Adaptive Optics is generally used with ground telescopes to correct for atmospheric distortions or seeing. Right now it is used mostly in the infrared but is starting to be used in longer visible wavelengths (see excellent answers here and here), and is applied in software to radio telescope arrays as well.

In Project Scientist John Mather's recent AMA #1 the use of adaptive optics in large aperture ground telescopes to potentially image exoplanets is mentioned as well. The JWST FAQ #7 touts putting a telescope in space as an alternative to adaptive optics on the ground.

In the Business Insider article NASA is trying to keep part of its giant golden telescope a secret the use of adaptive optics on the JWST secondary mirror is mentioned, and shown in the large image below with a red arrow. Also two screen shots from the NASA Goddard YouTube video are shown.

Since the JWST is outside the atmosphere and therefore doesn't need adaptive optics, what would the adaptive optics be needed for? Or is the BI article wrong and this is simply an active mirror mount sans AO?

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above: Two frame captures from the NASA Goddard YouTube video

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above: JWST secondary mirror mechanism annotated with arrow by Business Insider - credit NASA/Chris Gunn Original Image

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    $\begingroup$ Adaptive optics aren't only used to correct for atmospheric distortion, but also for clearing up any other possible errors caused by the telescope collecting surface, such as small imperfections in the mirror. $\endgroup$ – Phiteros Nov 4 '16 at 3:52
  • $\begingroup$ @Phiteros can you cite and link to an example please, or is it just a guess? Why not fix the imperfections before launch? $\endgroup$ – uhoh Nov 4 '16 at 3:55
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    $\begingroup$ I can probably find one, but I'm on mobile right now. $\endgroup$ – Phiteros Nov 4 '16 at 3:56
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    $\begingroup$ Imperfections caused by launch vibrations, or by inaccuracy of the unfolding process (mechanical systems always have some play), can't be corrected before launch. $\endgroup$ – Hobbes Nov 4 '16 at 8:53
  • $\begingroup$ @Hobbes theory or fact? I'm still not sure if JWST has AO or not. $\endgroup$ – uhoh Nov 4 '16 at 9:29

The primary and secondary mirrors on JWST can be adjusted:

Launching a mirror this large into space isn’t feasible. Instead, Webb engineers and scientists innovated a unique solution – building 18 mirrors that will act in unison as one large mirror. These mirrors are packaged together into three sections that fold up - much easier to fit inside a rocket. Each mirror is made from beryllium and weighs approximately 20 kilograms (46 pounds). Once in space, getting these mirrors to focus correctly on faraway galaxies is another challenge entirely. Actuators, or tiny mechanical motors, provide the answer to achieving a single perfect focus.

The primary and secondary mirror segments are both moved by six actuators that are attached to the back of the mirrors. The primary segment has an additional actuator at the center of the mirror that adjusts its curvature. The third mirror segment remains stationary.

Lee Feinberg, Webb Optical Telescope Element Manager at NASA's Goddard Space Flight Center in Greenbelt, Md. explained "Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair. This alignment has to be done at 50 degrees above absolute zero! What's even more amazing is that the engineers and scientists working on the Webb telescope literally had to invent how to do this."

Each mirror can be adjusted in only 7 spots: each corner of the hexagon and the middle. This seems to be too few spots to really do adaptive optics. In adaptive optics, you adjust the shape of the mirror itself in multiple locations to compensate for changes in the light wavefront as it's distorted.

Those 7 spots are adjustable for one reason: to align the 18 segments of the primary mirror to each other, and to adjust the primary and secondary mirrors to the rest of the optical path after launch.

Given the amount of information available on NASA's own website, the Business Insider article looks like clickbait. There's nothing being kept secret there.

  • $\begingroup$ The article seems to be triggered by the deliberate de-focus, blurring, or digitization of the secondary mirror, as can be clearly seen in the video at around 01:37, and in the smaller image on the left in my question, along with the quotes by NASA in that article. It's quite believable that the article is wrong about the AO, but the NASA Goddard YouTube video does contain deliberate blurring of something for some reason. $\endgroup$ – uhoh Nov 4 '16 at 10:36
  • $\begingroup$ Are the six actuators at the six corners, or is it a 3-point mount with two actuators on each corner, like a standard Steward platform? Also, considering that the seventh actually distorts the curvature of each mirror segment, it has potential to be "adaptive". The optical figure of each element itself is not necessarily static - the curvature of each segment's reflected wavefront can be independently changed. $\endgroup$ – uhoh Nov 4 '16 at 10:41
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    $\begingroup$ The blurring may be due to an overzealous ITAR officer. $\endgroup$ – Hobbes Nov 4 '16 at 11:06
  • $\begingroup$ Could be - overzealous and simultaneously undervigilant considering the second shot around 03:08 is larger and yet unblurred. $\endgroup$ – uhoh Nov 4 '16 at 11:21
  • $\begingroup$ In your linked site jwst.nasa.gov/mirrors.html and in the video linked there youtu.be/4gvPl3qWZIM it seems the primary mirror elements are mounted in three spots with "hexapod-like" actuation (plus the central "deforming" actuator). $\endgroup$ – uhoh Nov 5 '16 at 1:59

It appears that the actuators are there for initial alignment. Should the need arise, and I bet it will, the servomotors should be and will be used for adaptive optics during observations as well. Why waste all the hardware/software that went into alignment? NASA's always been good at killing several birds with one stone.

Kenny, P. Eng.

  • $\begingroup$ I have to agree with you about the ingenuity of the folks at NASA. I'm not sure there is the capability to do true adaptive optics - deforming the figure of the mirror to correct the wavefront though. Welcome to Stack Exchange by the way! I can see so far you have one up-vote and one down-vote. If you take a look at other answers, you'll see that generally an answer needs to have a bit of factual backup - usually some math, a link, or some detailed explanation. $\endgroup$ – uhoh Jan 31 '18 at 9:28
  • $\begingroup$ What you've written is pretty much a comment, and the site requires you to reach a reputation of 50 to post comments on posts that aren't your own. Usually new users pass 50 fairly quickly with one good quality answer or question. $\endgroup$ – uhoh Jan 31 '18 at 9:29

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