Spectrometer Slit Position
The blue dotted line marks the position of the spectrometer slit and is overlain on a series of MRI images taken at the same time as one of the spectrometer exposures. The red dot marks the position of one particular spectrum. Notice that the slit does not overlay the exact impact point, but is a little below it. This was quite fortuitous as the ejecta was very bright and would have saturated (overexposed) many more spectra instead of just the 2-3. Even in the MRI, portions of the image are saturated and show up as horizontal white lines/blobs (frames 7 and 8, where 1 is in the upper left and frame 12 is on the lower right). The shift of the slit off the impact point allowed us to watch the time evolution of the ejecta, as the material from the impact passed across the slit, material from later after impact came from deeper beneath the surface.
Another interesting item to notice is the shadow of the ejecta plume. It is best seen in frames 10-12 (bottom right) as the dark area just to the left of the ejecta cloud (white blob).