The United Kingdom Infrared Telescope

Like all large modern telescopes UKIRT is a reflecting telescope. Infrared and visible radiation from an astronomical object (e.g., a star) is collected, reflected and focussed by a concave primary mirror 3.8m in diameter. Just before it gets to the focus a smaller convex secondary mirror at the top of the telescope reflects this steeply converging beam of radiation and directs it downwards, now converging much more slowly, through a central hole in the primary mirror to a flat tertiary mirror, below, which directs it to one of four instrument focal stations. The telescope's current and future instrument suites are described in a separate document.

Although UKIRT was originally conceived and built as a low cost facility and its original performance requirements were quite relaxed relative to those of other telescopes, its actual performance has been steadily enhanced over the years. Since 1991 a determined programme of upgrades has been carried out, co-ordinated by the JAC with major hardware contributions from the Max Planck Institüt für Astronomie, Heidelberg (MPIA), the Royal Greenwich Observatory (RGO) and the Royal Observatory, Edinburgh (ROE) who also provided central project management. As a result the telescope's optical quality is good enough to provide images which, in sharpness, are close to the limit set by the wave nature of light (the "diffraction limit") at wavelengths of 2 microns or longer (0.11 arcsec across the width of a point source image). In good atmospheric conditions (good seeing) images of stars less than 2.5 times bigger than this limit have been seen.

Because seeing is better in the infrared than at visible wavelengths, and because the telescope optical performance has been enhanced to match, this image quality is better than that of optical telescopes of similar size. In fact at a wavelength of 2 microns (the K band) the best UKIRT images thus far recorded rival those of the Hubble Space Telescope's NICMOS infrared imager for sharpness: see the image of M100 (NGC 4321) in the image gallery.

Overall Design

Until very recently, large telescopes have normally been designed with thick primary mirrors to prevent the mirror sagging under its own weight. UKIRT, however, was designed to be as inexpensive as possible to build, and the 3.8m primary mirror is much thinner than was then customary, weighing only 6.5 tonnes. It and its supporting cell (a structure of welded steel plates weighing 20 tonnes) are mounted at one end of an open frame of steel beams. At the other end is the small convex secondary mirror (see below), which is supported on a "spider" of four narrow steel vanes attached at their outer edges to a removable top-end ring of sturdy steel tube. The whole open frame, the so-called tube, swings around an east-west axis inside an octagonal yoke, which itself rotates about a north-south axis parallel to that of the Earth, between two steel piers. It is rotated by its drive motors at the precise rate of the Earth's rotation but in the opposite direction. In this way the telescope can point at and track objects as they cross the sky. However this simple, robust (and inexpensive) support system, known as an English Equatorial mounting, does not allow the telescope to point at the pole of the sky (in fact UKIRT cannot point further north than +60 degrees declination).

The precise shape of the primary mirror was figured by the now-defunct UK company Grubb Parsons, of Newcastle-upon-Tyne, who were the first to develop techniques for the accurate manufacture of such large thin optical components. Similarly new and sophisticated engineering techniques were developed by the also-defunct engineering firm of Dunford Hadfields to support this lightweight mirror in the telescope while keeping its shape as close to the ideal as possible. As a result the telescope is very light for its size and employs many innovations recently adopted by telescope designers seeking to build instruments with the best possible optical performance and least possible locally-generated atmospheric turbulence ("facility seeing").

All movements are controlled by computer, which determines the telescope's position by means of encoders on the axes. The encoder readings are corrected for various effects (atmospheric refraction of light, flexure of the telescope structure, etc.) according to a "pointing model", which is regularly updated by tests, and a final accurate position is thereby determined. At present UKIRT can point at a specified position anywhere in the sky with an absolute (blind) accuracy of about 1.3 arcsecond.

Active Optical System and "tip-tilt" Secondary Mirror

The primary mirror support system has recently been modified to provide active control of its shape and thus maintain its surface figure with greater precision and in all telescope positions. This has been done using a very simple approach employing twelve actuators around the edge, which can apply upward or downward forces of up to 500 N (~50 kg weight) which thereby bend the mirror. By this means the simpler (low-order) optical aberrations (those most likely to be caused by the changing force and direction of gravity as the telescope moves, for example) can be corrected.

UKIRT's secondary mirror is only 314 mm (about 12 1/2 inches) in diameter and is supported by a custom-designed, computer-controlled hexapod built by Physik Instrumente of Germany. This six-legged mounting allows the mirror to be moved and positioned with great accuracy (about 2 microns, or 1/12000 inch) in any or all of five axes (x, y, z, tip, and tilt). The first two of these provide positioning of the secondary on the optical axis of the primary mirror, the third allows focussing of the telescope, and the fourth and fifth allow alignment of the reflected beam parallel to the optical axis.

The secondary mirror is attached to its hexapod support by three piezoelectric stacks, which can each push or pull on the mirror. These allow it to be slightly rotated in two directions ("tip" and "tilt") and moved up and down for fine focus. These movements can be extremely rapid. Under the control of the Fast Guider (see below) this system eliminates image motion due to the atmosphere (seeing fluctuations), windshake of the telescope, or tracking imperfections. Dramatic improvement in image quality is apparent when this fast tip/tilt system is used, as it now is on all possible occasions.

Up to four instruments are mounted below the mirror cell and can receive the infrared beam via by a four-position rotating mirror. This is a dichroic, i.e., it reflects infrared radiation (sideways) to the instruments, but transmits visible light (downward). Here it can be sent either to a TV camera, for target recognition, or to a Fast Guider system employing a low-noise fast-readout CCD detector. This tracks the image of the target (if brighter than a red magnitude of ~18.6) or of a nearby "guide star", measuring its location at up to 100 times per second and sending corrections to the tip tilt secondary mirror, and to the main telescope drive, so as to hold it accurately steady at all times. The guide star can be up to 3.5 arcmin away from the target on the sky, as the entire guider assembly is mounted on an X-Y crosshead driven by precision screws, and can be moved off-axis to reach the guide star.

Accumulated measurements of the telescope optics (using an instrument called a Wavefront Curvature Sensor which normally occupies one of the four instrument ports) have been used to construct a table of settings of the primary mirror figure control system and secondary hexapod system. This is used to maintain accurate optical figure and alignment at all telescope orientations.

A critical factor in achieving superb image quality is the telescope focus. Because it is made of steel, with a large coefficient of thermal expansion, this changes rapidly in the early part of the night as the telescope cools down. The length of the telescope also varies according to the position it points to on the sky, being longest when pointing near the zenith. A simple thermal-mechanical model of these effects is now used to correct them, and the focus is for the most part extremely stable.

Currently the wavefront delivered by the telescope optics deviates from its ideal shape by less than 400 nanometres RMS. Planned improvements, especially a new secondary mirror, should reduce this by a factor of four or more, when the telescope will be fully diffraction limited at the reference wavelength of 2.2 microns, and able to exploit the very best seeing to the full.

Dome, dome ventilation and mirror cooling.

The telescope is housed in a dome which opens to allow the telescope to view the sky and closes to protects it from the weather. Inside, on the north-east side, is the control room for operating the telescope and its instruments and in an adjacent extension are a computer room and laboratories and workshops for preparing and repairing instruments and electronics. A small kitchen and dining area are available downstairs, in the building basement.

Temperature differences between the air inside the dome and that outside result in the mixing of warm and cold air and consequent blurring of the images of stars seen through it. To reduce or prevent such dome seeing, UKIRT uses ventilation both forced (via a powerful extractor fan system) and natural (by wind through sixteen louvred openings in the dome) to maintain the night-time temperature of the air in the dome as close as possible to that of the air outside. The dome is very compact, which greatly assists this process. The main remaining night-time heat source is conduction through the floor, part of which is above the downstairs kitchen. The underside of the floor, over the kitchen, will be insulated in the near future.

Similar effects, termed mirror seeing, can occur when the primary mirror is significantly warmer than the air around it. Starting in the last quarter of 1998, it is planned that a primary mirror cooling system will be brought into use at UKIRT. During the daytime the mirror will be maintained close to the predicted night-time temperature by blowing cold dry air across its surface to provide up to 5 kW of cooling power.

By these methods it is planned to reduce the deleterious effects of telescope and dome (so-called "facility seeing") to a minimum, and with luck to eliminate them for much of the time. This will leave UKIRT able to exploit the superb natural seeing so common on Mauna Kea.