UKIRT is a 3.8m classical cassegrain telescope with a thin primary mirror utilising an “English” yoke mounting. It has been the subject of a concerted Upgrades Programme since 1990. It currently provides:

• Sky access from +60° 07′ to -40° (equinox of date).
• All-sky absolute pointing with RMS errors ~ 1.”3.
• “Encoder” offsets over a few degrees to ±0.”3.
• Cross-head offsetting up to 3.’5 off axis; currently to <0.”1.
• Image stabilisation to <0.”1 (in winds <45 mph) by a fast tip-tilt secondary mirror.
• Tip-tilt control by a CCD Fast Guider, on G or K stars with V > 18. ^m6 in dark conditions.
• Auto-focus using the fast guider; focus actively maintained using a thermal and elastic model of the telescope. 
• Active correction of primary figure and alignment by a lookup table. This, and optical performance, are regularly monitored.
• Active and passive dome ventilation and internal air circulation 

Between 13 February 1998 and 29 September 1998 the delivered K-band image quality (on exposures >tens of seconds) was systematically monitored and corrected for undersampling. The results showed:

• Median FWHM ~0.”433;
• Best quartile FWHM <0.”25;
• Best directly measured FWHM = 0.”171 (Strehl Ratio ~25%);
• About 10% of the time the image FWHM may be <0.”15 (NB: the FWHM of a diffraction limited image is ~0.”11 at K).
• In September 1998 the median corrected FWHM was 0.”265