Defining Telescope Pointing Model

At the start of every WHIRCAM run, and whenever the telescope structure is changed (eg if another instrument is changed during the day, which will cause a slight shift in the telecope balance), a pointing calibration must be performed on a set of astrometric stars. This is done by your TO, but it must be done with respect to the WHIRCAM ALICE display. Therefore, go into Movie mode (see § 3.5.4), choose the filter you'll be using most of the night, set the integration time to 0.3 sec, and aquire the first of the position standards (ask for the faint set, as the normal set saturate on WHIRCAM). You now need to define the the telescope rotation centre:

• (1) Position the telescope such that the star is near the top edge of the ALICE display, and mark this with a whiteboard marker pen.
• (2) Ask your TO to change the SKY PA to 180, and you'll see the star describe a half circle on the ALICE display, and mark this new position (If it looks like it will move off screen, mark the approximate centre of rotation, go back to SKY PA 0, move the star closer to the rotation centre, and start again.)
• (3) The telescope rotation centre will be the point halfway between your two marks.

The TO should then centre each of the position standard stars on this centre. You can find out where this centre is with respect to the WHIRCAM array by going into the `Set_graphics', `statistics_window' menus, and changing the X and Y position of the statistics box until it is centred on the rotation centre.