<title>Postupgrade performance of the 3.8-m United Kingdom Infrared Telescope (UKIRT)</title>

Abstract

The upgraded 3.8m UK Infrared Telescope (UKIRT) is now provided with:. Tip-tilt image stabilisation by a light-weighted secondary mirror on piezo-electric actuators, controlled by a fast guider sampling at 40 Hz on guide stars V 18.m6;. Active primary mirror figure and secondary mirror alignment control, via a regularly-maintained look-up table (LUT);. Active focus measurement and correction by the fast guider, supplementing a focus maintenance model which corrects for elastic and thermal changes;. Ventilation of the 2600 m dome by 16 apertures totalling 50m2;. Insulation of the underside of the concrete dome floor; . Internal air circulation during the day, to reduce heating of the upper telescope steelwork.In 1998 a campaign of K-band image quality (FWHM) measurements showed that after correction for undersampling the median FWHM was 0.433 and 10% of images were indistinguishable from the diffraction limit. Fully sampled images in September 1998 confirmed the correction function; these have FWHM = O."178 (Strehi ratio O.25) and are believed still to be the best ground-based images ever secured without the use of higher-order adaptive optics. Over 21 nights in that month the median FWHM was 0.265. A new secondary mirror has recently been installed and has reduced the RMS wavefront error from -360 nm to -180 nm (in the first instance). A new campaign of image quality measurements has been initiated. The primary mirror cooling system is complete and beginning its operational checkout.

Seeing statistics at the upgraded 3.8m UK Infrared Telescope (UKIRT)

“…Between 1991 and 1997 the UK Infrared Telescope (UKIRT) was the subject of a systematic campaign of improvements, the UKIRT upgrades programme, which had the explicit goal of providing imaging performance competitive with the best ground-based facilities (Hawarden et al 1994(Hawarden et al , 1996(Hawarden et al , 1998. By active control of the primary-mirror figure and secondary-mirror alignment (Chrysostomou et al 1998) and tip-tilt actuation of the secondary mirror under control of a fast guider, this goal has been met (Hawarden et al 1999(Hawarden et al , 2000. The telescope routinely delivers images with FWHM less than 0.…”