07-22-2015, 04:52 PM
Justin wrote a superb post covering infrastructure and politics, I have nothing to add to that other than to emphasize the infrastructure already exists on Mauna Kea and the island, so that cuts costs significantly. This saves me a lot of work! I'll concentrate on the science reasons for choosing Mauna Kea.
For an optical/infrared telescope such as the TMT, the main issues that astronomers will use to determine how good a potential site is (not necessarily in order of priority) are:
1) Atmospheric turbulence;
2) Cloud cover;
3) Precipitable water vapour above the site;
4) Atmospheric extinction;
5) Dark skies.
Atmospheric turbulence.
Astronomers measure image quality using a value called "natural seeing". Put simply, it's how much the atmospheres blurs a point source, such as a star, between the light entering the atmosphere and hitting the telescope. The smaller the number, the better. Good seeing means you can see greater detail and also means more photons are concentrated into a smaller area, so sensitivity is increased and it's easier to detect fainter objects.
Seeing is affected by atmospheric turbulence, both in the upper atmosphere and locally, i.e., just above the telescope. Mauna Kea has about the best seeing on the planet. Just one of the factors is that it's on a small island. Winds can easily go around the island rather than being forced to go over the top. Most other mountains are in mountain ranges and the winds can't go around them, they are forced over the top of the mountain, cause local turbulence which degrades seeing. Seeing is a very important factor for both optical and infrared telescopes.
Cloud cover.
I think this one doesn't need a lot of explanation. Astronomers need clear skies. Mauna Kea has more clear nights than just about every other potential site on the planet.
Precipitable water vapour.
The amount of water vapour above the observatory site is a critical factor for infrared observations, especially for those done at longer wavelengths than around 2.3-microns. Water molecules absorb and then re-emit infrared light, so the more water there is the brighter the sky becomes at infrared wavelengths. At a certain point, infrared observations become impossible.
Because of its height, the summit region of Mauna Kea is above most of the water in the atmosphere, so, for instance, it's a far better site than Haleakala for infrared work. Go even higher it gets better, but then you run into other problems. You don't have to go much higher than Mauna Kea to have to start building pressurized rooms at the observatory, so expenses suddenly start to escalate. Mauna Kea's altitude is just about a perfect balance between providing an excellent infrared site and providing expensive facilities to keep people comfortable and safe.
Atmospheric extinction.
Mauna Kea generally has a strong inversion layer a few thousand feet below the summit. As well as keeping most of the water vapour well below the summit, it also keeps atmospheric pollutants down below as well (e.g., vog). Pollutants scatter light, so reduce the number of photons reaching the telescope from astronomical targets. Mauna Kea isn't particularly unique here, but it's a consideration site selectors will include.
Dark skies.
Or light pollution. Although the lights from Big Island towns can be seen from the summit, their effect generally doesn't extend beyond about 30 degrees above the horizon, and most telescopes don't observe that low in the sky (for both mechanical and science reasons, it's very difficult to calibrate a measurement taken below 30 degrees above the horizon). This rules out many potential sites, especially on the mainland.
Hope this has helped. As you might imagine there are many details I've not touched on, but this covers the main areas site selectors look at. The TMT have made their site testing results and documents publicly available if anyone wants to do further research.
http://sitedata.tmt.org/Available_data/sites.html
ETA. Fixed a couple of obvious typos.
For an optical/infrared telescope such as the TMT, the main issues that astronomers will use to determine how good a potential site is (not necessarily in order of priority) are:
1) Atmospheric turbulence;
2) Cloud cover;
3) Precipitable water vapour above the site;
4) Atmospheric extinction;
5) Dark skies.
Atmospheric turbulence.
Astronomers measure image quality using a value called "natural seeing". Put simply, it's how much the atmospheres blurs a point source, such as a star, between the light entering the atmosphere and hitting the telescope. The smaller the number, the better. Good seeing means you can see greater detail and also means more photons are concentrated into a smaller area, so sensitivity is increased and it's easier to detect fainter objects.
Seeing is affected by atmospheric turbulence, both in the upper atmosphere and locally, i.e., just above the telescope. Mauna Kea has about the best seeing on the planet. Just one of the factors is that it's on a small island. Winds can easily go around the island rather than being forced to go over the top. Most other mountains are in mountain ranges and the winds can't go around them, they are forced over the top of the mountain, cause local turbulence which degrades seeing. Seeing is a very important factor for both optical and infrared telescopes.
Cloud cover.
I think this one doesn't need a lot of explanation. Astronomers need clear skies. Mauna Kea has more clear nights than just about every other potential site on the planet.
Precipitable water vapour.
The amount of water vapour above the observatory site is a critical factor for infrared observations, especially for those done at longer wavelengths than around 2.3-microns. Water molecules absorb and then re-emit infrared light, so the more water there is the brighter the sky becomes at infrared wavelengths. At a certain point, infrared observations become impossible.
Because of its height, the summit region of Mauna Kea is above most of the water in the atmosphere, so, for instance, it's a far better site than Haleakala for infrared work. Go even higher it gets better, but then you run into other problems. You don't have to go much higher than Mauna Kea to have to start building pressurized rooms at the observatory, so expenses suddenly start to escalate. Mauna Kea's altitude is just about a perfect balance between providing an excellent infrared site and providing expensive facilities to keep people comfortable and safe.
Atmospheric extinction.
Mauna Kea generally has a strong inversion layer a few thousand feet below the summit. As well as keeping most of the water vapour well below the summit, it also keeps atmospheric pollutants down below as well (e.g., vog). Pollutants scatter light, so reduce the number of photons reaching the telescope from astronomical targets. Mauna Kea isn't particularly unique here, but it's a consideration site selectors will include.
Dark skies.
Or light pollution. Although the lights from Big Island towns can be seen from the summit, their effect generally doesn't extend beyond about 30 degrees above the horizon, and most telescopes don't observe that low in the sky (for both mechanical and science reasons, it's very difficult to calibrate a measurement taken below 30 degrees above the horizon). This rules out many potential sites, especially on the mainland.
Hope this has helped. As you might imagine there are many details I've not touched on, but this covers the main areas site selectors look at. The TMT have made their site testing results and documents publicly available if anyone wants to do further research.
http://sitedata.tmt.org/Available_data/sites.html
ETA. Fixed a couple of obvious typos.
Pretty much all the same positives, except for being on an active volcano...
