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Somak Raychaudhury www.iucaa.in/~somak/teaching
ASTRONOMICAL TECHNIQUES 1 (Incoherent Detection)
Lecture 2: Mon 8 Jan 2018
The inversion layer
dT/dz>0
dT/dz<0
Important factors: dT/dz and dρ/dz The Temperature gradient changes sign at the “inversion layer”. This is where most clouds sit The height of the “inversion layer” is a strong function of latitude on the Earth- it can reach the ground at the South Pole (height of Polar cap= 3 km) The ozone layer peaks at about H=30 km Space observatories orbit at H=300 km
P ~ exp(-z/H), H=8 km
Atmosphere: large scale
Space Station
Atmospheric effects
• Absorption • reduced source flux • difficult calibrations
• Turbulence • increased object size (“seeing”)
• Emission (more important in the infrared than optical) • increased background noise • reduced integration times
• All effects vary with wavelength, time, altitude, line-of-sight
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• Particle number densities (n) for most absorbers fall off rapidly with increasing altitude.
• x0,H20 ∼ 2 km, x0,CO2
∼ 7 km, x0,O3 ∼ 15-30 km
• So, 95% of atmospheric water vapour is below the altitude of Mauna Kea.
Atmospheric absorption versus altitude
Iλ= I0,λe−τ λ ,whereτ λ is optical depth,
τλ ∝ ndx∫ ∝ e− x / x 0dx∫
Attenuation
“Seeing” due to variable refraction Diffraction and Spatial resolution
ϑ =1.22 λD(rad)
= 2.5×105 λ(m)D(m)
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Diffraction limited; Seeing limited • Seeing for ground-based telescopes ~ 1 arcsec
Dλ
θ22.1
=ΔBut resolution
Hubble Space Telescope Diameter 2.4 m For λ=500 nm, Δθ= 2.54 × 10-7
= 0.05 arcsec
Seeing limited on Earth, but diffraction limited in space
305m dish 1420 MHz
Arecibo
For λ=21cm Δθ= 9.15 × 10-4
= 189 arcsec
Diffraction limited!
• Seeing varies as wavelength as
• So at optical wavelength e.g. λ =500 nm, if seeing
is Φ, then at radio wavelengths, e.g. λ=21 cm, seeing would be
Wavelength dependence of “seeing”
�� � ⇥�0.2
Compare this with the Diffraction limit: �� = 1.22�D
�� =� 21� 10�2
500� 10�9
⇥�0.2 � ⇥
= 0.63 ⇥
Connection with Fried parameter later