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!

€

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