<p>That's normal - but it is not (random) noise.</p>
<p>The more sophisticated method does not only employ stacking of the images (light frames, <span class="math-container">$I$</span>), but it also corrects for the dark frame (due to sensor noise with identical exposure like the light frame, <span class="math-container">$D$</span>) and sometimes the bias frames (short duration exposure to capture the read-out noise, <span class="math-container">$B$</span>). The redish background likely results from a missing dark frame correction which will most often reduce any red captured when no light reaches the sensor (e.g. through the device's own thermal influence on its sensor which are somewhat sensitive into the near infrared).</p>
<p>Additionally the images are usually also corrected by <a href="https://en.wikipedia.org/wiki/Flat-field_correction" rel="noreferrer">flat fielding</a> which corrects for the different sensitivity of the pixels as well as the optics. Flat field images <span class="math-container">$F$</span> are aquired by taking images of a <a href="https://astrobackyard.com/how-to-take-flat-frames/" rel="noreferrer">perfectly white surface</a> with the same optical setup, such that no pixel is over-exposed, but nearly saturated.</p>
<p>The overall correction sequence to obtain a corrected astrophotography image <span class="math-container">$C$</span> can be:</p>
<p><span class="math-container">$$
C = \frac{(I - D)\cdot &lt;F - D&gt;}{F - D}
$$</span>
where <span class="math-container">$&lt;F - D&gt;$</span> denotes the average</p>
<p>Sequator should provide (according to the docs I find) means to correct for dark and flatfield frames as well. Personally I made good experience with <a href="https://siril.org/" rel="noreferrer">Siril</a> which allows to provide the stacking tool with all these different kind of frames. But there are <a href="https://nightskypix.com/astrophotography-stacking-software/" rel="noreferrer">other tools</a> which deal with this as well.</p>
<p>All this said: if you do astrophotography for the looks and not so much for science, there is no harm in loading the stacked result into Gimp, Photoshop or whatever you prefer and fine-tune the result in terms of colour to what you think should be the result; CCD and CMOS sensor sensitivities do not correspond to the sensitivity of the human eye - so there is even good reason to touch images in this regard.</p>
<p>See also</p>
<ul>
<li><a href="https://beltoforion.de/en/astrophotography/stacking.php" rel="noreferrer">https://beltoforion.de/en/astrophotography/stacking.php</a></li>
<li><a href="https://nightskypix.com/calibration-frames/" rel="noreferrer">https://nightskypix.com/calibration-frames/</a></li>
</ul>

CC BY-SA 4.0
2022-01-10T10:46:59.620Z
2022-01-10T10:55:13.270Z
2022-01-10T10:55:13.270Z
planetmaker
<p>I have always been fascinated by astrophotography and have been trying my hands at the skill. I have tried photographing the milky way with limited resources (smartphone with a tripod). I usually click individual photos with 30s exposure, 3200 ISO and stack the raw images in the free software Sequator. Here is how an individual photo looks like <a href="https://i.sstatic.net/2GLfd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2GLfd.png" alt="enter image description here" /></a></p>
<p>After stacking 10 of such photos, and playing with the settings on Sequator, the best photo according to me, which makes the milky way more prominently visible, looks like this</p>
<p><a href="https://i.sstatic.net/N5P9A.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/N5P9A.jpg" alt="enter image description here" /></a></p>
<p>Even though more stars are visible due to stacking and milky way looks more obvious (?), it does add a lot of noise to the image. I am specifically talking about the blue and purple colors that are added onto the image. Is there a problem in the way that I am stacking the images or do I need to next employ some photo editing software (Photoshop, GIMP) to get rid of this noise? Any help is appreciated! Thank you!</p>

CC BY-SA 4.0
2022-01-12T00:12:24.210Z
*Welcome to Stack Exchange!* There are [almost 200 sites](https://stackexchange.com/sites#name) to choose from. You may be interested in checking out [372+ questions tagged with astronphotography in Photography SE](https://photo.stackexchange.com/questions/tagged/astrophotography) where discussion of mainstream image processing applications can be found. Here in Astronomy SE people may be more familiar with processing astronomical images using very rigorous techniques which may be quite fun to learn but might be overkill if you are looking for aesthetic results rather than scientific results.
uhoh
<photography>
Avoiding noise while stacking multiple night sky photographs
Rishav Roy
# Avoiding noise while stacking multiple night sky photographs

(Asked by: username_0 on 2022-01-10)

I have always been fascinated by astrophotography and have been trying my hands at the skill. I have tried photographing the milky way with limited resources (smartphone with a tripod). I usually click individual photos with 30s exposure, 3200 ISO and stack the raw images in the free software Sequator. Here is how an individual photo looks like [![enter image description here](https://i.sstatic.net/2GLfd.png)](https://i.sstatic.net/2GLfd.png)

After stacking 10 of such photos, and playing with the settings on Sequator, the best photo according to me, which makes the milky way more prominently visible, looks like this

[![enter image description here](https://i.sstatic.net/N5P9A.jpg)](https://i.sstatic.net/N5P9A.jpg)

Even though more stars are visible due to stacking and milky way looks more obvious (?), it does add a lot of noise to the image. I am specifically talking about the blue and purple colors that are added onto the image. Is there a problem in the way that I am stacking the images or do I need to next employ some photo editing software (Photoshop, GIMP) to get rid of this noise? Any help is appreciated! Thank you!

**Tags:** photography
> **Comments:**
> **username_1**: *Welcome to Stack Exchange!* There are [almost 200 sites](https://stackexchange.com/sites#name) to choose from. You may be interested in checking out [372+ questions tagged with astronphotography in Photography SE](https://photo.stackexchange.com/questions/tagged/astrophotography) where discussion of mainstream image processing applications can be found. Here in Astronomy SE people may be more familiar with processing astronomical images using very rigorous techniques which may be quite fun to learn but might be overkill if you are looking for aesthetic results rather than scientific results.  

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## 1 answers

### Answer by username_2 on 2022-01-10. Score: 5

That's normal - but it is not (random) noise.

The more sophisticated method does not only employ stacking of the images (light frames, $I$), but it also corrects for the dark frame (due to sensor noise with identical exposure like the light frame, $D$) and sometimes the bias frames (short duration exposure to capture the read-out noise, $B$). The redish background likely results from a missing dark frame correction which will most often reduce any red captured when no light reaches the sensor (e.g. through the device's own thermal influence on its sensor which are somewhat sensitive into the near infrared).

Additionally the images are usually also corrected by [flat fielding](https://en.wikipedia.org/wiki/Flat-field_correction) which corrects for the different sensitivity of the pixels as well as the optics. Flat field images $F$ are aquired by taking images of a [perfectly white surface](https://astrobackyard.com/how-to-take-flat-frames/) with the same optical setup, such that no pixel is over-exposed, but nearly saturated.

The overall correction sequence to obtain a corrected astrophotography image $C$ can be:

$$
C = \frac{(I - D)\cdot }{F - D}
$$
where $$ denotes the average

Sequator should provide (according to the docs I find) means to correct for dark and flatfield frames as well. Personally I made good experience with [Siril](https://siril.org/) which allows to provide the stacking tool with all these different kind of frames. But there are [other tools](https://nightskypix.com/astrophotography-stacking-software/) which deal with this as well.

All this said: if you do astrophotography for the looks and not so much for science, there is no harm in loading the stacked result into Gimp, Photoshop or whatever you prefer and fine-tune the result in terms of colour to what you think should be the result; CCD and CMOS sensor sensitivities do not correspond to the sensitivity of the human eye - so there is even good reason to touch images in this regard.

See also

* <https://beltoforion.de/en/astrophotography/stacking.php>
* <https://nightskypix.com/calibration-frames/>


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