<p>Jansky is defined as  <span class="math-container">$1 Jy= 10^{-23}erg/s/cm^2/Hz$</span>. So all the energy coming from a given solid angle per time, per frequency bin and per detector area.</p>

<p>This is convenient for sources of small angular extent (smaller than observing beam), e.g. for point sources, because the flux remains constant for varying beam sizes. For extended sources, the surface brightness is often described with units of Jy per solid angle. Radio contour maps, e.g. of interstellar clouds, usually show objects that are larger than the beam, so that the observed fluxes are a function of the beam size. To clean the beam influence from the date Jy/beam (beam area in steradian) is used. </p>

<p>So it is a surface brightness. For a flux of 1 Jy measured with a beam of 1 sr solid angle we find:   <span class="math-container">$1 Jy/beam= 10^{-23} erg/s/cm^2/Hz/sr$</span>. </p>

<p>Note, that 1 sr is already a huge portion of the sky. Beams are much smaller, on the order of arcsec to arcmin diameters.</p>

CC BY-SA 3.0
2017-03-14T11:13:19.497Z
I think the OP's confusion stems from not knowing what the "beam" is. Perhaps you could include an explanation of this concept (and its relation to field of view). Also, I think the last sentence might be a bit confusing, since what you say there is essentially "So it is a surface brightness Jy/sr", which is not really the case, except for a beam of exactly 1 sr.
pela
CC BY-SA 3.0
2017-03-14T11:45:12.797Z
@pela Correct me if I am mistaken, but if we dived a flux by a solid angle, we get a surface brightness, i.e. the mean flux per sr, If we observe a point source having a flux of 10Jy with a 1 sr beam it has a s.b. of 10 Jy/beam.(or Jy/sr)  Observing with a 2 sr beam gives a s.b of 5 Jy/beam. Right?
Markus Roellig
CC BY-SA 3.0
2017-03-14T12:36:27.547Z
Yes, exactly. That's what I mean. So SB can be expressed in Jy/sr or in Jy/beam (or Jy/arcsec² or whatever), but the two numbers will differ by a constant. Sorry, I also see now that _my_ comment was a bit confusing, since "erg/s/cm²/Hz/sr" is not equal to "Jy/sr", but to "1e23 Jy/sr". Anyway, my point was that your last sentence might be interpreted as "1 Jy/beam is 1 erg/s/cm²/Hz/sr", which is not really correct.
pela
CC BY-SA 3.0
2017-03-14T13:22:32.627Z
@pela Thank you. I updated the text. Hope it is clearer now :)
Markus Roellig
CC BY-SA 3.0
2017-03-14T20:43:23.930Z
Yes, excellent!
pela
CC BY-SA 4.0
2023-08-27T23:43:16.040Z
any thoughts on [How do we get -0.4 mJy/beam in the the 1.28 GHz MeerKAT Galactic Center Mosaic?](https://astronomy.stackexchange.com/q/54563/7982)
uhoh
CC BY-SA 4.0
2017-03-14T10:31:10.523Z
2019-07-02T07:34:01.137Z
2019-07-02T07:34:01.137Z
Markus Roellig
<p>I know what a jansky is, but I don't understand what beam (in <span class="math-container">$\frac{1 Jy}{beam}$</span>) is.
If jansky is a unit of flux density, what kind of unit is <span class="math-container">$\frac{1 Jy}{beam}$</span>?</p>

CC BY-SA 3.0
2016-12-14T18:37:10.503Z
Welcome to Astronomy Stack Exchange! Can you edit your question to include an example of such a map, along with the terminology you mention? That would make it much easier to answer.
HDE 226868
CC BY-SA 3.0
2016-12-14T20:06:49.593Z
When talking about fluxes here, there are two areas involved. One is the area through which the source is emitting and that is wrapped up in the Janksy. The beam is the area of the sky the radar is observing (more or less). So you're basically saying the energy per emitted area per frequency per measured area. I believe, that's the high-level answer, but my background is not in radio astronomy so perhaps someone with more technical knowledge can provide a better answer.
zephyr
<radio-astronomy><units><radio>
Why levels of radio contours maps are given in mJy/beam and what does it mean?
mavic
# Why levels of radio contours maps are given in mJy/beam and what does it mean?

(Asked by: username_0 on 2016-12-14)

I know what a jansky is, but I don't understand what beam (in $\frac{1 Jy}{beam}$) is.
If jansky is a unit of flux density, what kind of unit is $\frac{1 Jy}{beam}$?

**Tags:** radio-astronomy, units, radio
> **Comments:**
> **username_1**: Welcome to Astronomy Stack Exchange! Can you edit your question to include an example of such a map, along with the terminology you mention? That would make it much easier to answer.  
> **username_2**: When talking about fluxes here, there are two areas involved. One is the area through which the source is emitting and that is wrapped up in the Janksy. The beam is the area of the sky the radar is observing (more or less). So you're basically saying the energy per emitted area per frequency per measured area. I believe, that's the high-level answer, but my background is not in radio astronomy so perhaps someone with more technical knowledge can provide a better answer.  

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## 1 answers

### Answer by username_3 on 2017-03-14. Score: 5

Jansky is defined as $1 Jy= 10^{-23}erg/s/cm^2/Hz$. So all the energy coming from a given solid angle per time, per frequency bin and per detector area.

This is convenient for sources of small angular extent (smaller than observing beam), e.g. for point sources, because the flux remains constant for varying beam sizes. For extended sources, the surface brightness is often described with units of Jy per solid angle. Radio contour maps, e.g. of interstellar clouds, usually show objects that are larger than the beam, so that the observed fluxes are a function of the beam size. To clean the beam influence from the date Jy/beam (beam area in steradian) is used.

So it is a surface brightness. For a flux of 1 Jy measured with a beam of 1 sr solid angle we find: $1 Jy/beam= 10^{-23} erg/s/cm^2/Hz/sr$.

Note, that 1 sr is already a huge portion of the sky. Beams are much smaller, on the order of arcsec to arcmin diameters.

> **Comments:**
> **username_4**: I think the OP's confusion stems from not knowing what the "beam" is. Perhaps you could include an explanation of this concept (and its relation to field of view). Also, I think the last sentence might be a bit confusing, since what you say there is essentially "So it is a surface brightness Jy/sr", which is not really the case, except for a beam of exactly 1 sr.  
> **username_3**: @pela Correct me if I am mistaken, but if we dived a flux by a solid angle, we get a surface brightness, i.e. the mean flux per sr, If we observe a point source having a flux of 10Jy with a 1 sr beam it has a s.b. of 10 Jy/beam.(or Jy/sr)  Observing with a 2 sr beam gives a s.b of 5 Jy/beam. Right?  
> **username_4**: Yes, exactly. That's what I mean. So SB can be expressed in Jy/sr or in Jy/beam (or Jy/arcsec² or whatever), but the two numbers will differ by a constant. Sorry, I also see now that _my_ comment was a bit confusing, since "erg/s/cm²/Hz/sr" is not equal to "Jy/sr", but to "1e23 Jy/sr". Anyway, my point was that your last sentence might be interpreted as "1 Jy/beam is 1 erg/s/cm²/Hz/sr", which is not really correct.  
> **username_3**: @pela Thank you. I updated the text. Hope it is clearer now :)  
> **username_4**: Yes, excellent!  
> **username_5**: any thoughts on [How do we get -0.4 mJy/beam in the the 1.28 GHz MeerKAT Galactic Center Mosaic?](https://astronomy.stackexchange.com/q/54563/7982)  

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