THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

4. Report on Operations

4.3. Telescope Performance

Until the installation of the new top-end in summer 1996 the telescope pointing accuracy and long term tracking accuracy slowly degraded, with rms blind pointing accuracy across the sky deteriorating to about 10 arcseconds by early summer 1996 (the best performance achieved in previous years was 3-4 arcseconds). The poorer than usual pointing accuracy negatively affected a few observing programs, mainly those requiring blind pointing to invisible and infrared-faint sources and blind tracking (no guide star available). On average the accuracy of short term tracking also deteriorated during 1995 and early 1996. The well known RA oscillation (0.7 Hz, 0.3 arcseconds rms) continued. This oscillation inevitably resulted in images of point sources being noticeably elongated in right ascension, even when autoguiding was employed. In addition, from mid-1995 to mid-1996 the telescope suffered a large amplitude (1 arcminute) oscillation at an hour angle of about 0.25 hr east of the meridian, which persisted for about two minutes and made observations impossible during that short time interval.

Several investigations of these problems were made. However, because of the need for engineering and technical staff to concentrate effort on implementing the Upgrades Programme, a conscious decision was taken not to begin lengthy investigations into the tracking and guiding anomalies. This decision also was made in part because installation of Upgrades components, in particular the new top-end, scheduled for 1996, were expected to have major effects on the telescope tracking and pointing. It was decided to address these problems only where repair actions could be easily and quickly identified and taken. Some of the limited investigative work during 1996 was successful. The short term tracking problems were alleviated considerably in early summer 1996 by replacing drive electronics components and by freeing some cables on the telescope which had been binding. This reduced the short term RA oscillation to 0.1-0.2 arcseconds rms and eliminated the high amplitude oscillation. However, during 1995 and 1996 several investigations failed to find the cause of the poor pointing.

During the scheduled telescope down time in August 1996 for installation of the new tip/tilt top-end, a loose mechanical connection in one of the Serrurier trusses of the telescope was identified and retightened. Following this and the installation of the top-end, the telescope pointing improved to 1.5 arcseconds rms. This accuracy, which also was accompanied by improved accuracy of long term blind tracking, is 2-3 times better than ever before measured at UKIRT, and once again makes UKIRT one of the most accurate ground based telescopes in the world. Implementation of fast tip/tilt guiding has resulted in considerable improvement in short term tracking, virtually eliminating the effects of RA motion and windshake. Since autumn 1996 observers consistently report round, sub-arcsecond diameter images of point-like objects, even on long exposures.

During 1995-1996 telescope emissivity generally was maintained in the 10-13 percent region by cleaning it as needed, using the CO snow pellet technique described in the previous Annual Report. The primary and secondary were not re-aluminised during 1995-1996; it is planned that they will be re-coated in 1998. It is believed that installation of the new top-end, which has thicker supports than the old top-end, and the new dielectric dichroic (which is required for fast guiding on faint objects), have led to an increase in overall telescope emissivity by perhaps a few percent. In the future it is planned to make the undersides of the top-end supports fully reflective (at present they are black) and, once the Gemini coating plant is completed and available (in about the year 2000), use it to deposit lower emissivity coatings on UKIRT's primary and secondary mirrors. These should result in a reduction of the telescope emissivity to a value significantly less than 10 percent.