A Study of Vignetting on Images Taken with the C3 Coronagraph Aboard the SORO Satellite

Abstract

I worked at the Naval Research Lab, in Washington DC, with the solar physics group. The solar physics group is responsible for conducting research using data from a number of solar satellites. My project consisted of working on the correction and measurement of vignetting present on images from an instrument on one of these satellites. Vignetting is an optical phenomenon in which light is focused or blocked in such a way as to wholly or partially miss a lens or imaging device. In practice this often means some feature of the data taking instrument, such as an occulting disk or a stop, blocks a section of the object from being recorded. This appears to us as a gradual fading out, or darkening, of a section of the final image. In fact vignetting may become so severe that some image sections may be blocked out entirely. This obviously causes difficulty with image analysis, and so I was instructed to gather data about the location and intensity of this vignetting so that it could be decided what course of action should be taken to correct it. My project revolved around working with images of the sun taken by the satellite. This was made more complicated because I had to use IDL, the programming language in which all work in the branch was done. IDL is an interactive language that is effective in image and data manipulation. It is also an array oriented language with many mathematical and graphical routines built into it. Since it is a language that is most often used in astronomy, and so is probably unknown to the reader, I did not make many specific programming references in the text. The programs that I wrote can be found in appendix A. The following two sections explain necessary background to the reader, such as information about the sun and the satellite. Following this the project procedure is explained in detail. The results of this analysis conclude the paper.