# What makes a really good observatory site, besides altitude?

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Let's limit this to optical telescopes.

I understand that the higher you are, the less atmosphere is above you to get in the way of observations. From reading around, there seem to be a few more but I'm not sure:

• Not near an active or dormant volcano (but Mauna Kea seems to disprove this?). This is kinda a bummer because a lot of tall mountains seem to be volcanic.

• Somewhere with clear and/or dry weather for as much of the year as possible

• Not near major light pollution like cities

• Cold weather is better than hot weather? Not sure I understand that; if it's uniformly hot or cold then I don't see how it makes a difference.

Am I missing any? Or are these wrong in some way?

As a concrete example, would the Tibetian plateau make a really good observatory site? (Not considering construction or transportation costs to a remote area, of course.)

• Not near an active or dormant volcano (but Mauna Kea seems to disprove this?). This is kinda a bummer because a lot of tall mountains seem to be volcanic.

There's nothing intrinsically wrong with dormant volcanos, as Mauna Kea and the observatories in the Canary Islands demonstrate. Not sure where you would have gotten that idea.

• Somewhere with clear and/or dry weather for as much of the year as possible

Clear weather is essential -- you can't observe when it's cloudy!

Dry weather is very good, especially for infrared observations (water vapor blocks a lot of infrared light).

• Not near major light pollution like cities

Yes.

• Cold weather is better than hot weather? Not sure I understand that; if it's uniformly hot or cold then I dont see how it makes a difference.

Cold climates tend to have drier air. (Antarctica is an extreme case.) Of course, being on top of a mountain means colder air, which is why Mauna Kea is good despite it's being in the tropics.

Am I missing any? Or are these wrong in some way?

You also want stable air with good "seeing", which rules out places with turbulent air and lots of wind (and is another reason mountaintops are good: they're usually above the most turbulent layers of the atmosphere). Quoting from this page (which has a good discussion of the general topic):

Seeing… requires at minimum a lack of extra turbulence at all atmospheric levels, and seems to be best satisfied in the convergence zones just outside the tropics, at latitudes about ± 30º. Also, minimal local turbulence is often associated with mountain peaks that reach into the otherwise undisturbed oceanic airflow, as on islands or coastal ranges (and given the direction of the planet's rotation, this generally favors western coast ranges).

The Brigham Young University (BYU) West Mountain Observatory (WMO) is a modern astronomical research facility located approximately one hour from the main BYU campus in Provo, Utah. West Mountain is a lone mountain in the center of Utah Valley on the southeast shore of Utah Lake. The observatory is located slightly west of Long Ridge about a quarter mile from the numerous communications towers seen near the center of the 6850 foot West Mountain. The site itself is far enough to the west of the developed areas along the Wasatch Mountains so that it continues to produce high quality research observations. Overhead and to the west, the sky brightness at West Mountain comparable to that of Kitt Peak National Observatory, while the median seeing conditions are as good or better.

What makes a good observing site? Nearly every kind of astronomical telescope benefits from a site that provides clear, dry, and dark conditions. While the stormy weather on the Antarctic coast is legendary, the high plateau in the center of the continent is quite the opposite. It lies at the eye of the storm it is a calm, clear polar desert that offers space-like observing conditions while comfortably standing on terra firma. Thus, siting a telescope at the summit of the Antarctic plateau has significant advantages over essentially anywhere else on Earth:

In particular, one's ability to do infrared, submillimeter-wave and terahertz astronomy hinges on the amount of water vapor in the atmosphere, since water molecules absorb this light very effectively before it reaches the ground.

Really! In the summer, there is enough moisture in the air at most midlatitude locations on Earth, at the frequency of the ionized carbon line (1900 GHz), light can only travel 10-50 meters before it is mostly absorbed by the intervening water vapor.

This effectively forces such astronomical observatories to the highest and driest sites where this atmospheric absorption is minimized. The bitterly cold air holds no water vapor, and what little remains freezes out into tiny ice crystals. This makes the summit of the Antarctic plateau the driest place on Earth.

We selected the Ridge A site from satellite data to be the best location for an astronomical observatory on the Antarctic plateau, and indeed, anywhere on Earth. It is located on the summit ridge of the ice plateau at 81:40:25 South latitude and 72:42:58 East longitude at a physical elevation of 13,260' (4040 m) with a typical pressure altitude of 15,200' (4650 m). It constitutes the origin of the continent's famous katabatic winds and is perhaps the calmest place on Earth, with typical winds of 4 knots (2 m/s). Even more important for infrared and terahertz telescopes is the extreme cold. Winter temperatures routinely drop below -100F (-70C), providing for a very dry, stable, clear atmosphere. The extremely low amount of water vapor that results allows observations to be routinely performed here that cannot be done reliably anywhere else on Earth. While it is perhaps the most remote site on the planet, it is nevertheless still accessible by aircraft or ground traverse.

## The Jet With a 17-Ton Telescope That NASA Uses as a Flying Observatory

SOFIA is a heavily modified flying observatory. German Aerospace Center

If you thought Boeing 747s weren’t useful for understanding how stars are formed, you don’t know about SOFIA.

Officially known as the Stratospheric Observatory for Infrared Astronomy, SOFIA is a heavily modified Boeing 747 Special Performance jetliner, with a 17-ton, 8-foot telescope mounted behind a 16-by-23-foot sliding door that reveals the infrared telescope to the skies.

The plane’s ability to fly near the edges of the atmosphere gives it better visibility than ground-based observatories. And the fact that it makes regular appearances on Earth's surface, unlike a space telescope, means it can easily be repaired or reprogrammed when necessary.

NASA and its partner on the project, the German Aerospace Center (DLR), expect SOFIA to keep flying for another 20 years. To that end, they’ve grounded the world’s only flying observatory for extensive maintenance that will take five months.

The 747SP was designed by Boeing in the 1970s to fly faster, higher, and farther than other versions of the 747. The company's engineers shortened the fuselage by 55 feet to cut weight, but left the power plants intact, giving the SP incredible performance statistics.

The plane can stay airborne for over 12 hours and its range is 6,625 nautical miles (7,624 miles). With a service ceiling of 45,000 feet, it can fly above the troposphere and 99.8 percent of the water vapor held in our atmosphere, which obscures infrared light. That gives its on-board infrared telescope a clear view into outer space.

NASA says the data provided by SOFIA "cannot be obtained by any other astronomical facility on the ground or in space." Unlike grounded telescopes and satellites fixed in orbit, SOFIA is mobile, so it can better spot transient space events like supernovae and comets.

The telescope on board is 10 times as sensitive and has triple the resolution of NASA’s Kuiper Airborne Observatory, originally launched in 1975 on a converted C-141 military cargo plane, and decommissioned in 1995. That telescope was the first to spot the rings around Uranus.

The SP that now serves NASA was first flown as a passenger aircraft by Pan Am, which dubbed it the “Clipper Lindbergh.” The failing airline sold the jet to United Airlines in 1986, which in turn passed it on to NASA in 1997.

## Introduction to Large High Altitude Air Shower Observatory (LHAASO) ☆

Since the century discovery of cosmic ray, the origin of cosmic ray is always a mystery. The study on the origin of high-energy cosmic ray is in an interdiscipline between the very high-energy (VHE) gamma-ray astronomy and the cosmic ray physics. The Large High Altitude Air Shower Observatory (LHAASO) is a unique and new generation cosmic-ray station with the advantages of high altitude, all-weather, and large-scale. It takes the function of hybrid technology to detect cosmic rays and to upgrade greatly the resolving power between gamma rays and cosmic rays. The LHAASO is expected to make the full-sky survey to find new gamma-ray sources, to obtain the highest sensitivity of gamma-ray detection at the high energy band of > 30 TeV, and to make the very high precision measurement on the component energy spectra of cosmic rays in a broad energy range of 5 orders of magnitude, in order to provide the evidence for revealing the mystery of the origin of cosmic ray. This paper describes the detector structure, performance superiority and scientific motivation of the LHAASO.

## Onizuka Center for International Astronomy

Astronomers and technicians must acclimatize when coming from sea level to work at the summit. For this reason, "mid-level" facilities are provided at an altitude of 2,800 meters (9,300 ft). These facilities were constructed in 1982 and have been named in honor of Ellison Onizuka, an astronaut from the Big Island who died in the 1986 Challenger disaster.

The Onizuka Center also includes a Visitor Information Station which is open to the public. It contains exhibits about the mountain and its observatories, offers evening sky-viewing opportunities, and provides guided tours of the summit.