Anomalous Line Shifts From Local Intensity Gradients on the Soho/cds nis Detector

Abstract

Line shifts for some emission lines on the SOHO/CDS NIS detector appear to be strongly correlated with local intensity gradients along the slit in a way that seems impossible to explain with a physical solar model. Line widths also show a correlation with local intensity gradients. The most plausible instrumental explanation seems to be an elliptical, tilted point-spread function inducing the line shifts. A toy model demonstrating the essentials of the observed behaviour is presented. The effective point-spread function of the instrument appears to modify the line shape into something other than a Gaussian, leaving highly structured residuals after line fitting, including 'ghost' images in some pixel planes. The cause of these effects is yet unknown, but they should warrant experiments on the engineering model to reproduce the observed effects, shedding light on the nature of the aberrations.