<p>A priori it is very difficult to distinguish the origin of any particular feature from just one image.</p>

<p>For that reason it is established workflow, especially in astronomical context, to create 4 kind of images:</p>

<ul>
<li>the light frame R. That's the actual image of what you are interested
in </li>
<li>the dark frame D. That's an exposure of identical length and at
identical conditions, but with closed apperture. That takes care to
assess the sensor reading in the absense of anything visible</li>
<li>the bias frame B. That's a short-duration image which allows to assess the
read-out noise </li>
<li>the flat field F. That's an image of a perfectly white /
grey area which allows to assess the different sensitivity of the
individual pixels.</li>
</ul>

<p>Then you can obtain the corrected image C via <span class="math-container">$ C = (R-D-B) \cdot \frac{&lt;F - D - B&gt;}{F - D - B}$</span></p>

<p><a href="https://en.wikipedia.org/wiki/Flat-field_correction" rel="noreferrer">https://en.wikipedia.org/wiki/Flat-field_correction</a></p>

CC BY-SA 4.0
2022-12-14T12:03:03.260Z
One can also take two or more images with the pointing slightly offset. Astronomical objects will all shift the same amount, in the same direction, between the two images; instrumental features will either be missing from one image or will keep the same (pixel) location.
Peter Erwin
CC BY-SA 4.0
2020-04-27T08:41:04.060Z
2020-04-27T08:41:04.060Z
planetmaker
<p>Here is an image of hcg79 taken from APO. I'm just curious if there is any way to separate astronomical and instrumental features in this picture?</p>
<p><a href="https://i.sstatic.net/mnfhF.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mnfhF.png" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/c3G7W.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/c3G7W.jpg" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/L6n7E.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/L6n7E.jpg" alt="enter image description here" /></a></p>

<observational-astronomy>
Is there any way to separate astronomical and instrumental objects?
Boris
# Is there any way to separate astronomical and instrumental objects?

(Asked by: username_0 on 2020-04-27)

Here is an image of hcg79 taken from APO. I'm just curious if there is any way to separate astronomical and instrumental features in this picture?

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

[![enter image description here](https://i.sstatic.net/c3G7W.jpg)](https://i.sstatic.net/c3G7W.jpg)

[![enter image description here](https://i.sstatic.net/L6n7E.jpg)](https://i.sstatic.net/L6n7E.jpg)

**Tags:** observational-astronomy


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## 1 answers

### Answer by username_1 on 2020-04-27. Score: 5 | Accepted answer

A priori it is very difficult to distinguish the origin of any particular feature from just one image.

For that reason it is established workflow, especially in astronomical context, to create 4 kind of images:

* the light frame R. That's the actual image of what you are interested
  in
* the dark frame D. That's an exposure of identical length and at
  identical conditions, but with closed apperture. That takes care to
  assess the sensor reading in the absense of anything visible
* the bias frame B. That's a short-duration image which allows to assess the
  read-out noise
* the flat field F. That's an image of a perfectly white /
  grey area which allows to assess the different sensitivity of the
  individual pixels.

Then you can obtain the corrected image C via $ C = (R-D-B) \cdot \frac{<F - D - B>}{F - D - B}$

<https://en.wikipedia.org/wiki/Flat-field_correction>

> **Comments:**
> **username_2**: One can also take two or more images with the pointing slightly offset. Astronomical objects will all shift the same amount, in the same direction, between the two images; instrumental features will either be missing from one image or will keep the same (pixel) location.  

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