Tests of Solar X-Ray Image Reconstruction: A New Index for Assessing Image Quality

Abstract Indirect X-ray modulation imaging has been adopted in a number of solar missions and provided reconstructed X-ray images of solar flares that are of great scientific importance. However, the assessment of the image quality of the reconstruction is still difficult, which is particularly useful for scheme design of X-ray imaging systems, testing and improvement of imaging algorithms, and scientific research of X-ray sources. Currently, there is no specified method to quantitatively evaluate the quality of X-ray image reconstruction and the point-spread function (PSF) of an X-ray imager. In this paper, we propose percentage proximity degree (PPD) by considering the imaging characteristics of X-ray image reconstruction and in particular, sidelobes and their effects on imaging quality. After testing a variety of imaging quality assessments in six aspects, we utilized the technique for order preference by similarity to ideal solution to the indices that meet the requirements. Then we develop the final quality index for X-ray image reconstruction, QuIX, which consists of the selected indices and the new PPD. QuIX performs well in a series of tests, including assessment of instrument PSF and simulation tests under different grid configurations, as well as imaging tests with RHESSI data. It is also a useful tool for testing of imaging algorithms, and determination of imaging parameters for both RHESSI and ASO-S/Hard X-ray Imager, such as field of view, beam width factor, and detector selection.