AAS Meeting #193 - Austin, Texas, January 1999
Session 11. Observatories, Telescopes and Instruments
Display, Wednesday, January 6, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[11.08] The UK 3.8m Infrared Telescope (UKIRT): from Light Bucket to the Diffraction Limit

T.G. Hawarden (JAC Hilo Hawaii), D.G. Pettie (UK Astronomy Technology Centre Edinburgh Scotland), C.P. Cavedoni (Gemini Telescopes Hilo Hawaii)

The UK Infrared Telescope (UKIRT) was built in the 1970's as a ``light bucket'' with very relaxed performance specifications, costing less than $ 5M. The design employed a thin primary mirror on 80 pneumatic supports, lightweight structure and a very compact dome, foreshadowing the designs of modern high-performance telescopes. Since 1991 a programme of upgrades has exploited this fact, and outstanding imaging performance is now being achieved.

The primary figure is actively adjusted by 12 peripheral force actuators. These, and the secondary mirror position, are controlled by a lookup table to correct astigmatism, coma, trefoil and sperical aberration. Focus is maintained by a thermal/elastic model. Currently about 360 nm of RMS wavefront error remains uncorrected, but should be largely eliminated by an athermally-mounted replacement secondary, making UKIRT's optics diffraction limited down to the J-band.

Images are stabilised by the high-bandwidth tip-tilt secondary, controlled by a fast guider CCD sampling at 40-100 Hz using guide stars with V<18.6. Dome seeing is reduced by extraction of 13 volumes/hour and by 40m2 of ventilation apertures; dome and outside air temperatures usually converge by midnight, and shortly even faster when insulation and daytime air cooling and circulation are implemented. Cooling of the primary will also commence in early 1999.

K-band image sets from 139 half-nights between February and October 1998 have median (undersampling-corrected) FWHM of 0.444 arcseconds; around 9% have FWHM near the 0.11 arcseconds diffraction limit. September's median is 0.265 arcseconds, including a fully-sampled set with FWHM of 0.18 arcseconds which may be the best ground-based images without higher-order adaptive optics yet measured. We expect even better images than these to be even more frequent in the near future.

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