Anomalous line shifts from local intensity gradients on the SOHO/CDS NIS detector

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 w ay that seems impossible to explain with a physical solar model. Line width s also show a correlation with local intensity gradients. The most plausibl e instrumental explanation seems to be an elliptical, tilted point-spread f unction 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 th an a Gaussian, leaving highly structured residuals after line fitting, incl uding `ghost' images in some pixel planes. The cause of these effects is ye t unknown, but they should warrant experiments on the engineering model to reproduce the observed effects, shedding light on the nature of the aberrat ions.