<p>This is a really interesting question!</p>
<h1>Atmospheric distortions:</h1>
<p>As the discussion in comments points out, terrestrial telescopes use wavefront correction to undo Earth's <em>atmospheric</em> distortion, an in general atmospheric correction is not needed for telescopes in space.</p>
<p>There is a caveat for a telescope trying to observe the surface of a planet through its atmosphere. The most obvious case would be <em>non-astronomical</em> observations of Earth:</p>
<ul>
<li><a href="https://space.stackexchange.com/q/20107/12102">Atmospheric influences on earth to satellite visibility and vice versa (e.g. atmospheric seeing)</a></li>
</ul>
<p>There was a mild &quot;conspiracy theory&quot; that JWST had wavefront correction optics in its secondary mirror, but that was quickly dispelled.</p>
<ul>
<li><a href="https://space.stackexchange.com/q/18938/12102">Why would the James Webb Space Telescope need adaptive optics outside the atmosphere?</a></li>
</ul>
<h1>Astronomical distortions</h1>
<p>There are two that I can think of, and both are important fields of research.</p>
<h3>1. Plasma vs. Radio</h3>
<p>In radio astronomy, there is (primarily interplanetary) scintillation. See for example:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/a/34579/7982">this answer to <em>What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?</em></a></li>
</ul>
<p>The timescale for changes in this kind of wavefront distortion (be it from interplanetary plasma or Earth atmosphere (ionospheric and water vapor)) is longer than for atmospheric optical measurements - minutes to months. And because radio astronomy data is recorded electronically in a way that <strong>preserves phase information</strong> for off-line implementation of interferometry and image construction, the closed loop will look a little different. For example, when the Event Horizon Telescope recorded data for several days to image galactic center objects involving black holes and their accretion disks, there was always one or two other radio objects in the same electronic fields of view which were used as reference, analogous to &quot;guide stars&quot;.</p>
<h3>2. Gravity vs. all wavelengths</h3>
<p>Gravitational lensing and microlensing phenomena are interesting in their own right, but they are also a very useful tool to make observations of things so distant that they would be too dim to observe or too small to resolve.</p>
<p>Generally the timescales for any <em>changes</em> in wavefront distortion are &quot;astronomically&quot; long, but transient effects are observed, say when a closer star passes in front of a more distant point-like object.</p>
<p>Once again, the wavefront distortion is <em>computational</em> and done offline. Usually this is done for optical images, where we have intensity only and have lost phase information. <strong>In this case phase information is lost and there is no such analog to the wavefront sensor.</strong> We're then left with a very fancy &quot;trial-and-error&quot; program that varies the correction until a sharp image is realized.</p>
<p>For more on that, see:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/50029/7982">If I can&#39;t unscramble an egg, how do Astronomers unscramble views gravitationally lensed by complex mass distributions?</a></li>
</ul>

CC BY-SA 4.0
2024-06-18T11:28:38.653Z
Thank you! I found a lot of answers to some questions. It seems to me that extra-atmospheric optics is the next layer in optical sciences; some models have probably also been developed (the influence of dust, plasma, cosmic rays, etc. on the distortion of electromagnetic waves and image restoration methods). These can be purely computational tasks (based on neural networks, for example).
ayr
CC BY-SA 4.0
2024-06-16T00:44:28.593Z
2024-06-16T00:44:28.593Z
uhoh
<p>Closed-loop control systems in general have the following form:</p>
<p><a href="https://i.sstatic.net/19LqZiy3.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/19LqZiy3.png" alt="enter image description here" /></a></p>
<p>Here the controller seeks to eliminate or minimize the difference between a given signal (this can be either a constant signal of the desired physical nature or time-varying) and the desired state of the control object-system measured using a sensor.</p>
<p>The generalized diagram of an adaptive optical system is as follows:</p>
<p><a href="https://i.sstatic.net/ZQZNPsmS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZQZNPsmS.png" alt="enter image description here" /></a></p>
<p>This is how this system works. The wave front of electromagnetic waves from the object of observation passes through outer space and is distorted (small cosmic bodies, dust, etc. can introduce aberrations). It is then reflected from a mirror with a customizable spatial configuration and hits a “light splitter.” One branch goes to the image recording and storage device. The other branch goes to the wavefront sensor, after which the signal enters the control system. An internal &quot;wavefront corrector&quot; adjusts the spatial configuration of the mirror so that...</p>
<p><strong>And this is where my question begins:</strong> The fact is that in such a system we cannot set the reference signal in the form “the controlled output signal of the control object must be equal to 5 V (roughly speaking).” We also cannot set the desired image, because... such an adaptive optics system would not make sense, and besides, we do not know in advance what the object of observation looks like (a high-quality image of, for example, a star cluster is the result of the system’s operation). The term &quot;wavefront corrector&quot; contains the necessary instructions. But what kind of correction are we talking about? How do you know if the wavefront is corrected correctly? Are the properties of the wavefront sensor used for this? Or are integral indicators used (for example, the Strehl number or others)? Or does the system not use a reference signal, as in classical control systems, but maximizes some indicators of the quality of the adaptive optical system and the image obtained with its help?</p>
<p><em>Generally speaking, how does an adaptive optics system know that the image on the left is bad and the image on the right is good? How does this rate?</em></p>
<hr />
<p><strong>UPDATE:</strong> (based on comments below) I just realized that the problems that adaptive optical systems solve are not so acute for spacecraft, because there are no “atmospheric” distortions like on Earth. But at the same time, what if we are talking about observing exoplanets, or observing space objects “contaminated” with cosmic dust or other factors. How would adaptive optics work there? Also with reference lasers? Is it possible to do without lasers?</p>
<p><a href="https://i.sstatic.net/bjljMtUr.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bjljMtUr.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2024-06-14T18:07:54.020Z
What would you want an extra-atmospheric optical system adapt to? There's no systematic distortion.
Zac67
CC BY-SA 4.0
2024-06-14T18:20:51.460Z
@Zac67 First of all, I would like to know the answer to the question about the technical properties of such a system.  And what to adapt to...in fact, the last picture, at such objects I would check the performance of the system.
ayr
CC BY-SA 4.0
2024-06-14T19:22:41.087Z
My point is: no distortion, no reference signal.
Zac67
CC BY-SA 4.0
2024-06-14T20:34:04.090Z
@Zac67 Then let's take a look at the last picture.  The left image is very muddy.  Why is that?  And how are adaptive optics adjusted to produce the right image?
ayr
CC BY-SA 4.0
2024-06-14T21:33:47.170Z
That example is a *terrestrial* telescope disturbed by atmospheric turbulence. The trick is to use a reference signal (laser beam, guide star) to 'modulate' a flexible mirror to counter the disturbance. Without the atmosphere there's no disturbance to counter (at least nothing you can do with a flexible mirror). For starters: https://en.wikipedia.org/wiki/Adaptive_optics
Zac67
CC BY-SA 4.0
2024-06-15T04:14:19.063Z
@Zac67 I just realized that the problems that adaptive optical systems solve are not so acute for spacecraft, because there are no “atmospheric” distortions like on Earth. But at the same time, what if we are talking about observing exoplanets, or observing space objects “contaminated” with cosmic dust or other factors. How would adaptive optics work there? Also with reference lasers? Is it possible to do without lasers?
ayr
<observational-astronomy><optics><instruments><adaptive-optics>
What are the reference signal(s) for a closed-loop space-based adaptive optical system?
ayr
# What are the reference signal(s) for a closed-loop space-based adaptive optical system?

(Asked by: username_0 on 2024-06-14)

Closed-loop control systems in general have the following form:

[![enter image description here](https://i.sstatic.net/19LqZiy3.png)](https://i.sstatic.net/19LqZiy3.png)

Here the controller seeks to eliminate or minimize the difference between a given signal (this can be either a constant signal of the desired physical nature or time-varying) and the desired state of the control object-system measured using a sensor.

The generalized diagram of an adaptive optical system is as follows:

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

This is how this system works. The wave front of electromagnetic waves from the object of observation passes through outer space and is distorted (small cosmic bodies, dust, etc. can introduce aberrations). It is then reflected from a mirror with a customizable spatial configuration and hits a “light splitter.” One branch goes to the image recording and storage device. The other branch goes to the wavefront sensor, after which the signal enters the control system. An internal "wavefront corrector" adjusts the spatial configuration of the mirror so that...

**And this is where my question begins:** The fact is that in such a system we cannot set the reference signal in the form “the controlled output signal of the control object must be equal to 5 V (roughly speaking).” We also cannot set the desired image, because... such an adaptive optics system would not make sense, and besides, we do not know in advance what the object of observation looks like (a high-quality image of, for example, a star cluster is the result of the system’s operation). The term "wavefront corrector" contains the necessary instructions. But what kind of correction are we talking about? How do you know if the wavefront is corrected correctly? Are the properties of the wavefront sensor used for this? Or are integral indicators used (for example, the Strehl number or others)? Or does the system not use a reference signal, as in classical control systems, but maximizes some indicators of the quality of the adaptive optical system and the image obtained with its help?

*Generally speaking, how does an adaptive optics system know that the image on the left is bad and the image on the right is good? How does this rate?*

---

**UPDATE:** (based on comments below) I just realized that the problems that adaptive optical systems solve are not so acute for spacecraft, because there are no “atmospheric” distortions like on Earth. But at the same time, what if we are talking about observing exoplanets, or observing space objects “contaminated” with cosmic dust or other factors. How would adaptive optics work there? Also with reference lasers? Is it possible to do without lasers?

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

**Tags:** observational-astronomy, optics, instruments, adaptive-optics

> **Comments:**
> **username_1**: What would you want an extra-atmospheric optical system adapt to? There's no systematic distortion.  
> **username_0**: @username_1 First of all, I would like to know the answer to the question about the technical properties of such a system.  And what to adapt to...in fact, the last picture, at such objects I would check the performance of the system.  
> **username_1**: My point is: no distortion, no reference signal.  
> **username_0**: @username_1 Then let's take a look at the last picture.  The left image is very muddy.  Why is that?  And how are adaptive optics adjusted to produce the right image?  
> **username_1**: That example is a *terrestrial* telescope disturbed by atmospheric turbulence. The trick is to use a reference signal (laser beam, guide star) to 'modulate' a flexible mirror to counter the disturbance. Without the atmosphere there's no disturbance to counter (at least nothing you can do with a flexible mirror). For starters: https://en.wikipedia.org/wiki/Adaptive_optics  
> **username_0**: @username_1 I just realized that the problems that adaptive optical systems solve are not so acute for spacecraft, because there are no “atmospheric” distortions like on Earth. But at the same time, what if we are talking about observing exoplanets, or observing space objects “contaminated” with cosmic dust or other factors. How would adaptive optics work there? Also with reference lasers? Is it possible to do without lasers?  

---

## 1 answers

### Answer by username_2 on 2024-06-16. Score: 1 | Accepted answer

This is a really interesting question!

# Atmospheric distortions:

As the discussion in comments points out, terrestrial telescopes use wavefront correction to undo Earth's *atmospheric* distortion, an in general atmospheric correction is not needed for telescopes in space.

There is a caveat for a telescope trying to observe the surface of a planet through its atmosphere. The most obvious case would be *non-astronomical* observations of Earth:

* [Atmospheric influences on earth to satellite visibility and vice versa (e.g. atmospheric seeing)](https://space.stackexchange.com/q/20107/12102)

There was a mild "conspiracy theory" that JWST had wavefront correction optics in its secondary mirror, but that was quickly dispelled.

* [Why would the James Webb Space Telescope need adaptive optics outside the atmosphere?](https://space.stackexchange.com/q/18938/12102)

# Astronomical distortions

There are two that I can think of, and both are important fields of research.

### 1. Plasma vs. Radio

In radio astronomy, there is (primarily interplanetary) scintillation. See for example:

* [this answer to *What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?*](https://astronomy.stackexchange.com/a/34579/7982)

The timescale for changes in this kind of wavefront distortion (be it from interplanetary plasma or Earth atmosphere (ionospheric and water vapor)) is longer than for atmospheric optical measurements - minutes to months. And because radio astronomy data is recorded electronically in a way that **preserves phase information** for off-line implementation of interferometry and image construction, the closed loop will look a little different. For example, when the Event Horizon Telescope recorded data for several days to image galactic center objects involving black holes and their accretion disks, there was always one or two other radio objects in the same electronic fields of view which were used as reference, analogous to "guide stars".

### 2. Gravity vs. all wavelengths

Gravitational lensing and microlensing phenomena are interesting in their own right, but they are also a very useful tool to make observations of things so distant that they would be too dim to observe or too small to resolve.

Generally the timescales for any *changes* in wavefront distortion are "astronomically" long, but transient effects are observed, say when a closer star passes in front of a more distant point-like object.

Once again, the wavefront distortion is *computational* and done offline. Usually this is done for optical images, where we have intensity only and have lost phase information. **In this case phase information is lost and there is no such analog to the wavefront sensor.** We're then left with a very fancy "trial-and-error" program that varies the correction until a sharp image is realized.

For more on that, see:

* [If I can't unscramble an egg, how do Astronomers unscramble views gravitationally lensed by complex mass distributions?](https://astronomy.stackexchange.com/q/50029/7982)

> **Comments:**
> **username_0**: Thank you! I found a lot of answers to some questions. It seems to me that extra-atmospheric optics is the next layer in optical sciences; some models have probably also been developed (the influence of dust, plasma, cosmic rays, etc. on the distortion of electromagnetic waves and image restoration methods). These can be purely computational tasks (based on neural networks, for example).  

---

