The 3.8m United Kingdom Infrared Telescope (UKIRT) is the largest
telescope in the world dedicated solely to observations at infrared
wavelengths between 1 micron and 30 microns.
The United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, Hawaii.
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
been carried out, co-ordinated by the JAC
with major hardware contributions from the Max Planck Institüt
für Astronomie, Heidelberg (MPIA), the Royal Greenwich
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
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.
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
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
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
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
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
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.