Vignetting                                                                    
                                                                              
The use of occulting disks in the LASCO coronagraphs causes vignetting        
of the images where the support arm holding the occulting disk obstructs      
the field of view, in the lower left quadrant of the images. The              
vignetting is different for each telescope due to different construction.
The C3 vignetting changed slightly after the interruption in 1998. The
C2 vignetting was not affected by the interruption.
                                                                              
To properly correct for vignetting, raw images must be multiplied by a
vignetting function. The vignetting function is an image which corrects       
for the obstruction by the occulter supports and the radial decrease in       
sensitivity.                                                                  
                                                                              
Vignetting images used in PDS calibration are (located in the 
/calib/vignetting/ directory):                                
                                                                              
C2   
     whole mission    c2vig_final.fit

C3                                                                         
     pre-interrupt    c3vig_preint_final.fit
     post-interrupt   c3vig_postint_final.fit
                                                                              
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Downloaded:                                                                   
http://sohowww.nascom.nasa.gov/solarsoft/soho/lasco/lasco/data/calib/         
Modified:       21 Feb 2006, Matthew Knight
                31 Jan 2008, Matthew Knight