150-in mirror, UKIRT, Hawaii, 1976

UKIRT website: http://www.jach.hawaii.edu/UKIRT/

The United Kingdom Infrared Telescope (UKIRT) (Notes by John NIchol)

This telescope, brought into operation in 1979, was a joint undertaking between Grubb Parsons who made the optics, and Dunford Hadfields who were responsible for the telescope structure and mount. What follows is a brief account of the making of the optics under the guidance of David Brown who was technical director at Grubb Parsons, but I will begin with the historical background which resulted in the birth of UKIRT and influenced the design of the great telescopes which came later.

In 1967 Professor Jim Ring set up an infrared astronomy group including instrument and telescope design. Infrared Astronomy is the detection and study of infrared radiation (heat energy) emitted from objects in the universe. All objects emit infrared radiation. So, Infrared astronomy involves the study of just about everything in the universe. In astronomy the infrared region lies within the range of infrared detectors, which is between the wavelengths of about 1 and 300 microns (a micron is one millionth of a meter). The human eye detects only 1% of the light at .69 microns, and 0.01% at 0.75 microns, and so effectively we cannot see wavelengths longer than about .75 microns unless the light source is extremely bright.

The group wanted to investigate the viability of thin mirrors; traditionally telescope mirrors had a diameter to thickness ratio of 6:1. This helped control flexure in the glass and reduced distortion of the mirror when it was at different angles in the telescope. As a result large telescope mirrors were massive. If thinner mirrors were viable their mass would be reduced which would result in a less massive telescope structure, and crucially, a reduced cost to manufacture. Calculations by John Long, one of Ring’s team, showed that with the correct axial support system a ‘thin’ mirror could give diffraction limited performance at mid-infrared wavelengths. An optical system with the ability to produce images with angular resolution as good as the instrument's theoretical limit is said to be diffraction limited. The poor detectors used at the time for infrared astronomy would be the limiting factor. So, a telescope with a thin mirror (sometimes referred to as a flux collector or light bucket) would be acceptable for IR astronomy. As a result a 1.5 m telescope saw first light in 1972 and was used successfully for IR observations. The path for a larger instrument was now clear.

Design studies for a 4m IR telescope were begun in 1973 with Grubb Parsons (Newcastle) and Dunford Hadfields (Sheffield) taking part, both had previous involvement with the 1.5m telescope. The studies resulted in Jim Ring proposing a larger telescope and in 1974 the 3.8m IR flux collector proposal was accepted in principle, it was approved in 1974 with Jim Ring as project scientist. The telescope would eventually be known as the United Kingdom Infrared Telescope, or UKIRT. The final specification for image quality was that 98% of the encircled energy would fall within a 2.4 arc second circle. The distance between two stars in the sky may be stated in degrees, one arc second is equal to 1/3600 of one degree.

Pyrex, a low expansion glass, was once the choice for large telescope mirrors but by the 1970’s glass technology had moved on and materials with much better thermal properties were now available. The Schott Company produced Zerodur which was ideal for telescope mirrors, but a blank would not be available before May 1976. An alternative was Cer-Vit made by Owens Illinois, they had a blank in stock but it was cracked. When sliced in half a blank free from defects and of the correct thickness resulted. UKIRT had a mirror blank weighing 7 tonnes, about one third the weight of a traditional mirror blank. David Brown of Grubb Parsons was initially uncertain about the accuracy attainable with such a thin mirror, the contract for the mirror allowed for a more accurate specification than originally stated if this was found to be attainable. The mirror blank was delivered to Grubbs in 1975. During working the blank was to be supported on an 80 pad axial system to prevent flexure. A 1 m diameter hole was cut in the middle of the blank. 24 Invar pads were cemented round the edge of the blank to provide contact with the counterweight outer radial lever arms of the mirror cell. Invar is a Nickel – Iron alloy noted for its very low thermal coefficient of expansion meaning that it will remain stable over a wide temperature range. The front surface of the mirror blank was ground to a curve with a central depth of about 10 cms. After being fully polished the process of changing the mirrors surface to the exact curve required was started, this process is called figuring. The figuring cycles lasted 2-3 days after which the mirror was tested from above in a test tower at the centre of curvature of the mirror using a combination of the Hartman test and wave shearing interferometry. The Hartman test involves placing a perforated screen in front of the mirror under test, a light source is directed at the mirror from the centre of curvature. Light returning through the perforated screen forms a pattern of dots; these are measured at different positions to determine the profile of the mirrors surface. The wave shearing interferometer works by measuring differences in surface contour between adjacent points on the mirror. David Brown introduced shearing interferometry to the industry; it became the standard test method for Grubbs.

During figuring the mirror would be tested, the results analysed and the next figuring cycle computed. During each test cycle the mirrors’ surface would be examined for small bubbles which might break through to the surface releasing small shards of glass might scratch the mirrors surface under the action of the polishing lap. If found such bubbles were drilled out to prevent this. Much care was taken to ensure that the mirrors’ surface remained scratch free however, one scratch did occur due to unforeseen circumstances. In the workshop high above the mirror one of many fluorescent lights failed resulting in the surface of the mirror being showered with splinters of glass from the tube. Despite this the subsequent clean up and examination revealed only one small scratch on the mirrors surface. As a result fine mesh covers were fitted to all lighting which posed a potential threat. In addition to making the 3.8m diameter primary mirror for UKIRT Grubbs also made a 1m diameter secondary mirror and a flat mirror for a Coudé focus.

The above diagram shows the arrangement of the optical components in the UKIRT and many modern telescopes. On the right is the main or primary mirror, 3.8m in diameter in the UKIRT, this is concave and has a parabolic shape (in a Ritchey-Chretien Cassegrain telescope the shape is hyperbolic). On the left is the secondary mirror which is convex and has a hyperbolic shape. In between the two, closer to the primary mirror, is a flat mirror used to deflect the light beam out to one side of the telescope tube. Instrumentation may be located at the Cassegrain, prime or Coudé focus points.

By October 1976 the polishing of the primary mirror had reached a point at which Grubbs were confident of reaching the optical performance specified in the contract. David Brown was also confident that the quality could be improved to the point were 95% of the incident light would fall into a diameter of 1 arc sec, the equivalent to the angle subtended by an 18mm diameter circle at a distance of 4Kms. It was deemed that the mirror support system was good enough to handle this improvement so the go ahead was given. In the event the improvement was made in just two figuring cycles with the mirror being completed in the autumn of 1977. The final bill for the improvement in quality was £9325.00; the equivalent to £40,000 in 2017.The decision to improve the mirror specifications was not without its risks. The skill and experience of David Brown and his team resulted in a successful conclusion. UKIRT began operations in 1979. Much of the telescope time was spent undertaking the UKIRT Infrared Deep Sky Survey. In 2011 the most distant quasar yet observed was discovered as part of the survey. The Quasar could not be seen in visible light, but could in the longer wavelengths observed by UKIRT.

Sadly, in 2014 the UK withdrew all funding from UKIRT; it was taken over by the University of Hawaii. In 2015 the university announced that, along with two other telescopes, it would be decommissioned. The site of the telescope, Mauna Kea, Hawaii, has been at the centre of a long running dispute over land usage for observatories; the natives consider this land sacred and want restricted development. Mauna Kea is the preferred site for the 30 meter telescope currently under development, but there have been protests about it being sited there. The decommissioning of three telescopes may show protesters that the authorities are serious about limiting development on the mountain and as a result they may withdraw their objections to the proposed thirty meter telescope being sited there. Time will tell.

References

The Life Story of an Infrared Telescope. John K Davies

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/importance.htmlhttp://www.grattavetro.it/hartmann-test/?lang=en

The Creation of the Anglo-Australian Observatory

http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_coude.html

https://en.wikipedia.org/wiki/Minute_and_second_of_arc

https://en.wikipedia.org/wiki/United_Kingdom_Infrared_Telescope

Video featuring the work of UKIRT: http://www.ustream.tv/recorded/1336197

1st June 2012, I read that the UKIRT will cease operations in 2013, a sad loss. http://www.ras.org.uk/news-and-press/219-news-2012/2131-stfc-island-site-telescopes-response-from-the-royal-astronomical-society

Here is the response form the UKIRT board.....http://www.jach.hawaii.edu/UKIRT/news/Statement_from_the_UKIRT_Board.pdf