These are done after lunch to ensure the telescopes, instruments and image detectors (usually a CCD camera) are performing correctly. If everything is ok, an entry is made in the telescope log book formally handing over the systems for observing.
This also includes the mundane job of filling the CCD camera cryostats with liquid nitrogen (LN2). The INT has only one CCD camera in use. Either up at the prime focus for wide field imaging or mounted on the Intermediate Dispersion Spectrograph (IDS) at the Cassegrain focus. The WHT depending on what instruments are in use may have several cryostats that need to be filled and this can be quite time consuming.
Update Since I wrote this page back in 2006, an LN2 AUTO FILLING system has been installed on the WHT. This is connected by the Observing Support Assistant (OSA) at the end of the night and left running during the day. The OSA (in the WHT) is now the only staff member who is on site at night.
Click here to read about some of my experiences working as a night duty engineer.