Basic Radiomm Astronomy

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Slide 1

A Crash Course in Radio Astronomy and Interferometry: 1. Basic Radio/mm Astronomy

James Di Francesco

National Research Council of Canada

North American ALMA Regional Center – Victoria

(thanks to S. Dougherty, C. Chandler, D. Wilner & C. Brogan)

Slide 2

Intensity & Flux Density

EM power in bandwidth dn from solid angle dW intercepted by surface dA is:

Defines surface brightness Iv (W m-2 Hz-1 sr-1 ; aka specific intensity)

Flux density Sv (W m-2 Hz-1) – integrate brightness over solid angle of source

Convenient unit – the Jansky  1 Jy = 10-26 W m-2 Hz-1 = 10-23 erg s-1 cm-2 Hz-1

Basic Radio/mm Astronomy

Slide 3

Surface Brightness

In general surface brightness is position dependent, ie. In = In(q,f)

(if In described by a blackbody in the Rayleigh-Jeans limit; hn/kT << 1)

Back to flux:

In general, a radio telescope maps the temperature distribution of the sky

Basic Radio/mm Astronomy

Slide 4

Basic Radio Astronomy

Many astronomical sources DO NOT emit as blackbodies!

However….

Brightness temperature (TB) of a source is defined as the temperature of a blackbody with the same surface brightness at a given frequency:

This implies that the flux density

Brightness Temperature

Slide 5

What does a Radio Telescope Detect?

Recall :

Telescope of effective area Ae receives power Prec per unit frequency from an unpolarized source but is only sensitive to one mode of polarization:

Telescope is sensitive to radiation from more than one direction with relative sensitivity given by the normalized antenna pattern PN(q,j):

Basic Radio Astronomy

Slide 6

Antenna Temperature

Johnson-Nyquist theorem (1928):

Antenna temperature is what is observed by the radio telescope.

Power received by the antenna:

A “convolution” of sky brightness with the beam pattern

It is an inversion problem to determine the source temperature distribution.

Basic Radio Astronomy

Slide 7

Radio Telescopes

The antenna collects the E-field over the aperture at the focus

The feed horn at the focus adds the fields together, guides signal to the front end

Basic Radio/mm Astronomy

Slide 8

Components of a Heterodyne System