The Vignetting Effect of the Soft X-Ray Telescope Onboard Yohkoh: II. Pre-Launch Data Analysis

The vignetting effect in the Soft X-ray Telescope (SXT) onboard Yohkoh is studied using a two-dimensional distribution of the effective area in the field of view taken from a pre-launch experiment. Our initial estimation of the vignetting in SXT was carried out by fitting the data with a second-order polynomial function. The results show that a two-dimensional vignetting effect in SXT is not rotationally symmetric, particularly in the y\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$y$\end{document}- (north–south) direction, which is probably due to the offset of SXT’s optical center from the location of its CCD center. We adopted a combined functional form consisting of a second-order polynomial function and a Gaussian function to take this asymmetric distribution of the effective area into account. We also considered the steep gradient of the effective area variation at the region near the vignetting center for the case of higher photon energy with this form. We completed a two-dimensional description of the vignetting effect in SXT by a spline surface fit using the “cleaned” effective area data whose noise was reduced satisfactorily by the fitting of our combined function.