Tuesday, 23 September 2008

Why are we here?

Sorry, I'm not going to answer the Ultimate Question nor could I, so if you were expecting an answer, stop reading now. I would, however, like to attempt a lay person's explanation of why the summit of Mauna Kea looks so "unworldly" (first-time visitors often say they feel they're on another planet up there) and why there are about a dozen world-class astronomical observatories at the summit.

Clouds: One of the banes of your common-or-garden amateur astronomer is the significant amount of time they sit around, freezing to death, waiting for the clouds to clear. No matter where you live, most days you look into the sky you'll see clouds of some sort. Professional observatories suffer in the same way (unless you work at a radio telescope or for Hubble) so why not build them above the clouds? The inversion layer at Mauna Kea is typically around 6,000-8,000 feet so most of the time clouds form below the summit allowing us access to a clear sky. This doesn't happen all the time, obviously, and you get the most stunning sunsets when we suffer high-level clouds, but apparently there are 300 nights per year when the sky is clear at the summit - I suppose it depends on your definition of clear though.

Seeing: Those of you who are lucky or unlucky enough to know an astronomer, amateur or professional, will no doubt have heard the word "seeing" on many occasions. I know most people don't know what it means and assume, incorrectly, that it refers to how many things you can see in the sky. It's actually a measure of how small a star appears in the sky. The smaller the number, the better the seeing (it's normally measured in arcseconds, or 1/3600 of a degree, i.e., it's an angular measurement).

As a young lad (sigh), I spent many awful hours in the middle of a cow pat infested field training to be an astronomer. Why didn't I just become a banker? (Oh, I remember now.) Our typical seeing was 5 arcseconds on the three nights a year it was clear. If you are lucky enough to see a star from a city, the seeing is probably much worse than that. The average seeing on the summit, however, is 0.5 arcseconds (and clever optics means you can get much better than this). This has two great advantages: 1) you can see much more detail in an astronomical object and 2) the light from a star is concentrated into a much smaller area which means your telescope and instrument become much more sensitive and so you can detect fainter objects.

Why is the seeing so good? There are several reasons, but mainly it's because the observatories are above much of the atmosphere - about 40% of it is below the summit. Turbulence in the atmosphere causes bad seeing, so if you're above much of it you'll benefit. Secondly, Hawai`i is an island in the middle of an ocean rather than a large land mass like North America or Europe. You could build an observatory in the Rockies or Alps, but imagine where the low-level winds blow. A large mountain range forces the winds up and over the summits causing turbulence and therefore bad seeing. On the other hand, the low-level winds just pass around the flanks of Mauna Kea leaving the summit alone. So why was it so windy last night?

Part II later this week which will hopefully include a bit about water, as the lack of it is crucial to most telescopes at the summit.

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