New LT Upgrades

Two important improvements to the LT control systems have taken place over the last few weeks. In summary these are:

The physical centres of the RATCam CCD and SupIRCam arrays are slightly displaced from the optical axis. The LT now introduces an offset to centre the requested coordinates on to the centre of the instrument field of view (subject to the normal telescope blind pointing error of around 10 arcseconds).

Due to flexibility problems with the coolant pipes for RATCam and SupIRCam, we were forced to dispense with the practice of routing them through the cable wrap in the cassegrain axis rotator. As a consequence of this, for the last several months we have effectively been operating with random detector orientations on the sky (in fact tracking from a fixed mount angle).

A new algorithim has now been implemented to move the rotator to one of the "cardinal" sky angles of 0, 90, 180 or 270 degrees. This means that images will be rotated to one of these angles, the choice being the one that will give the longest possible observing time before reaching a mechanical rotator limit. It is possible that the sky angle will change during a (very) long series of exposures - but not during an exposure!

The combination of these two changes means that the overlap between repeated images of the same field with the LT is now maximized, which is important for programmes relying on secondary standards within the fields. As a side effect, it also means that a standard "North at the top" image orientation is now simple to achieve in an image viewer, using only 90 degree rotations and "flips".