracy, etc, is obtained by simulation study. The simulation study also compares the performances of pointed observations and scanning imaging of different schemes of collimators. This comparison provides insights for the optimization of HE/HXMT, indicating that reducing the number of the types of FOVs from 18 to 2 would not degrade the imaging performance, but would decrease the complication of the data processing and increase the accuracy of off-axis pointed observations. Quick-look imaging, which is based on DDM, is proposed to obtain near-real-time two-dimensional images from one-dimensional scanning data. Finally, DDM library is designed and developed under the object-oriented programming methodology. Users of DDM library can write high-performance programs without much knowledge of DDM and the methods to accelerate it.