THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

6. UKIRT Upgrades

6.1. Top-End

The new top-end ring and vanes, and the secondary support and actuation systems, were delivered by the MPIA and installed on the telescope in mid-1996 and checked out with the bottom-end in August 1996 (see below). The hexapod positioning system allows precise positioning ( 2 microns) of the secondary mirror relative to the telescope structure and the primary. It has proved to be very stiff and no attitude-dependent coma is currently seen. Through the end of 1996 it has performed with complete reliability.

The tip/tilt system has performed well since installation. Its only problem has been a temperature-dependent failure to close the internal servo loop for large chop throws: the maximum throw available for chopping with tip/tilting on both images varies through the night, decreasing with the temperature of the top-end from about 30 arcseconds at top-end temperatures of 10 C to perhaps 15 arcseconds at -5C. Currently this can be worked around by resetting the unit at the new operating temperature.

The low-profile vanes are believed to have led to a significant reduction of wind-shake relative to the performance with the old top-end. However the data on which this assertion is based is scanty, deriving from the period between August 1996, when the top-end was installed, and November of that year, when the tip/tilt system came into routine use, since when windshake has been effectively imperceptible as long as a guide star is available. No-one, now, voluntarily observes without the tip/tilt system, so ``with vs. without'' comparisons are lacking.

The old top-end has been cut up and disposed of, while the secondary mirror systems are being prepared for display in the Lyman Museum in Hilo (along with the old f/9 secondary).