LHC portal

Does anybody know how the "cameras" work that are used to take the pictures of the beamline intersections that appear from time to time on the "LHC1" page?
I read that they are movable with one camera for low intensities, one for high intensities and one position to let the beam pass. They are in transfer lines, in the circle (but these should only be used when the beam is dumped immediately, without passing the camera a second time) and in the dump-tube. One can for example see on the pictures in the dump-tube that the beam is distributed on a circle to reduce intensity per area.
But I still wonder if the cameras are only moved when there is no beam, or also when there is, why there appear several spots sometimes, how the picture is read out, and what effect passing the screen has for the beam....

Here is what I have guessed...

When there is one bunch in the beam (or perhaps the beam is unbunched) there is only 1 spot... The 4 spots are, I assume, each of the 4 bunches in the beam - looking at other screens it seems the beam wanders about in the tube quite a bit which would explain their rather large scatter - I don't think the distribution is completely deliberate; when you want the collisions you probably want all bunches in line or as close to that as possible.

When the beam is kicked to dump you get a nice swirl on the output... there was a pptx file somewhere in CERN (I think it was in the progress link from here) which explained that thinking about it there was a post on here that mentioned the cameras... It was one in the threads about "what is page 1?"

edit: look at the pptx for 11th down in the progress link.... some really nice camera shots of the dump cameras showing the 4 bunches and the pilot beam

B.