Tj

Z 104

Wavelength (/im)

Figure 7.9. Calculated disk-integrated irradiance spectra of the giant planets in the visible and near-infrared as seen from the Earth at opposition, in Janskys (1 Jy = 10~26 Wm~2 Hz-1). Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dot-dashed line.

Wavelength (/im)

Jupiter

Figure 7.9. Calculated disk-integrated irradiance spectra of the giant planets in the visible and near-infrared as seen from the Earth at opposition, in Janskys (1 Jy = 10~26 Wm~2 Hz-1). Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dot-dashed line.

magnitude-6 stars) is defined as m = 2.512 log(B0/B) (7.15)

where B is the measured irradiance from the source; and B0 is a reference irradiance approximately equal to B0 = 7 x 1010Wm~2nm_1 at visible wavelengths. The calculated disk-averaged spectral irradiances at the Earth of the giant planets at opposition in the visible/near-IR, and mid-IR to far-IR spectral ranges are shown in Figures 7.9 and 7.10, where spectral irradiance has been expressed in terms of Janskys, a unit often used in ground-based IR spectroscopy and defined as 1Jy= 10 ~26W m 2 Hz 1. The rapid decrease in irradiance with distance is clearly apparent in both spectral ranges.

7.4 GROUND-BASED VISIBLE/IR OBSERVATORIES

We saw earlier that the problems of atmospheric absorption, especially due to water vapor, may be limited by placing telescopes at high altitudes and thus limiting the mass of air above the telescope. In addition, it helps to place telescopes in regions where the air is statically stable and thus atmospheric turbulence is minimal. Very

Figure 7.10. Calculated disk-integrated irradiance spectra of the giant planets in the mid-infrared to far-infrared as seen from the Earth at opposition (in Janskys). Jupiter: solid line; Saturn: dotted line; Uranus: dashed line;Neptune: dot-dashed line.

good regions for ground-based telescopes are thus near the tropics of Cancer and Capricorn since these are latitudes where air which has risen near the equator at the Inter-Tropical Convergence Zone (ITCZ) and traveled towards the poles in the terrestrial equatorial Hadley cell circulation then descends and heats adiabatically forming a particularly stable air column. The air is also very dry as most of its moisture precipitated in the ITCZ as rain, which has additional advantages for ground-based telescopes as we have just seen, and is also the reason the Earth's major deserts are found to lie at these latitudes. Such regions are also clearly relatively free of clouds, an obvious advantage for visible/IR astronomy! It is thus no accident that most of the major telescope sites on the Earth: Hawaii, the Canary Islands, Chile, Southwest U.S.A, etc. are near these desert latitudes. In this section we will review some of the major astronomical observatories that among many other things are currently undertaking observations of the giant planets.

7.4.1 European Southern Observatory (ESO); Very Large Telescope (VLT)

ESO is an intergovernmental organization of 15 member states that operates two astronomical observatories in the Atacama desert of Chile, one of the driest places on Earth. The La Silla observatory (29° 15'S, 70°44'W), at an altitude of 2,400 m, is the

Table 7.1. La Silla telescopes.

ESO-operated telescopes

National telescopes

3.6m

DENIS 1 m

2.2m

Geneva 1.2 m

Danish 1.5 m

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