<p>I found this image on space.com and it can also be found in NASA's <a href="https://blogs.nasa.gov/webb/2022/02/18/webb-team-brings-18-dots-of-starlight-into-hexagonal-formation/" rel="nofollow noreferrer">JWST blog</a>. This is one step in the process of aligning the 18 mirrors on JWST. A single relatively-isolated star has been selected. At this point, the orientation of each mirror has been adjusted so that the image it forms falls at a position on the <a href="https://blogs.nasa.gov/webb/2022/02/11/photons-received-webb-sees-its-first-star-18-times/" rel="nofollow noreferrer">Near Infrared Camera (NIRCam)</a> sensor that corresponds directly to the position of the mirror in the array.  <br></p>
<p><a href="https://i.sstatic.net/mRjpu.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mRjpu.jpg" alt="image from JWST during the alignment process" /></a></p>
<blockquote>
<p>This early Webb alignment image, with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, is called an “image array.” Credit: NASA/STScI/J. DePasquale</p>
</blockquote>
<p>This question is a follow up to the earlier <a href="https://astronomy.stackexchange.com/questions/48685/jwst-mirrors-each-can-be-positioned-in-tip-tilt-piston-horizontal-vertical">question about the mirrors' actuators</a>. Apparently the position and orientation of each mirror segment can be adjusted in all six degrees of freedom using a <a href="https://en.wikipedia.org/wiki/Stewart_platform" rel="nofollow noreferrer">hexapod actuator</a>. In addition, one more actuator is available to make small changes to the spherical curvature component (focal-length).</p>
<p>The picture above is the second in a sequence of three pictures. This first shows the 18 images wildly scattered. The third picture shows the same array but with each of the 18 images of the star now almost perfectly focussed.</p>
<p>I have two questions: <br></p>
<ol>
<li>Several of the 18 images show a lot of astigmatism (ellipses rather than circles) which would require cylindrical correction.  But there is no obvious means of providing this. How is this achieved? <br></li>
<li>Some of the 18 images show marked diffraction fringes - how do these arise?</li>
</ol>

CC BY-SA 4.0
2022-03-11T07:21:14.903Z
Spherical and chromatic are on-axis aberrations; coma and astigmatism are off-axis aberrations. One solution for astigmatism is to add a cylindrical component but the previous sentence suggest a simpler solution.
uhoh
CC BY-SA 4.0
2022-03-11T13:47:03.663Z
I’m voting to close this question because this is better suited to space.SE
Carl Witthoft
CC BY-SA 4.0
2022-03-11T13:47:56.853Z
Diffraction occurs any time you have an aperture - be it positive or negative (see "Spot of Arago").
Carl Witthoft
CC BY-SA 4.0
2022-03-11T13:49:02.887Z
BTW, many of the segments in fact have cylindrical power, since they are designed to be part of a collective figure.
Carl Witthoft
CC BY-SA 4.0
2022-03-11T19:30:00.967Z
@CarlWitthoft Is space.SE a better place to ask questions about telescopes?
Roger Wood
CC BY-SA 4.0
2022-03-12T14:10:54.963Z
@CarlWitthoft Suppose the question was instead asked about how a multisegment ground-based telescope such as the Thirty Meter Telescope will be brought into alignment and focus. Such a question would be off-topic at space.SE but on-topic here. This is essentially the same question applied to a telescope in space. Questions about astronomical equipment, whether on Earth or in space, are on-topic at this site. This question is on topic at this site.
David Hammen
CC BY-SA 4.0
2022-03-13T07:02:44.480Z
@CarlWitthoft "is better suited" is NEVER a close reason. OP decides where to ask. Can you help clarify how a question about an astronomical telescope's optical performance is off-topic in Astronomy SE? In absence of any evidence of this being off-topic I'll **vote to leave open**.
uhoh
CC BY-SA 4.0
2022-03-13T07:05:03.517Z
@RogerWood if you'd like to go down a rabbit hole there's [this JWST PSF toolbox](https://www.stsci.edu/jwst/science-planning/proposal-planning-toolbox/psf-simulation-tool) but in the mean time I think I can try to simulate those patterns.
uhoh
CC BY-SA 4.0
2022-03-13T17:18:57.163Z
@uhoh excellent link, but I think I'd be starting on a whole new career trying to use it. 
 (PS. thanks for the nice edits to the question)
Roger Wood
CC BY-SA 4.0
2022-03-13T17:52:15.103Z
@CarlWitthoft I can't find any detail about the primary mirror shape but it's presumably somewhere between spherical and parabolic. Given the long focal length compared to the diameter, the cylindrical component of curvature must be very small compared with the spherical component. Maybe if the mirror is misplaced in rotation or in the radial direction, the error in the cylindrical component will become apparent?
Roger Wood
CC BY-SA 4.0
2022-03-14T11:20:50.740Z
@RogerWood that would be my guess as well.
Carl Witthoft
<telescope><optics><james-webb-space-telescope><adaptive-optics><cassegrain-telescope>
How are the aberrations seen in early JWST images corrected?
Roger Wood
# How are the aberrations seen in early JWST images corrected?

(Asked by: username_0 on 2022-03-11)

I found this image on space.com and it can also be found in NASA's [JWST blog](https://blogs.nasa.gov/webb/2022/02/18/webb-team-brings-18-dots-of-starlight-into-hexagonal-formation/). This is one step in the process of aligning the 18 mirrors on JWST. A single relatively-isolated star has been selected. At this point, the orientation of each mirror has been adjusted so that the image it forms falls at a position on the [Near Infrared Camera (NIRCam)](https://blogs.nasa.gov/webb/2022/02/11/photons-received-webb-sees-its-first-star-18-times/) sensor that corresponds directly to the position of the mirror in the array.

[![image from JWST during the alignment process](https://i.sstatic.net/mRjpu.jpg)](https://i.sstatic.net/mRjpu.jpg)

> This early Webb alignment image, with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, is called an “image array.” Credit: NASA/STScI/J. DePasquale

This question is a follow up to the earlier [question about the mirrors' actuators](https://astronomy.stackexchange.com/questions/48685/jwst-mirrors-each-can-be-positioned-in-tip-tilt-piston-horizontal-vertical). Apparently the position and orientation of each mirror segment can be adjusted in all six degrees of freedom using a [hexapod actuator](https://en.wikipedia.org/wiki/Stewart_platform). In addition, one more actuator is available to make small changes to the spherical curvature component (focal-length).

The picture above is the second in a sequence of three pictures. This first shows the 18 images wildly scattered. The third picture shows the same array but with each of the 18 images of the star now almost perfectly focussed.

I have two questions:

1. Several of the 18 images show a lot of astigmatism (ellipses rather than circles) which would require cylindrical correction. But there is no obvious means of providing this. How is this achieved?
2. Some of the 18 images show marked diffraction fringes - how do these arise?

**Tags:** telescope, optics, james-webb-space-telescope, adaptive-optics, cassegrain-telescope

> **Comments:**
> **username_1**: Spherical and chromatic are on-axis aberrations; coma and astigmatism are off-axis aberrations. One solution for astigmatism is to add a cylindrical component but the previous sentence suggest a simpler solution.  
> **username_2**: I’m voting to close this question because this is better suited to space.SE  
> **username_2**: Diffraction occurs any time you have an aperture - be it positive or negative (see "Spot of Arago").  
> **username_2**: BTW, many of the segments in fact have cylindrical power, since they are designed to be part of a collective figure.  
> **username_0**: @username_2 Is space.SE a better place to ask questions about telescopes?  
> **username_3**: @username_2 Suppose the question was instead asked about how a multisegment ground-based telescope such as the Thirty Meter Telescope will be brought into alignment and focus. Such a question would be off-topic at space.SE but on-topic here. This is essentially the same question applied to a telescope in space. Questions about astronomical equipment, whether on Earth or in space, are on-topic at this site. This question is on topic at this site.  
> **username_1**: @username_2 "is better suited" is NEVER a close reason. OP decides where to ask. Can you help clarify how a question about an astronomical telescope's optical performance is off-topic in Astronomy SE? In absence of any evidence of this being off-topic I'll **vote to leave open**.  
> **username_1**: @username_0 if you'd like to go down a rabbit hole there's [this JWST PSF toolbox](https://www.stsci.edu/jwst/science-planning/proposal-planning-toolbox/psf-simulation-tool) but in the mean time I think I can try to simulate those patterns.  
> **username_0**: @username_1 excellent link, but I think I'd be starting on a whole new career trying to use it.   (PS. thanks for the nice edits to the question)  
> **username_0**: @username_2 I can't find any detail about the primary mirror shape but it's presumably somewhere between spherical and parabolic. Given the long focal length compared to the diameter, the cylindrical component of curvature must be very small compared with the spherical component. Maybe if the mirror is misplaced in rotation or in the radial direction, the error in the cylindrical component will become apparent?  
> **username_2**: @username_0 that would be my guess as well.  

---

## 0 answers

