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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