A coronagraph blocks out the disk of the Sun in order to observe the faint emission coming from the surrounding corona. This means that there is an uncertainty in knowing exactly where the Sun is. There is also an uncertainty in the value of the roll angle, the angle between the "top" of the image and solar north. Operationally, the pointing legs on LASCO are commanded to move the entire LASCO optical box to make the corona appear to be symmetric around the occulter. This can be done only for one of the three telescopes and C2 has been selected as the primary determinant of the pointing because it has the most stringent pointing requirements. The last time the legs were moved was April, 1996. To calculate the position of the Sun and the roll angle the stars that are present in each image are used. This is possible for the C2 and C3 images in virtually every coronal image because each image has many stars. For C1, the passage of Alpha Leo, which occurs every August, is used to determine the same information. An IDL program has been written to transform the position of the stars and the Sun to the focal plane for the SOHO orbit. The output of the program is used to determine the geometric distortion of the optical systems, the row and column positions of the Sun center and the roll of the image with respect to solar north. In addition, it also provides an experimental determination of the vignetting function. You may view PNG files of the Sun center and roll angle determinations: Plots of Roll Angle for January 1996 to August 2007 from SOHO attitude files C2 Sun Center X position (Column) - c2_xpos_960124_070829.png Sun Center Y position (Row) - c2_ypos_960124_070829.png Solar Roll Angle (Full Scale) - c2_roll_960124_070829.png Solar Roll Angle (Expanded Y-Scale) - c2_roll_exp_960124_070829.png C3 Sun Center X position (Column) - c3_xpos_960116_070829.png Sun Center Y position (Row) - c3_ypos_960116_070829.png Solar Roll Angle (Full Scale) - c3_roll_960116_070829.png Solar Roll Angle (Expanded Y-Scale) - c3_roll_exp_960116_070829.png Also provided are PNG files of the LASCO pointing eyes (BoreSighters and Pointing Error Sensors) plots: LASCO BS X and Y - bs.gif LASCO PES X and Y - pes_a.gif From the analysis C2 and C3 are found to have a slight roll offset from each other. C2 is offset from solar north by +0.50 degrees and C3 is offset by -0.24 degrees. Notice that there is a slight variation (<0.1 degrees) throughout the mission. The plots also indicate variations in the X and Y positions of the Sun. However the variations are so large that they are suspect, since it is known that the spacecraft doesn't experience such large variations, its pointing eyes (the PES and boresighters) and the images themselves don't show those excursions. The large excursions may be a problem with the orbit positions or the translation of the stellar ephemerides to the SOHO orbit or to the procedures. The images do indicate small excursions which are presumably due to thermal distortions in the instrument (not in the spacecraft). Therefore only the overall average of the row and column positions of the Sun are believed. 25 August 1999 R.A. Howard Text downloaded: http://lasco-www.nrl.navy.mil/stars/sunpos.html updated: M. Knight 2006-03-15; 2008-01-27 Images downloaded: http://lasco-www.nrl.navy.mil/images/stars/ M. Knight 2008-01-21