A.LLebaria
LAS (CNRS)
S.Aubert
LAS (CNRS)
P.Lamy
LAS (CNRS)
S.Plunkett
SRG (Univ. of Birmingham)
The LASCO coronagraph experiment aboard the SOHO spacecraft has been in sucessful operation since January 1996. The instrument consists of three coronographs of 3, 6 and 30 solar radius of field of view. The intermediate coronagraph, LASCO-C2, suffers from moderate distortion effects which need to be corrected in order to properly locate the stars crossing its field-of-view and get critical attitude parameters and in-orbit photometric calibrations. Practical considerations prevented a accurate calibration before launch. This has been done in orbit using stars as absolute reference.Stars of V magnitude brighter than 8 were used as references. Calibration procedure consistes of several steps: 1) detection of stars by correlation of successive frames, 2) star identification and initial estimation of optical parameters (focal length) and orientation from independent frames, 3) fine estimation from a large sequence of images, 4) estimation of intrinsic distortion parameters. This paper describes these procedures wich extensively use image processing methods and adresses interesting issues related to specific problems of externally occulted coronographs (center of field blindnees, strong vignetting and stray light limits) and spatial instrumentation (cosmics rays and limited number of reference points).