Thursday, 30 July 2009

Submillimeter Valley

Although it's not an official name, everyone who works at the summit of Mauna Kea knows "Submillimeter Valley". It's the name given to the area that's home to all the submillimeter telescopes on the mountain: the CSO, the JCMT and the SMA. This region is a few hundred feet below the summit and the optical-infrared telescopes. I'm not entirely sure I know the reason why these observatories are in the valley rather than a little higher up where the other telescopes are, it may be a scientific one or purely logistical, or a combination of both. I haven't really thought about it before but now I have, I will ask and find out!

Submillimeter telescopes bridge the gap between observing in the optical/infrared and the radio regions of the electromagnetic spectrum. Unlike radio telescopes, they need a dry atmosphere in order to work hence the reason for placing them on high mountains such as Mauna Kea in Hawai`i or the high Atacama desert in northern Chile where most of the atmospheric water is below them. That's partly the reason you'll find infrared telescopes at similar sites as they are also affected by telluric water but often suffer a disadvantage: more often than not submillimeter telescopes can look right through high cirrus clouds as the water is frozen into ice particles and is undetectable at submillimeter wavelengths. On the other hand, ice shines rather brightly in the infrared so all we can do on those cloudy nights is detect cloud! I've spent many nights on Mauna Kea during which we can't work yet we get called by our colleagues at the JCMT telling us what a wonderful night it is!

The view above was taken just outside one of the mountain's infrared telescopes, UKIRT, looking down at Submillimeter Valley in May earlier this year. The shot below, from roughly the same place, was taken in March during one of the most awful few months of summit weather I've known in the last decade.


punagreek said...

I believe the sub-mm dishes are in the valley to reduce wind exposure, since their wind tolerances are much lower than the traditional telescopes due to the large surfaces the dishes present to the wind. Wind speeds in the valley are lower because it is more sheltered. That is also part of the reason for the JCMT gore-tex membrane.

I wonder where the TMT is going...

Tom said...

Punagreek - that's definitely a possibility although, for example, the JCMT's opening wind limits are not that much different to ours and we're on the summit ridge. The SMA dishes are certainly unprotected but on the other hand are quite small. Sometimes the wind is actually higher in submm valley compared to the summit region although usually not - might be some wind-channeling effect on occasion. I promise to find the answer though!

The pictures in this post don't show where the TMT will be but it's to the right and at a lower level than submm valley - it's on a large plateau that extends approximately westward from submm valley and from where the Kecks, Subaru and the IRTF are. At least that's where they were doing the the site testing.

Thanks for your comment!

Protege said...

Although a scientific post and I am not following it all, I still like the picture.;))

Tom said...

Turns out the reason that the submm telescopes are in Submillimeter Valley is because there is no advantage for them being on the summit. It makes sense, there's no improvement in submm seeing at the summit so why not just leave that space to the optical/IR telescopes?

Punagreek - happy to explain this more if you want.