Thermal performance and facility seeing at the upgraded 3.8-m UK Infrared Telescope (UKIRT)

The upgraded 3.8 m UK Infrared Telescope employs active control of the primary mirror figure and secondary mirror alignment to constrain intrinsic wavefront errors, currently to approximately 180 nm, while a fast guider controls a tip- tilt secondary to remove telescope vibrations and tracking errors. It routinely produces images with FWHM below 0.'5 at 2.2 micrometers wavelength (the K-band). The best fully-sampled image yet recorded has FWHM equals 0.'171 and is believed still to be the best ever achieved by a ground-based telescope without the use of higher-order adaptive optics.