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Atmosphere
ASTR 3010
Lecture 5
not from the Textbook
Seeing and scintillation
Atmospheric extinction
• Three major componentso Rayleigh scattering : scattering by small particle
o Blue light gets scattered much more than red light blue sky!
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I ∝ I01
λ4
Atmospheric extinction
• Three major componentso Mie scattering : scattering by larger particles
o Wavelength independent white cloud
Atmospheric extinction
• Three major componentso molecular absorption Fraunhoffer lines
Airmass
Airmass python program of calculating airmass for a given elevation.
Kasten and Young (1989)
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()
Atmospheric window
Atmospheric absorption bands
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
High mountains are best!
14,000 ft
45,000 ft
and dry weather!
and dry weather!
and dry weather!
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
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
Correcting Atmospheric Effects
• How do we correct the effect from the atmosphere?
Effect of Atmosphere in Photometry
• Airmass (extinction) correction (measure a standard star over large airmass)
Effect of Atmosphere in Spectroscopy
• Telluric correction : division by a smooth spectrum standard star (WD or …)
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