Friday, 26 September 2008

Why are we here? Part II

I wanted to title this Dark sky and no water but that would be confusing as this is a follow-up to my earlier post.

A dark sky: There are no large cities on the Island of Hawai`i (no, Honolulu is on another island), in fact most of the island is unpopulated and therefore is lucky enough not to suffer too greatly from a common problem around the world - light pollution. Those of you that live in large cities and don't take the opportunity to travel to remote places will likely have never experienced the wonder of a dark sky; seeing more stars than you could ever imagine, the dark lanes in the Milky Way and perhaps even the Andromeda Galaxy with your own naked eye.

In order to observe the faintest astronomical objects you need to have a very dark background, and that background generally comes from two sources: electronic noise in the instrument detectors, which we can take care of most of the time, or the sky itself. There are other sources and those can depend on what type of observatory you're using - more of that in a later post perhaps.

The Mauna Kea sky is dark, very dark, except when the moon's up of course. We can schedule observations around those occasions though.

Water: Actually, it's the lack of water in the atmosphere which makes Mauna Kea a superb observing site for infrared (IR) and submillimetre telescopes. It's probably second only to the South Pole (Antarctica) as an IR observing site and that's not the most inviting place in the world, so Mauna Kea is probably the premier site on this planet.

Although about 40% of the atmosphere is below Mauna Kea's summit, that includes about 90% of the atmospheric water. Water molecules are very good at absorbing IR radiation which means that if you have a lot of water in the atmosphere above you, the IR light from space gets blocked long before you can see it. This is an even greater problem for submillimetre telescopes (such as the CSO, JCMT and SMA) because water really loves light with a wavelength of a few hundred microns which is the "submillimetre regime".

There are other molecules which are a complete pain: carbon dioxide is such a strong absorber of IR radiation that we have to go into space to avoid that one, but it only absorbs at very specific wavelengths so generally it doesn't have too much of an impact on observations - unless you want to observe carbon dioxide in space which is something I'd actually like to be able to do.

The other stuff: I think I covered the important scientific reason to observe on Mauna Kea albeit extremely briefly and in a fairly unscientific way. There are a whole bunch of other reasons the mountain is such an excellent site including access (there's a road to the summit), vicinity to small but important population centres and their airports and sea ports, an energy infrastructure (i.e., there's electricity here) and a drinking water supply although it has to get trucked to the summit.

This is all such a big change from the days I spent observing in a muddy cow pasture in the UK.

No comments: