Radio Sources in the Sky
Joachim Köppen Kiel July 2016
This tool shows the continuum radio fluxes of various celestial sources.
Given the effective diameter of an antenna and the system temperature, the
antenna temperature and the signal to noise ratio are computed for the present
position of the mouse. If the angular size of the source is comparable or
larger than the antenna HPBW, the antenna's main lobe picks up only a fraction
of the true flux. This fraction is given by the beam filling
factor fill ≈ ang.diam²/(ang.diam² + HPBW²).
[This formula is exact for a gaussian antenna beam and a gaussian object,
but it also quite good for a uniformly bright disc object observed by the main
lobe from a uniformly illuminated disc antenna.
For a uniformly bright disc viewed by a gaussian beam:
fill = 1-2-x*x with x = ang.diam/HPBW.].
The effective diameter Deff of an antenna is the diameter of a uniformly illuminated dish antenna which gives the same HPBW. The corresponding area Aeff = π D2eff/4 is the antenna's effective capture area. Thus a source with radio flux F in unpolarized radiation gives a received power P = F * Aeff/2, as the feed (dipole or waveguide) in the antenna's focus is sensitive to only one state of polarization.
The data for the quiet Sun are from: A.O.Benz: Radio Emission of the Quiet Sun in Solar System, Landolt-Börnstein - Group VI Astronomy and Astrophysics, Volume B, Springer-Verlag, Heidelberg, 2009 (DOI 10.1007/978-3-540-88055-4_5).
The additional black curve for the Sun and the Moon shows the brightness temperature as a function of frequency. For the Moon this is the physical temperature on its surface, while for the Sun it is the temperature in the layer from where the radio emission at that frequency comes from.
Emission from Galactic Centre and Anticentre is estimated only for a patch of about 1° square; an antenna with a wider beam will pick up more emission from adjacent areas.
When entering a value in the text fields, hit the return key to display the new plot. The receiving system noise may be entered as system temperature or as noise figure. The light green background colour indicates the current choice.
Vertical lines mark frequencies of 144, 432, 1420, 10380, 24000, 47000, and 77000 MHz. Horizontal lines indicate the levels of the Cosmic Microwave Background (2.7 K) and ground radiation (290 K). The dashed line marks the receiver noise (system) temperature.
Absorption and thermal emission from the Earth atmosphere is about 5 K at the zenith, but increase towards the horizon (like 1/sin(elevation)) and also for frequencies above about 20 GHz. Note that if a value for the air absorption is given, it is taken into account in the displayed numerical data, but not in the plots ;-)
The angular diameter of each source is given in parentheses.