THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1999

3.3. Telescope and Enclosure

In July 1999, the MPIA guider system was extended to allow rapid adjustment of telescope focus. This configuration employs a 2 x 2 Shack-Hartmann lenslet array to measure focus at 60 Hz. The individual measures are averaged and then applied, and the process repeated, over a series of increasingly long integrations, ending with 32 s. The whole process takes about 62 s, replacing a less reliable procedure taking tens of minutes, greatly enhancing the efficiency and convenience of focus checks. The procedure gives the best-focus position of the autoguider assembly as set for the particular instrument: this (internal) setting is carefully remeasured after each engineering period. As a byproduct the RMS focus variation (the ``z-rms'') is determined. This is a pure function of seeing, which can therefore be investigated independently of telescope and instrument performance. This was used to show a clear ``dome seeing'' dependence on the temperature difference between dome and outside air, and also to aid in the flexible scheduling experiments (see §3.6.3). A new secondary mirror was installed in June 1999. The previously installed mirror suffered from lightweighting print-through and a considerable degree of trefoil aberration due to thermal effects in the mounting pads; the new secondary has none of these aberrations. The typical RMS wavefront error has been reduced from 350 nm to about 180 nm, only about 30% from formal diffraction-limited performance at 2µm.

The dome ventilation system worked well throughout 1999; the protocol for operating this system is now firmly established. There were few problems with telescope control, apart from some imbalances which occasionally caused slewing difficulties at large airmasses. It was decided to prepare for future implementation of the JCMT's Portable Telescope Control on UKIRT by installing an interface to the existing TCS. Work on this system progressed very well, and the TCS is not a critical item in the plan to have ORAC working for all instruments in summer 2000. In the run-up to the arrival of Michelle, efforts were made to better characterize changes in the telescope emissivity. The measurement protocol was tightened considerably, and detailed investigations of the emissivity spectrum were begun. A hydrocarbon emission feature, possibly originating in the mirror covers, was identified, which might account for some few percent additional emissivity when the 150 grating is used to perform the measurement. The comparison between emissivity and photometric zero-points proved powerful in analyzing the causes of emissivity changes, and the patchy nature of photometric zero-point determinations (which are largely tied to standards nights) will be improved with increasing use of ORAC to control observations. Once ORAC is fully in place, all aspects of telescope performance assessment will be consolidated through continual, semi-automated monitoring of focus measurements from the Shack-Hartmann system, photometric zero points and the seeing FWHM from every standard-star measurement.

Finally, the UKIRT control room was refurbished in early August 1999. It is widely agreed that the new room is a great improvement, and it certainly conforms more closely to recognized ergonomic and health/safety standards.