<p>Welcome &quot;akostadinov&quot;, and welcome to me, too!  Indeed, a great question..  In an attempt to start an answering discussion, understand each of the Webb's 18 primary mirrors have I believe 5 motorized adjustment points (correction - 6 actuator motors!), for just what you're asking!  Yes, once everything is locked down and deployed, there will be an alignment/collimation period to get all the optics in the right position, something essential in a multiple mirror primary system.  I can imagine over time, these mirrors may get occasional tweaking, but I don't know with what regularity.  With the multi-mirror Keck observatory here on Earth, they actually &quot;warp&quot; the primary segments based on what they're observing!  Hope this helps!  Cheers..twg.</p>

CC BY-SA 4.0
2022-01-05T14:19:14.253Z
2022-01-05T17:23:17.240Z
2022-01-05T17:23:17.240Z
T W
<p>Each of James Webb's mirror segments are adjustable through seven small motors, called <em>actuators</em>. Due to thermal variations, the segments do indeed need to be re-adjusted occasionally.</p>
<p>Exactly how often is a compromise between desired image quality and valuable science observation time, as argued by <a href="https://ui.adsabs.harvard.edu/abs/2017SPIE10374E..06P" rel="noreferrer">Park et al. (2017)</a> who develop a thermal model, but don't seem to actually recommend any interval between re-adjustments.</p>
<p>An older proceeding by <a href="https://ui.adsabs.harvard.edu/abs/2010hstc.workE..38M" rel="noreferrer">Mather (2010)</a> says that</p>
<blockquote>
<p>it is […] expected the JWST primary mirror may be readjusted on the order of every two weeks.</p>
</blockquote>
<p>which is backed up by this recent <a href="https://spie.org/news/building-the-james-webb-space-telescope?SSO=1" rel="noreferrer">SPIE article</a>.</p>

CC BY-SA 4.0
2022-01-05T16:13:32.370Z
I suspect that 14-day "sampling period" is a tradeoff between the bandwidth (calculated) of all thermal effects, the amount of power neede to issue a correction, and the image degradation due to loss of primary figure.   By comparison, ground-based adaptive optics, with the same control loop,  need to run on the order of a kHz to compensate for atmospheric bandwidths up to 100 Hz or so.
Carl Witthoft
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2022-01-05T22:27:27.857Z
Amazing answer, thank you, pela. @CarlWitthoft, isn't ground based adaptive optics working in a very different way?
akostadinov
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2022-01-06T07:32:13.740Z
Good point with the power needed, @CarlWitthoft, which isn't a problem for ground-based or LEO telescopes. My un-educated guess would be that the power needed to adjust the mirrors (typically by fractions of a wavelength I think) is negligible compared to slewing the telescope and positioning instruments, filters, coronagraphs, slits, etc., but I don't actually know.
pela
CC BY-SA 4.0
2022-01-06T13:22:59.287Z
@akostadinov  yes and no :-) .  Both involve analyzing sources of phase error and manipulating a mirror (bendable or segmented) to remove that error.
Carl Witthoft
CC BY-SA 4.0
2022-01-05T15:41:02.090Z
2022-01-05T15:41:02.090Z
pela
<p>I couldn't find an answer to this question.</p>
<p>James Webb Space Telescope will <a href="https://www.nasa.gov/feature/goddard/2017/aligning-the-primary-mirror-segments-of-nasa-s-james-webb-space-telescope-with-light" rel="noreferrer">align primary mirror segments</a> and secondary mirror in space.</p>
<p>Question is whether these will need re-alignment during the course of operation? And how often if so?</p>

CC BY-SA 4.0
2022-01-04T22:41:15.873Z
*This is a great question!* With such a large structure and even with low coefficient of thermal expansion materials, it's possible that even small changes of temperature (or its gradient) due to orientation changes might require some retuning. It's also possible that after some period of data collection that might become predictable and even preemptive.
uhoh
CC BY-SA 4.0
2022-01-05T09:59:08.357Z
I'm pretty sure it would, though "how often" is much more difficult to guess at. (Presumably this is something that will be regularly monitored and adjusted as needed.)
Peter Erwin
<telescope><space-telescope><james-webb-space-telescope>
Will the James Webb Space Telescope need re-alignment of primary mirror segments?
akostadinov
# Will the James Webb Space Telescope need re-alignment of primary mirror segments?

(Asked by: username_0 on 2022-01-04)

I couldn't find an answer to this question.

James Webb Space Telescope will [align primary mirror segments](https://www.nasa.gov/feature/goddard/2017/aligning-the-primary-mirror-segments-of-nasa-s-james-webb-space-telescope-with-light) and secondary mirror in space.

Question is whether these will need re-alignment during the course of operation? And how often if so?

**Tags:** telescope, space-telescope, james-webb-space-telescope

> **Comments:**
> **username_1**: *This is a great question!* With such a large structure and even with low coefficient of thermal expansion materials, it's possible that even small changes of temperature (or its gradient) due to orientation changes might require some retuning. It's also possible that after some period of data collection that might become predictable and even preemptive.  
> **username_2**: I'm pretty sure it would, though "how often" is much more difficult to guess at. (Presumably this is something that will be regularly monitored and adjusted as needed.)  

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## 2 answers

### Answer by username_4 on 2022-01-05. Score: 6 | Accepted answer

Each of James Webb's mirror segments are adjustable through seven small motors, called *actuators*. Due to thermal variations, the segments do indeed need to be re-adjusted occasionally.

Exactly how often is a compromise between desired image quality and valuable science observation time, as argued by [Park et al. (2017)](https://ui.adsabs.harvard.edu/abs/2017SPIE10374E..06P) who develop a thermal model, but don't seem to actually recommend any interval between re-adjustments.

An older proceeding by [Mather (2010)](https://ui.adsabs.harvard.edu/abs/2010hstc.workE..38M) says that

> it is […] expected the JWST primary mirror may be readjusted on the order of every two weeks.

which is backed up by this recent [SPIE article](https://spie.org/news/building-the-james-webb-space-telescope?SSO=1).

> **Comments:**
> **username_5**: I suspect that 14-day "sampling period" is a tradeoff between the bandwidth (calculated) of all thermal effects, the amount of power neede to issue a correction, and the image degradation due to loss of primary figure.   By comparison, ground-based adaptive optics, with the same control loop,  need to run on the order of a kHz to compensate for atmospheric bandwidths up to 100 Hz or so.  
> **username_0**: Amazing answer, thank you, username_4. @CarlWitthoft, isn't ground based adaptive optics working in a very different way?  
> **username_4**: Good point with the power needed, @CarlWitthoft, which isn't a problem for ground-based or LEO telescopes. My un-educated guess would be that the power needed to adjust the mirrors (typically by fractions of a wavelength I think) is negligible compared to slewing the telescope and positioning instruments, filters, coronagraphs, slits, etc., but I don't actually know.  
> **username_5**: @username_0  yes and no :-) .  Both involve analyzing sources of phase error and manipulating a mirror (bendable or segmented) to remove that error.  

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### Answer by username_3 on 2022-01-05. Score: 0

Welcome "username_0", and welcome to me, too! Indeed, a great question.. In an attempt to start an answering discussion, understand each of the Webb's 18 primary mirrors have I believe 5 motorized adjustment points (correction - 6 actuator motors!), for just what you're asking! Yes, once everything is locked down and deployed, there will be an alignment/collimation period to get all the optics in the right position, something essential in a multiple mirror primary system. I can imagine over time, these mirrors may get occasional tweaking, but I don't know with what regularity. With the multi-mirror Keck observatory here on Earth, they actually "warp" the primary segments based on what they're observing! Hope this helps! Cheers..twg.


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