When the two STEREO spacecraft move into positions on opposite sides of the Sun, we will have the capability to see a full 360 degrees around the solar sphere (there will probably still be some gaps in visibility near the poles of the Sun). Combined with solar observing satellites near the Earth, such as SDO and SOHO, this coverage will last for about eight years and the STEREO spacecraft move along in their orbits.
In this movie, we zoom in towards the Sun, fading from a visible light view to data from the 304 Ångstrom filters aboard SDO and both STEREO spacecraft. We swing the camera around to a view of the side of the Sun NOT visible from the Earth. With the STEREO and SDO data mapped to the sphere representing the Sun, we see the dark sliver of "No Data" which slowly shrinks as the STEREO spacecraft move into position 180 degrees apart on opposite sides of the Sun (and 90 degrees from Earth).
STEREO data near the edge of the solar disk get stretched when projected onto a sphere and is responsible for the streaking on either side of the dark sliver. These data are sampled roughly six hours apart for each frame of the movie. Slight differenences in the six hour time step creates a slight 'jitter' of the dark sliver.