Fig 1: The view of Uranus from Earth, watching as the planet rotates under the slit
Though Uranus is tilted onto its side from the plane of the ecliptic, it doesn't point towards us. The North pole is facing away from the sun, and is close to the limb, so that we cannot use a slit rotation in order find our required positions.
Also we align the slit with the rotational axis as close as possible, to reduce the rotational component to zero. By aligning the slit along the rotational axis, the North Pole P.A. ~58° for June. We then can watch the planet rotate underneath the slit.
Fig 1: The view of Uranus from Earth, watching as the planet
rotates under the slit
The reason for taking a number of nights of exposure is to allow the planet to have rotated underneath us, in order to sample a reasonable variety of planetary CML's. We must have calculated wrongly for the rotation rate produced, but exact timing isn't so important, as long as there is a general span of time that can be covered. One way that you could adjust the CML would be to alter the time you take each observation, so that Uranus has rotated in the sky, but this isn't as easy, because Uranus only appears in the morning in June, with transit times of 5:30->3:30 (the later in June the better), so there isn't much room to play with. Every hour adds ~20°, so you could delay each observation by an hour from the last, which is possible within the time needed, but I would be worried about getting into either too high airmass at the start, or too much light at the end.
So - it was a mistake I should have caught in my rush to get the proposal out, but it doesn't really effect our ability to get good science. We don't get that control night, but do get a wider coverage - actually, looking at the potential positions, I should have asked for five nights, but I wanted to keep the hours used as low as possible. Nevermind.
Below is an image that shows the predicted positions with the same time each night, with guess positions for the the magnetic poles. You can see that even if the one pole lands in the middle of the gap between the 2 and 3 night, the other should be close to 1 and 4, because of our wide coverage. I've included 5 and 6 to give you an idea of the coverage provided by possible extra nights. The 6th night covers well for the 1st night, the 5th night covers for the other three (in order to provide the widest coverage).
Fig 2: Looking up from the south pole, and seeing a schema of
the coverage provided
Hope this is a clarification of what we intended the observations to be... Any more questions, please do e-mail me back.
Tom.