Tuesday, 18 August 2009

Dedicated to the infrared

Of the dozen observatories on Mauna Kea, two are dedicated infrared (IR) observatories meaning they only observe at IR wavelengths. Most of the other optical telescopes are very capable in the infrared and Gemini, for instance, was specifically designed to be optimised for IR observing but it also operates at visible wavelengths. The NASA Infrared Telescope Facility (IRTF - pictured above) and the United Kingdom Infrared Telescope (UKIRT - pictured below) were designed and built solely to operate at IR wavelengths, approximately between 1 and 30 microns (our eyes are most sensitive to optical radiation of about 0.5 to 0.6 microns).

Mauna Kea is one of the best sites in the world for infrared observing, probably only bettered by the Antarctic. In a couple of my early posts on this blog I explained why Mauna Kea is such a sought-after site by astronomers (here and here) so I won't rehash that here but will try and explain, briefly, why IR telescopes are a little different to their optical counterparts.

On the whole, infrared telescopes look the same as optical ones, they have a primary mirror (the big mirror at the bottom of the telescope), a secondary mirror at the top and instrumentation below the primary mirror. There are subtle differences though due to the nature of IR radiation and these are best summarized by the need to reduce the amount of IR light given off by the telescope and surrounding structure.

Any mass at room temperature shines very brightly in the infrared so telescopes that observe at these wavelengths tend to have as little structure as they can get away with. Imagine an optical telescope that needs a dark sky and environment to detect the faintest astronomical target, and then imagine it has a bunch of christmas lights all over its structure - that'll really play havoc with observations! You don't need those lights to create the same problem in the infrared, all the metal that forms the telescope, the dome itself, the dome floor and anything in that space glows brightly in the IR. So IR telescopes are built with as little structure as possible. Even the secondary mirrors, which on optical telescopes are large to catch every bit of light possible, are under-sized on IR telescopes. That means that the effective size of the primary mirror is reduced (some light is lost as it passes by the edge of the secondary), but it avoids the instrumentation seeing the glow from the dome floor, the metal holding the primary mirror in place or even the instrumentation itself.

The primary mirror itself glows and so is also a source of IR radiation the astronomer doesn't want to see. The IRTF overcomes that problem by cooling the whole dome during the day so everything inside should be around the same temperature as the night-time air. UKIRT doesn't cool the dome but instead actively cools the primary mirror so it's kept at typical night-time temperatures and the dome is cooled by opening several vents when the observing team gets to the summit. In fact both telescopes make determined efforts to open up as early as possible in the evening to help equalize the dome and outside air temperature which not only reduces the IR glow, but also helps to improve seeing (image quality). A warm primary and dome will cause air to rise which creates turbulence in the local atmosphere and makes the images much larger and more unstable than they should be.

Why observe in the infrared? Light at those wavelengths is less affected by extinction. The galaxy and cosmos is full of dust and it absorbs and scatters radiation (astronomers use the word extinction to measure the amount of light absorbed and scattered). With less extinction, IR telescopes can probe further into regions that have a lot of dust such as molecular clouds and the centre of our own Galaxy and so can see details optical telescopes cannot. Distant galaxies whose optical light is shifted to the infrared via the expansion of the universe are also much easier to detect in the IR. Many molecules either emit or absorb radiation in the infrared and again the only way to detect them or understand the state they are in is by observing them in the infrared. My PhD was spent doing exactly that!

I hope this gives you a taste of infrared astronomy. I've obviously left countless details out but just wanted to give a brief overview of what IR telescopes are and what they do.

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