THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
2001-2002

3.3. Telescope and Enclosure

3.3.1. Telescope and Dome Control

The telescope performed reasonably well although telescope faults did lead to lost time during the report period. An RA drive fuse failed in semester 01A, producing serious tracking problems which took some time to trace. In 01B corrosion led to a failure of the primary mirror support, followed by a fault caused by a tip-tilt piezo shorting after winter storm damage. In 2002 the major telescope faults were jammed mirror covers, a drive failure due to a broken connection (which lost an entire night) and a failure of the primary mirror support system leading to oscillations in the mirror (again losing an entire night). The latter fault was traced to a bad valve, which was replaced.

In 2002 there were also major dome faults. One occurred after a storm with exceedingly high winds, which tore the exterior dome cladding. The tear was repaired by riveting an aluminium sheet over the broken section. Also, two of the three dome drives failed, leaving the dome running on one drive only, with no spare. Modifications to the dome software were made to allow the dome to run smoothly in this mode, and a replacement system will be installed in September 2003.

Major advances were made in 2001 with the Portable Telescope Control System, bringing us closer to replacement of the existing system, commonality between UKIRT and JCMT telescope control, and compatibility with UIST. Also in 2001 the telescope pointing was much improved when the new pointing model was installed. A new dome tracking routine, running under EPICS, was implemented. Considerable progress was made with the autoguiding systems. Autofocus mode was made to automatically offset the "guide telescope" to put the star in the Shack-Hartmann box, and the fast-guider acquisition mode was successfully commissioned. The crosshead code was updated in 2002 and the changes mitigated many of the time-out problems which have been in this system from its inception.

Cooling of the primary mirror was advanced in engineering in April 2001. Control was transferred to the control room Allen-Bradley panel; the system now tracks the dome temperature but further characterization and engineering work is needed. In 2002 wavefront sensing runs were carried out with both secondary mirrors, to determine their contribution to the telescope's spherical aberration; the difference was small and indicated that the primary is the main contributor. Top-end lab tests showed that the performance of tip-tilt with the heavier WFCAM secondary will be acceptable.

3.3.2. Emissivity

In the summer of 2001 the emissivity had risen to an unusually high level of 17% (in CGS4 echelle measurements). Dichroics were sent off to Zeiss for resurfacing; in the meantime the gold-patch dichroic was installed which resulted in a 5% improvement. Early in 2002, the standard dichroics were returned from Zeiss but found to be faulty. In 02A dichroics with acceptable coatings were delivered and installed, leading to an emissivity of 14% when measured with MICHELLE. This figure is consistent with the long period since the last aluminizing; after realuminizing in summer 2003 a major improvement is expected.

3.3.3. Image Quality

Seeing of quarter-arcsecond quality was reported on a number of occasions in late summer 2001. Statistically there is a clear improvement in the seeing through the night, suggesting some contribution from mirror seeing, or other local source; it is also possible that this is site-related, since Subaru has reported a similar effect. Early in 02A images were elongated after a long period during which poor weather precluded a quality assurance (QA) night. QA measurements were obtained as early as possible and the primary mirror support lookup table corrected.