SOHO's View of Mercury Transit
Since SOHO in its halo orbit was a bit "ahead" of Earth at the time, Mercury's shadow caught up with SOHO a little later than with observers on Earth. The transit itself started at 7:48 UT and ended at 13:16 UT; Mercury was, however, visible in EIT images against the extended corona for some hours before and after. Three instruments made a series of special observations:
MDI took a series of full disk continuum images at varying focus positions during the entire transit, in order to better determine absolute spacecraft roll and MDI absolute plate scale, and to better understand MDI image distortion. The varying focus positions are what causes the Sun to "breathe", or "pulsate" in size, in the movie.
CDS took a wide variety of observations (wide slit movies, narrow slit rasters, and narrow slit sit-and-stare sequences). One of the aims is to better characterize the three-dimensional (wavelength, X and Y) point spread function of the optics.
EIT took series of images in all four wavelengths, to improve models of stray light and their flatfielding.
