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Site selection is critical for a telescope. As long as observations are ground based, the observation performance is strongly affected by the Earth's atmosphere. A resolution of a telescope is determined by intensity of atmospheric disturbance. The background radiation in infrared wavelength is strongly affected by the amount of water vapor and air temperature. Although resolution can be improved through use of an adaptive optics system, better performance will be obtained by ensuring atmospheric disturbances are inherently weaker.

Conditions that should be satisfied by a site

Factors to be considered in selecting a telescope site are summarized in Table shown below. Clear weather must be available for a large proportion of night time, and seeing (stellar image diameter in a long exposure) must be good. Beyond those factors, for a large telescope in which adaptive optics will be frequently used, a height distribution of layers in which atmospheric disturbance (including disturbance of refractive index, related to temperature disturbance) occurs, a speed of disturbance, and a spatial spectrum of the disturbance (particularly in a low spatial frequency region) are also important. The height distribution of the layers affects a width of a field of view in which adaptive optics correction can be applied, while the speed of disturbance change and the spectrum in a low frequency region have a considerable effect on difficulty of designing an adaptive optics system ( a speed of control and amount of correction required). A good site has weak disturbances that are distributed mainly at low altitude, exhibit slow fluctuation, and which have small low spatial frequency components. In addition, it is important in telescope design that these conditions do not fluctuate over time.
In a case of infrared observations, it is important not only that a stellar image discussed above be clear, but also that background light be dim and that there be little absorption by an atmosphere. Principal components of background light are that emitted from molecules containing OH at wavelengths up to 1.6μm, and thermal radiation from the atmosphere itself at longer wavelengths. Consequently, a site with relatively little overlying air (i.e., a high-altitude site) and low air temperature is generally preferable. Atmospheric absorption is dominated by absorption due to water vapor, the effects of which are relatively small at high altitudes and low temperatures. Significant absorption in infrared wavelength also occurs due to molecules such as CO2, CO and O3. Sources of background light in optical wavelength include, in addition to air glow, artificial light from cities, and auroras in polar regions.
Conditions on the ground, as well as sky conditions, are important to telescopes and observation instruments. A difficulty of designing a large telescope is reduced somewhat if a wind at ground level is weak and a wind speed and direction do not fluctuate to a considerable degree. It is also important from the point of view of structural design that the site be unaffected by strong earthquakes. Adjustment of instruments is also simplified if diurnal and seasonal temperature variations are small.
In addition to the natural conditions discussed above, human activity must be considered in construction. Even if observations are conducted remotely, occasional access by staff will be necessary for maintenance and modification. In addition, safety and possible future expansion depend on maintaining good relations with local residents and government, and on freedom from civil strife. It is also necessary to collect information on a wide variety of local conditions such as whether the site is in a flight path (or will be in future) and whether there are plans for mine development, which could, for example, raise dust and bring urban development with accompanying pollution.

Factors to be considered in selecting a telescope site:
proportion of clear weather at night dust, aerosol
seeing wind speed at ground
precipitation variation of wind direction and speed
altitude distribution of atmospheric disturbance earthquake
temporal spectrum of atmospheric disturbance variation of temperature
spatial spectrum of atmospheric disturbance infrastructure (road, electric power etc.)
infrared background cooperation with regional government and poeple
airglow, aurora flight route (in future)
visibility developement status/plan

Methods of site evaluation

In evaluating a site, a large number of meteorological statistics will become necessary. For this purpose, historical artificial satellite data should be used first to determine a proportion of clear weather, upper level winds, and a distribution of water vapor as a basis for site selection. However, uncertainties exist in the process of converting satellite data to meteorological quantities, and ground monitoring at candidate sites remains essential.

Proportion of clear weather
Locations having high proportions of clear weather can be selected using global meteorological satellite data. Once candidate sites are selected, ground-based visible and infrared cloud monitoring is necessary.

Precipitation data are generally obtained by conversion from water vapor absorption near 1μm or from optical thickness of the atmosphere at submillimeter wavelengths. The latter method is more accurate when precipitation is less than 1mm. These measurements can also be made using GPS.

Seeing, altitude distribution and spectrum of atmospheric disturbance
Seeing through all layers of the atmosphere is measured using a differential image motion monitor (DIMM). Since turbulence near the ground accounts for a large proportion of atmospheric disturbances, it is desirable for DIMM measurements to be carried out at the height at which the telescope will be constructed. Even at the same location, a difference of height of only 2-3 m can lead to a greatly different measurement result. Consequently, when only DIMM measurements are used, comparison among different sites is difficult. It is necessary to measure an effect of the layer adjacent to the ground at the same time.
If height distribution of the disturbances can be measured, comparison among different sites will become more accurate. For this reason, observations should be carried out with SCIDAR or MASS, which use star scintillation as an indicator, or SODAR, which uses scattering of sound waves. Another possibility is direct measurement using balloons. As SCIDAR uses double stars as light sources, directions and time of observations are limited. In addition, a telescope with aperture on an order of 1m is needed to measure a turbulent layer at altitudes of about 10km. Hence, measurements with SCIDAR are not easy. It should be noted that a type of SCIDAR that uses single stars as light sources has recently been proposed. MASS is small and simple, making the method useful for measurements at locations without good infrastructure, but the sensitivity for low-altitude turbulence is relatively low. For measurements of an atmospheric boundary layer, which accounts for a large component of disturbance, it is necessary to use SODAR together with other instruments. As SODAR does not detect features of the lower air layer below a height of about 30m, a weak heat turbulence meter (for example) is installed on a tower to measure turbulence near the ground.
Due to the difficulty of measuring a turbulence spectrum, it is usual to assume a shape for the spectrum when measuring a coherence time and a wavefront outer scale, which are important parameters affecting the design of an adaptive optics system. The coherence time is the time scale of speed of fluctuations of atmospheric disturbances, and is measured from magnitude of star scintillation. The wavefront outer scale is the space scale on the low-frequency side at which the Kolmogorov law of a power of 2/3 breaks down, and is estimated from the movement of a star measured along several baseline lengths. Further development in techniques for measuring the outer scale is needed for evaluation of the turbulence spectrum. It is also necessary to rigorously calibrate measurement instruments used in these evaluations to ensure accurate comparison among sites.

Locations suitable for construction of an astronomical observatory

Mauna Kea
More than 10 telescopes are now in operation on Mauna Kea, which is today one of the main centers of astronomical observation in the world. This is a typical isolated mountain on an oceanic island. The altitude is greater than 4100m, which is well above the upper boundary of the temperature inversion layer (ca. 2000m), meaning that there is little water vapor despite being surrounded by ocean. The proportion of clear weather is less than in Chile. The infrastructure is well developed, but on the peak of the ridge, where observation conditions are best, there are already many telescopes, and the remaining available land is limited. There are also restrictions on the construction of new structures and modification to existing structures for environmental protection reasons.

Central and northern Chile are located at latitudes with a cold ocean current flowing along the coast, leaving a large expanse of dry land. The ocean side of the Andes, including Cerro Tololo, La Silla, Paranal and Atacama, is characterized by high altitudes and a dry climate, which have made this area another major center for astronomical observations, with sites comparable to Mauna Kea.
The Chajnantor area of Atacama has a broad expanse of flat plain at altitudes near 5000m. Construction of Atacama Large Millimeter/Submillimeter Array (ALMA) through cooperation among North American, European and Japanese radio observatories has already begun. It is thus expected that this area will have a well developed infrastructure. ALMA is being built on a flat plain, but nearby there are peaks higher than 5500m. Among these is Cerro Chajnantor (5700m), which the University of Tokyo is surveying as a possible telescope construction site. Data on seeing are as yet sparse, but there are indications that the seeing can be expected to be as good as that on Mauna Kea or better. This site is near the Bolivian border, where a proportion of clear weather is somewhat less than that on Cerro Paranal. Additional surveys are necessary.

Southwestern United States and northern Mexico
This region is dry for reasons similar to those in Chile. CELT group is searching for possible sites in this area as well as on Mauna Kea and in Chile. UNAM, a Mexican group planning a 6.5m telescope, is surveying San Pedro Martir as a candidate site.

La Palma
La Palma is located in the Canary Islands off the coast of Morocco. It is an isolated mountain on an oceanic island, like Mauna Kea. The peak altitude is 2400m, which although not high is above the upper boundary of the temperature inversion layer (1500m), meaning there is relatively little water vapor. At present, GTC (Gran Telescopio Canarias, a telescope with 10m segmented primary mirror) is under construction at this site. As the site is near the Sahara Desert, it is affected by dust, particularly in summer, when dust can reach an altitude of 3000m.

Southeastern Uzbekistan
This location is near the Pamirs and the western end of the Altai Mountains. This site has been based for astronomical observatories since the 1960s. In the late 1990s, seeing was reevaluated, and it became widely known that there are good sites even in this interior continental area, which does not belong to either of the above categories of site generally considered to offer good conditions. The area is characterized by relatively slow changes in atmospheric disturbance (time scales 2 - 4 times longer than at other principal sites), making this area very favorable for adaptive optics.

The South Pole is very cold, and the air is very dry. Infrared and submillimeter telescopes have now been operating there for more than 10 years. However, as the seeing is poor, the location is not suitable for large telescope requiring good spatial resolution. France and Italy are jointly developing Dome C (75° 06' S, 123° 21' E) as a site, and an Australian group is now cooperating in those efforts. The altitude is 3280m, and an amount of water vapor is very low.
The Antarctic plateau is located in a region of air subsidence, resulting in a high proportion of clear weather and little snowfall (ca. 10cm per year). Measurements of seeing in winter have been initiated. Although data are as yet sparse, the preliminary indications are that seeing is better than 0.3”, giving this site possibly the best observation conditions in the world. In addition, disturbances primarily occur at low altitude, resulting in a large isoplanatic angle and good coherence time. Surveys are continuing. Although auroras can become a source of background light in Antarctica, Dome C is sited near the present location of the South Magnetic Pole where auroras are infrequent.
As a bedrock is not exposed at Dome C, all structures must be built on top of the thick Icecap. In contrast to other areas, it is necessary to deal with problems specific to Antarctica, including subsidence and movement of structures. In addition, it is necessary to consider that only the southern sky can be observed in Antarctica and that the sun does not set in summer, requiring observation objectives to be limited accordingly. Dome C is accessible by aircraft. It is planned that it will become possible to winter over at Dome C from 2005.
Japan operates a station at Dome Fuji (77° 19' S, 39° 42' E), which sits at an elevation 3810 m higher than Dome C. Scientists have now wintered over there for 9 years. It is possible that Dome Fuji offers conditions as good as those at Dome C or better. However, as Dome Fuji is closer to the aurora belt than Dome C, background light is expected to be a problem, particularly in optical wavelength.

Other areas
China has begun a survey to find the best possible observation site in the country. Attention is being focused on western Tibet. Japan is cooperating in this survey. Western Tibet is a high plateau, exceeding 4000m, and the amount of water vapor is relatively small. There is a monsoon effect, but from satellite data the proportion of clear weather is equal to or better than 70% experienced at Hanle in India. Although there is the problem of turbulence generated by the Pamirs and Karakoram mountains, there remains a possibility of finding good sites, and surveys are expected to continue.
The Greenland Icecap can be expected to have weather conditions similar to those in Antarctica. The altitude is 2000m in the north, and 3000m in central Greenland. Toward the south, the chances of being affected by aurora increase. This area has not yet been surveyed.

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