Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources; for example, the
Third Cambridge Catalogue of Radio Sources (3C) reports results in janskys.
For extended sources, the surface brightness is often described with units of janskys per solid angle; for example, far-infrared (FIR) maps from the
IRAS satellite are in megajanskys per
steradian (MJy⋅sr−1).
Although extended sources at all wavelengths can be reported with these units, for radio-frequency maps, extended sources have traditionally been described in terms of a
brightness temperature; for example the Haslam et al. 408 MHz all-sky continuum survey is reported in terms of a brightness temperature in
kelvin.[3]
Unit conversions
Jansky units are not a standard SI unit, so it may be necessary to convert the measurements made in the unit to the SI equivalent in terms of watts per square metre per hertz (W·m−2·Hz−1). However, other unit conversions are possible with respect to measuring this unit.
AB magnitude
The flux density in janskys can be converted to a magnitude basis, for suitable assumptions about the spectrum. For instance, converting an
AB magnitude to a flux density in microjanskys is straightforward:[4]
dBW·m−2·Hz−1
The linear flux density in janskys can be converted to a
decibel basis, suitable for use in fields of telecommunication and radio engineering.
1 jansky is equal to −260
dBW·m−2·Hz−1, or −230
dBm·m−2·Hz−1:[5]
Gravitational waves also carry energy, so their flux density can also be expressed in terms of janskys. Typical signals on Earth are expected to be 1020 Jy or more.[6] However, because of the poor coupling of gravitational waves to matter, such signals are difficult to detect.
When measuring broadband continuum emissions, where the energy is roughly evenly distributed across the detector
bandwidth, the detected signal will increase in proportion to the bandwidth of the detector (as opposed to signals with bandwidth narrower than the detector bandpass). To calculate the flux density in janskys, the total power detected (in watts) is divided by the receiver collecting area (in square meters), and then divided by the detector bandwidth (in hertz). The flux density of astronomical sources is many orders of magnitude below 1 W·m−2·Hz−1, so the result is multiplied by 1026 to get a more appropriate unit for natural astrophysical phenomena.[7]
The millijansky, mJy, was sometimes referred to as a milli-flux unit (mfu) in older astronomical literature.[8]
^Kraus, John Daniel (1986).
Radio Astronomy. Table: Radio spectrum of astronomical sources.
ISBN1882484002. Archived from
the original on 16 May 2013. Retrieved 24 August 2013.