Introduction to radio astronomy Flashcards
optical and radio astronomy correspond to the two
electromagnetically transparent windows in the Earth’s atmosphere
optical wavelengths
0.4 to 0.8 microns
radio wavelengths
1mm to 30m
photon counting is not an option in radio astronomy so we can almost always think
classically in terms of waves and electric fields rather than photons
the transition to quanta occurs in
sub-mm wave radio astronomy
in radio astronomy we usually work in terms of
spectral flux density (usually just called flux)
flux
symbol S
units Wm^-2Hz^-1
1 jansky
10^-26 Wm^-2Hz^-1
maximum power out=
S A delta v
flux density corresponds loosely to
apparent magnitude in optical astronomy, in the sense that it is a measure of the strength of the signal we see from the source, at Earth
extended sources
sources that do not appear as just a point on the sky
extended sources have both a
total flux density s(v) and a flux density per steradian B(v)
B(v) is a function of
position (theta, thi) on the sky
a map of B(v) across the sky is an image of the source
B(v) is also known as
specific intensity
spectral intensity
sky brightness
surface brightness
units of B(v)
Wm^-2Hz^-1sr^-1
over a small patch of sky, the intensity of a sky pixel of solid angle d omega giving a flux density dS is
B=dS/d omega
total flux density is given by
S = integral of B(theta,thi) d omega
a source of uniform intensity B and total solid angle omega has a total flux
S=B omega
why does the measured surface brightness of an object not depend on its distance from the observer
B= delta S/ delta omega
both an inverse square so B is not a function of r
usually in radio astronomy, hv«
kbT for a blackbody source
rayleigh jeans law
exp(hv/kbt) = 1 + hv/kbT
B(v) = 1kbTv^2/c^2
can think of a radio telescope as
mapping the temperature of the sky
most radio sources do not emit via a thermal blackbody mechanism so
can’t assign a temperature to them
brightness temperature or effective temperature
a blackbody at Tb would have the same surface brightness as the source at the frequency in question
Tb is the real thermodynamic temperature of the source if and only if
the object is a blackbody
T»hv/kb
