Background subtraction for the SECCHI/COR1 telescope aboard STEREO

COR1 is an internally occulted Lyot coronagraph, part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. Because the front objective lens is subjected to a full solar flux, the images are dominated by instrumental scattered light which has to be removed to uncover the underlying K corona data. We describe a procedure for removing the instrumental background from COR1 images. F coronal emission is subtracted at the same time. The resulting images are compared with simultaneous data from the Mauna Loa Solar Observatory Mk4 coronagraph. We find that the background subtraction technique is successful in coronal streamers, while the baseline emission in coronal holes ( i.e. between plumes) is suppressed. This is an expected behavior of the background subtraction technique. The COR1 radiometric calibration is found to be either 10 - 15% lower, or 5 - 10% higher than that of the Mk4, depending on what value is used for the Mk4 plate scale, while an earlier study found the COR1 radiometric response to be ˜ 20% higher than that of the Large Angle Spectroscopic Coronagraph (LASCO) C2 telescope. Thus, the COR1 calibration is solidly within the range of other operating coronagraphs. The background levels in both COR1 telescopes have been quite steady in time, with the exception of a single contamination event on 30 January 2009. Barring too many additional events of this kind, there is every reason to believe that both COR1 telescopes will maintain usable levels of scattered light for the remainder of the STEREO mission.