THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1997

4. Approved Programme

4.1. Completion of the UKIRT Upgrades Programme

The core elements of the programme which were brought into operation in 1996 have continued to function with gratifyingly little trouble (especially considering that the Upgrades tripled the complexity of the telescope systems). The tip/tilt system is used at all times except on the rare occasions when no guide star can be found. The Dome Ventilation System came into routine operation in late 1997, and has been shown to have a significant and beneficial effect on dome air temperatures. Sub-arcsecond images are routinely delivered. ¹

4.1.1. Telescope Optical Performance

Roughly once a month since late 1996 measurements of the telescope optical properties are secured at around 60 points over the sky using the wavefront curvature sensor. The results have been used to set up a lookup table by which the active optical control system of the primary mirror can correct for astigmatism, low-order (r) trefoil and spherical aberration, while the five-axis precision positioning system of the secondary (the hexapod) eliminates misalignment coma.

As it turns out, constant corrections suffice to remove all aberrations except astigmatism, which is removed by a simple hour-angle dependent correction. The stability of the alignment of the secondary and primary mirrors, in particular, is a major tribute to the excellence of the basic mechanical-optical design of the telescope by Dunford Hadfields and Grubb Parsons. As a result, the main use to which the telescope's powerful active optical capability (to insert and remove aberrations with a range of amplitudes) has so far been put, has been to test the operation of the Gemini Prime Focus wavefront sensor, which was calibrated in parallel observations with the UKIRT WFS on December 1997 and January 1998. This was done by providing aberrations of known amplitude for it to measure.

UKIRT's overall wavefront error is now typically around 300 nm rms over 95% of the optical diameter. This is mostly contributed by the secondary mirror, which as noted last year suffers from print-through of the lightweighting pattern from the rear surface to the figured front surface (up to about 200 nm peak-to-peak), a significant turned-down edge (reducing the effective diameter by 5%) and distortion by thermal effects in its mountings, which injects low- and higher-order (r) trefoil (the latter uncorrectable by the active-optical system) and spherical aberrations. The aggregation of these exceeds the ability of the primary mirror force actuators to correct them and so a compromise setting is currently adopted, which eliminates all astigmatism and most of the low-order trefoil but only a part of the spherical aberration.

In late 1997 agreement was reached with the MPIA for the provision of a new mirror, to be correctly figured right out to the edge, stress-relieved by rear-surface etching after lightweighting to reduce or eliminate printhrough, and equipped with athermal mounts. The procurement process proposed by the MPIA was verified to be both likely to succeed and very cost-effective. At the time of writing manufacture is well underway, and we are hoping for delivery of the new mirror in early 1999. If the new mirror measures up to expectations, the wavefront error is expected to be reduced to well below 100 nm peak-to-peak, substantially better than the goal of the upgrades programme of diffraction limited performance at 2 microns.

After the new mirror is checked out and installed, the present mirror may in turn be etched for stress relief. This, with athermal mounts, would leave it suffering only from the turned down edge. With a good-quality backup mirror available we can plan to employ advanced (low-emissivity) coatings, even if this requires the mirror to be sent away for quite long periods.

4.1.2. Focus Stabilisation

An automatic routine for maintaining telescope focus has been in operation since mid-1997. This applies corrections for the temperature of the steel trusses of which the telescope is built and for elastic deformations due to gravity (the telescope is longest when pointed at the zenith). The routine has performed remarkably well considering its simplicity. Users have sometimes to be reminded that focus checks (using semi-automatic routines now provided for both CGS4 and IRCAM) are still necessary if the best images are to be exploited, since defocus is still typically the largest degrading factor affecting imaging performance.

4.1.3. Dome Ventilation System

This critical but demanding system (which must function well if it is to be employed at all!) came into operation by stages in the second half of 1997 and early 1998. At the time of writing three of the 16 apertures await correction of mechanical problems, but use of the rest is routine and their control reliable. The DVS produces an obvious and dramatic increase in the flow of air through the dome; the vents have to be closed to reduce wind-shake in winds above 25 mph.²

4.1.4. Dome Floor Insulation

The proposal to insulate the dome floor, to prevent heat from the 8-inch concrete slab - with its long thermal time constant - being transferred to the dome air at night, has been complicated by safety issues. Tests at Keck after the disastrous Subaru fire revealed that all flammable materials tested caught fire much more easily on Mauna Kea than at sea level. (This is because, while there is still plenty of oxygen for combustion of most materials, there is only half as much air to cool the igniting object, making the process easier.) A serious fire at the CTIO 4m telescope occurred in an ethylene glycol gel in insulating material, which had formed after spilled coolant solution lost water by evaporation. Since we plan to cool the UKIRT primary using ethylene glycol solution, the combination with the proposed foam floor insulation caused uneasiness.

Neutral advice from a consultant, the above considerations and others of general engineering practicability led us, early in 1998, to abandon the overfloor insulation project. We will substitute partial underfloor insulation to isolate the warm crew room and may use surplus cooling power from the primary mirror system to cool critical areas of the dome.

4.1.5. Primary Mirror Cooling

Staffing shortages have been affecting the programme since late 1996. One of the main consequences has been a complete lack of progress during 1997 towards completion of the mirror cooling system. Work on this system has recommenced in 1998 and it is expected to be under test by the end of the year.