The results on the vertical distribution of optical turbulence above the five mountains which were investigated by the site testing for the Thirty Meter Telescope (TMT) are reported. On San Pedro Martir in Mexico, the 13 North site on Mauna Kea and three mountains in northern Chile Cerro Tolar, Cerro Armazones and Cerro Tolonchar, MASS-DIMM turbulence profilers have been operated over at least two years. Acoustic turbulence profilers - SODARs - were also operated at these sites. The obtained turbulence profiles indicate that at all sites the lowest 200m are the main source of the total seeing observed, with the Chilean sites showing a weaker ground layer than the other two sites. The two northern hemisphere sites have weaker turbulence at altitudes above 500m, with 13N showing the weakest 16km turbulence, being responsible for the large isoplanatic angle at this site. The influence of the jetstream and wind speeds close to the ground on the clear sky turbulence strength throughout the atmosphere are discussed, as well as seasonal and nocturnal variations. This is the sixth article in a series discussing the TMT site testing project.
Deep Dive into Thirty Meter Telescope Site Testing VI: Turbulence Profiles.
The results on the vertical distribution of optical turbulence above the five mountains which were investigated by the site testing for the Thirty Meter Telescope (TMT) are reported. On San Pedro Martir in Mexico, the 13 North site on Mauna Kea and three mountains in northern Chile Cerro Tolar, Cerro Armazones and Cerro Tolonchar, MASS-DIMM turbulence profilers have been operated over at least two years. Acoustic turbulence profilers - SODARs - were also operated at these sites. The obtained turbulence profiles indicate that at all sites the lowest 200m are the main source of the total seeing observed, with the Chilean sites showing a weaker ground layer than the other two sites. The two northern hemisphere sites have weaker turbulence at altitudes above 500m, with 13N showing the weakest 16km turbulence, being responsible for the large isoplanatic angle at this site. The influence of the jetstream and wind speeds close to the ground on the clear sky turbulence strength throughout the
The TMT project conducted an extensive site monitoring campaign to identify the most suitable site to host the observatory. Knowing the vertical distribution of the optical turbulence strength (TS), as expressed by the structure constant of the refractive index C 2 n , in Earth's atmosphere -the turbulence profile (TP or C 2 n (h)) -is of great importance for modern astronomical observing techniques. Especially adaptive optics (AO) instrumentation in its various forms relies in its inital design on a good assumption of the typical altitude distribution of the TS above the telescope. AO will become an important observing technique of the future extremely large telescopes (ELTs) and it is therefore essential to obtain measurements of the TPs above the ELT candidate sites. Such measurements can in principle be obtained in-situ by means of balloon borne turbulence sensors (Barletti et al. 1977). But a continuous monitoring of the TP using balloons would be very costly. Therefore, site testing programs use remote sensing techniques to obtain measures of the total TS. The instruments used during such campaigns have to be rugged and should only require small size telescopes. These requirements limited early site testing programes to measurements of the total seeing only and were first based on visual observations (Stock 1964), photoelectric (Irwin 1966) or photographic techniques (Birkle et al. 1976). A radical improvement of measuring the total seeing was obtained by the development of the Differential Image Motion Monitor (DIMM, Sarazin & Roddier 1990). Remote sensing of the actual TP based on measurements of stellar scintillations has been done since some time by means of the SCIntillation Detection And Ranging (SCIDAR) technique (Vernin & Roddier 1973). This technique, however, requires telescopes with apertures of at least 1 m for being effective. Therefore, in recent years several other techniques, like SLOpe Detection And Ranging (SLODAR, Butterley et al. 2006) or Multi Aperture Scintillation Sensor (MASS, Tokovinin 2002) have been developed to overcome the requirement for such large apertures and still obtain TPs with reasonable altitude coverage and resolution. The TMT site testing employed combined MASS-DIMM instruments (Kornilov et al. 2007) to assess the low resolution TP through the entire atmosphere. In order to measure the TP within the lowest few hundred meters above the sites, acoustic turbulence profilers SODARs (SOund Detection And Ranging, Crescenti 1997) were also employed.
Here we report on the results obtained with these turbulence profilers during the TMT site testing. In Section 2 the candidate sites, the deployed instrumentation and data are described. Sections 3 and 4 show and discuss the observed “typical” TPs. Sections 5, 6 and 7 discuss seasonal and nighttime changes and the dependence on the ground wind speed of the TS at the different altitudes.
After a preselection based on satellite data and general topographic conditions (Walker et al. 2009) , TMT deployed site testing stations on five candidate sites: 13N on Mauna Kea (Hawaii), San Pedro Mártir or SPM (Mexico), Cerro Tolar, Cerro Armazones and Cerro Tolonchar (all in northern Chile). The general properties of these mountains are summarized in Table 1. More detailed information about these sites is given in Schöck et al. (2007) and Schöck et al. (2009). These stations have been operated for at least two years on each mountain, with a considerable time span being simultaneous between all stations (see Table 1).
All TMT site testing stations were equipped with a custom built 35 cm Cassegrain telescope, manufactured by Teleskoptechnik Halfmann. These telescopes were mounted on 6.5 m tall towers, resulting in an elevation of the telescopes of approximately 7 m above ground. All components of the TMT site testing equipment are summarized in Schöck et al. (2007) and Riddle et al. (2009). On each telescope a combined MASS-DIMM device (Kornilov et al. 2007) was deployed. The DIMM channel of these instruments measures the differential image motion of the star, which then results in the total seeing from the telescope level to the top of the atmosphere (Sarazin & Roddier 1990). The used DIMM system and its precision is described in detail by Wang et al. (2007), who reported the seeing precision -the comparability between the TMT DIMMs -to be better than 0. ′′ 02.
The MASS (Multi Aperture Scintillation Sensor) reconstructs, by measuring the spatial structure of the flying shadows, a TP at six altitudes h i=1…6 = 0.5, 1, 2, 4, 8, 16 km above the telescope (Tokovinin & Kornilov 2007). The MASS TPs consist of the integrals of C 2 n over each altitude bin, weighted with functions which peak at the the layer height h i and go to zero at the neighboring MASS layer altitudes (thus causing some overlap between neighboring layers). Therefore, the MASS TPs are given in C 2 n (h)dh, which is equivalent to the seeing of the layer to the power of
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