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Atmosphere
ASTR 3010
Lecture 5
not from the Textbook
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Seeing and scintillation
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Atmospheric extinction
• Three major componentso Rayleigh scattering : scattering by small particle
o Blue light gets scattered much more than red light blue sky!
€
I ∝ I01
λ4
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Atmospheric extinction
• Three major componentso Mie scattering : scattering by larger particles
o Wavelength independent white cloud
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Atmospheric extinction
• Three major componentso molecular absorption Fraunhoffer lines
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Airmass
Airmass python program of calculating airmass for a given elevation.
Kasten and Young (1989)
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Airmass Calculation with Python
from pylab import *from numpy import *…za = arange(0.0,90.0,0.1)airmass = 1.0 / (cos(radians(za)) + 0.50572*(96.07995 – za)**(-1.6364))
plot(za,airmass, ‘ro-’)xlim([80,90])xlabel(‘Zenith Distance’)ylabel(‘Airmass’)show()
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Atmospheric window
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Atmospheric absorption bands
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Atmospheric absorption bands
NJ KH QL MY
near-IR Mid-IR
UBVRI
• Optical bands : science driven (e.g., B and V to separate the Balmer Jump)• Near-IR and mid-IR bands : based on availability
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High mountains are best!
14,000 ft
45,000 ft
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and dry weather!
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and dry weather!
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and dry weather!
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Python Homework
• Using data from http://www.gemini.edu/?q=node/10789• Create a plot of atmospheric transmission for airmass=1.0 & 2.0 and
PWV=1.0 & 5.0mm
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Effect of Dust
• Gas manifest only in few allowed transitions (emission lines or absorption lines). It does not change the shape of the spectrum (unless they are optically thick!)!!
However, dust particles can change the shape of spectrao extinction (interstellar,
telluric) advantages of different wavelength astronomy e.g., extinction toward
the center of Galaxyo reddening (telluric
airmass, e.g., twilight)o scattering
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Correcting Atmospheric Effects
• How do we correct the effect from the atmosphere?
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Effect of Atmosphere in Photometry
• Airmass (extinction) correction (measure a standard star over large airmass)
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Effect of Atmosphere in Spectroscopy
• Telluric correction : division by a smooth spectrum standard star (WD or …)
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In summary…
Important Concepts• Atmospheric transmission• Effect of altitude and PWV• Photometric bands w.r.t.
Atmospheric windows• Airmass• Removing the effect of
atmosphere in photometry and spectroscopy
Important Terms• Scintillation• Airmass• PWV• Scattering (Rayleigh and Mie)• Extinction
Chapter/sections covered in this lecture : Not from the textbook