<p>The answer is in the <a href="https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-performance/nircam-absolute-flux-calibration-and-zeropoints#gsc.tab=0" rel="nofollow noreferrer">NIRcam docs</a>, but I’ll quote the relevant bit here:</p>
<blockquote>
<p>To measure photometry in MJy, convert the image to units of MJy/pixel by multiplying by the average area of a pixel in sr, a constant provided in the FITS header keyword PIXAR_SR. Note this value varies slightly (by a few percent) among the 8 short wavelength detectors and between the 2 long wavelength detectors. They correspond approximately to the nominal pixel sizes: 0.031&quot; and 0.063&quot;, for the short and long wavelength channels, respectively.</p>
</blockquote>
<p>So find that <code>PIXAR_SR</code> value in the image header, and you’ll have your conversion to MJy; then just convert MJy -&gt; uJy if desired.</p>

CC BY-SA 4.0
2024-08-31T14:01:50.250Z
It's maybe worth pointing out that it's maybe easiest to do this to the image *before* running SExtractor. Then the "counts" values reported by SExtractor will be MJy.
Peter Erwin
CC BY-SA 4.0
2024-09-01T13:20:25.657Z
@PeterErwin could you please explain this a bit further, so that I could catch up.
Alan
CC BY-SA 4.0
2024-09-02T17:17:50.380Z
@Alan In case it isn't clear, SExtractor treats pixel values as "counts" (it doesn't know about different flux units). If the pixel values are actually MJr/sr, then that's what "counts" means.  But MJy/sr is a *surface brightness* unit; you have to divide by the pixel area, as Eric Jensen notes. My suggestion was that you do this to the image before running SExtractor; then the SE "counts" are precisely MJy.
Peter Erwin
CC BY-SA 4.0
2024-09-07T10:51:51.253Z
@PeterErwin Thanks for the commands.
Alan
CC BY-SA 4.0
2024-08-31T12:44:54.600Z
2024-08-31T12:44:54.600Z
Eric Jensen
<p>I have a jwst nircam image of a galactic field. The science images are in units of MegaJansky per steradian (MJy/sr), and so the zeropoint is 28.08652 AB mag . When I ran sextractor on this image , I got my flux in units of counts(as per the output of Sextractor catalogue)</p>
<pre><code>#   6 FLUX_APER              Flux vector within fixed circular aperture(s)              [count]
</code></pre>
<p>I wanted to convert my flux into units of uJy which could be used with eazy for redshift determination.How can I convert the sextractor output flux into units of uJy</p>

<photometry><hubble-telescope><james-webb-space-telescope><units>
Flux unit conversion
Alan
# Flux unit conversion

(Asked by: username_0 on 2024-08-31)

I have a jwst nircam image of a galactic field. The science images are in units of MegaJansky per steradian (MJy/sr), and so the zeropoint is 28.08652 AB mag . When I ran sextractor on this image , I got my flux in units of counts(as per the output of Sextractor catalogue)
#   6 FLUX_APER              Flux vector within fixed circular aperture(s)              [count]

I wanted to convert my flux into units of uJy which could be used with eazy for redshift determination.How can I convert the sextractor output flux into units of uJy

**Tags:** photometry, hubble-telescope, james-webb-space-telescope, units

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## 1 answers

### Answer by username_1 on 2024-08-31. Score: 2

The answer is in the [NIRcam docs](https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-performance/nircam-absolute-flux-calibration-and-zeropoints#gsc.tab=0), but I’ll quote the relevant bit here:

> To measure photometry in MJy, convert the image to units of MJy/pixel by multiplying by the average area of a pixel in sr, a constant provided in the FITS header keyword PIXAR\_SR. Note this value varies slightly (by a few percent) among the 8 short wavelength detectors and between the 2 long wavelength detectors. They correspond approximately to the nominal pixel sizes: 0.031" and 0.063", for the short and long wavelength channels, respectively.

So find that `PIXAR_SR` value in the image header, and you’ll have your conversion to MJy; then just convert MJy -> uJy if desired.

> **Comments:**
> **username_2**: It's maybe worth pointing out that it's maybe easiest to do this to the image *before* running SExtractor. Then the "counts" values reported by SExtractor will be MJy.  
> **username_0**: @PeterErwin could you please explain this a bit further, so that I could catch up.  
> **username_2**: @Alan In case it isn't clear, SExtractor treats pixel values as "counts" (it doesn't know about different flux units). If the pixel values are actually MJr/sr, then that's what "counts" means.  But MJy/sr is a *surface brightness* unit; you have to divide by the pixel area, as Eric Jensen notes. My suggestion was that you do this to the image before running SExtractor; then the SE "counts" are precisely MJy.  
> **username_0**: @PeterErwin Thanks for the commands.  

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