Page 9 - 83 basic knowledge of astronomy

Figure 7: Frequency designations.

• L–band : GPS satellite, observations of OH masers,

• S– and X–bands : geodetic VLBI observations,

• K–band : observations of H 2 O masers, and
• Q–band : observations of SiO masers.

7 Basic Quantities in Radio Astronomy

The following quantities are commonly used in radio astronomy to charac-
terize radio waves coming from celestial bodies.

7.1 Intensity (Specific/Monochromatic Intensity) I or
ν
Brightness B ν

The intensity I ν (or brightness B ν ) is the quantity of electromagnetic radia-
tion energy incoming from a certain direction in the sky, per unit solid angle,
per unit time, per unit area perpendicular to this direction, and per unit
frequency bandwidth with center frequency ν. The SI unit of this specific
(or monochromatic) intensity is thus: W m −2 Hz −1 sr −1 (Figure 8).
In terms of the monochromatic intensity I ν , the power dW of radiation
coming from a direction s within a solid angle dΩ, through a cross-section of
area dσ with a normal inclined by an angle θ from the direction s, within a
frequency bandwidth dν centered at ν, is given by:

dW = I ν (s) cos θ dΩ dσ dν. (1)

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