Postupgrade performance of the 3.8-m United Kingdom Infrared Telescope (UKIRT)

The upgraded 3.8 m UK Infrared Telescope is now provided with: (1) tip-tilt image stabilization by a light-weighted secondary mirror on piezo-electric actuators, controlled by a fast guider sampling at >= 40 Hz on guide stars V m6; (2) active primary mirror figure and secondary mirror alignment control, via a regularly-maintained look-up table; (3) active focus measurements and correction by the fast guider, supplementing a focus maintenance model which corrects for elastic and thermal changes; (4) ventilation of the 2600 m3 dome by 16 apertures totalling 50 m2; (5) insulation of the underside of the concrete dome floor; and (6) internal air circulation during the day, to reduce heating of the upper telescope steelwork.