<p>This looks like readout noise to me. Such noise is a consequence of the electric machinery used to convert the individual pixel detections into a digital picture, and really is an unavoidable part of CCD technology.</p>

<p>Take a look at <a href="http://spiff.rit.edu/classes/phys373/lectures/readout/readout.html" rel="nofollow">this page</a> if you want to know more about the technical details.</p>

CC BY-SA 3.0
2014-10-02T15:00:50.330Z
There's definitely a lot of read noise, but read noise would not cause a fixed pattern (horizontal and vertical lines) like the one in the image. I should have adjusted the contrast some more - the pattern is hard to see on some monitors.
user181339
CC BY-SA 3.0
2014-08-04T20:43:41.287Z
2014-08-04T20:43:41.287Z
Michael B.
<p>Yes, it's some sort of CCD structure originated from the manufacturing process. CCD chips with large area were produced by chunks of several hundred pixels. see <a href="http://heasarc.gsfc.nasa.gov/Images/astroe/about/xis_config.gif" rel="nofollow noreferrer">http://heasarc.gsfc.nasa.gov/Images/astroe/about/xis_config.gif</a></p>

<p>And sometimes position offsets between these chunks can be detected.</p>

<p><img src="https://i.sstatic.net/fjbSl.png" alt="CCD pixel blocks"></p>

<p>from <a href="http://adsabs.harvard.edu/abs/2010MNRAS.405L..16W" rel="nofollow noreferrer">Wilken et al., 2010, MNRAS, 405, L16</a></p>

CC BY-SA 3.0
2014-08-07T01:22:34.763Z
2014-08-07T01:22:34.763Z
Asterion
<p>The image below is a small region of a twilight flat field. A grid of darker pixels with 728x36 cell dimensions can be seen.</p>
<p><a href="https://i.sstatic.net/Suzw6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Suzw6.png" alt="enter image description here" /></a></p>
<p>The camera used was the SBIG STL-6303 according to the FIT headers, which uses a Kodak KAF-6303E CCD according to the online data sheet.</p>
<p>The CCD dimensions, these cell dimensions, and all the binning options are all perfectly divisible, so I'm assuming this artifact is due to the structure of the CCD.</p>
<p>I plotted the mean pixel value of each row to make sure this wasn't an artifact from the image viewing software. The plot clearly shows a drop every 36 pixels.</p>
<p>So, I'm hoping for some confirmation: is this decreased sensitivity due to the structure of the CCD?</p>
<p>If so, what about the CCD structure actually causes this?</p>

CC BY-SA 3.0
2014-08-04T21:54:35.550Z
Could be worth posting over on Photography.SE?
Rory Alsop
CC BY-SA 3.0
2014-08-08T15:18:25.730Z
@RoryAlsop It [started over there](http://photo.stackexchange.com/questions/53262/why-is-there-a-grid-pattern-of-darker-pixels-in-this-twilight-sky-flat-field) before "we" (Photo.SE) sent it over here as we didn't have much clue :-)
Philip Kendall
<photography>
Why does this twilight sky flat field have a grid of dark pixels?
user181339
# Why does this twilight sky flat field have a grid of dark pixels?

(Asked by: username_0 on 2014-08-04)

The image below is a small region of a twilight flat field. A grid of darker pixels with 728x36 cell dimensions can be seen.

[![enter image description here](https://i.sstatic.net/Suzw6.png)](https://i.sstatic.net/Suzw6.png)

The camera used was the SBIG STL-6303 according to the FIT headers, which uses a Kodak KAF-6303E CCD according to the online data sheet.

The CCD dimensions, these cell dimensions, and all the binning options are all perfectly divisible, so I'm assuming this artifact is due to the structure of the CCD.

I plotted the mean pixel value of each row to make sure this wasn't an artifact from the image viewing software. The plot clearly shows a drop every 36 pixels.

So, I'm hoping for some confirmation: is this decreased sensitivity due to the structure of the CCD?

If so, what about the CCD structure actually causes this?

**Tags:** photography
> **Comments:**
> **username_1**: Could be worth posting over on Photography.SE?  
> **username_2**: @RoryAlsop It [started over there](http://photo.stackexchange.com/questions/53262/why-is-there-a-grid-pattern-of-darker-pixels-in-this-twilight-sky-flat-field) before "we" (Photo.SE) sent it over here as we didn't have much clue :-)  

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## 2 answers

### Answer by username_4 on 2014-08-07. Score: 6 | Accepted answer

Yes, it's some sort of CCD structure originated from the manufacturing process. CCD chips with large area were produced by chunks of several hundred pixels. see <http://heasarc.gsfc.nasa.gov/Images/astroe/about/xis_config.gif>

And sometimes position offsets between these chunks can be detected.

![CCD pixel blocks](https://i.sstatic.net/fjbSl.png)

from [Wilken et al., 2010, MNRAS, 405, L16](http://adsabs.harvard.edu/abs/2010MNRAS.405L..16W)


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### Answer by username_3 on 2014-08-04. Score: 4

This looks like readout noise to me. Such noise is a consequence of the electric machinery used to convert the individual pixel detections into a digital picture, and really is an unavoidable part of CCD technology.

Take a look at [this page](http://spiff.rit.edu/classes/phys373/lectures/readout/readout.html) if you want to know more about the technical details.

> **Comments:**
> **username_0**: There's definitely a lot of read noise, but read noise would not cause a fixed pattern (horizontal and vertical lines) like the one in the image. I should have adjusted the contrast some more - the pattern is hard to see on some monitors.  

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