6
$\begingroup$

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?

enter image description here enter image description here

above: Two frame captures from the NASA Goddard YouTube video

enter image description here

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

$\endgroup$
8
  • 1
    $\begingroup$ 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, 2016 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, 2016 at 3:55
  • 2
    $\begingroup$ I can probably find one, but I'm on mobile right now. $\endgroup$
    – Phiteros
    Nov 4, 2016 at 3:56
  • 2
    $\begingroup$ 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, 2016 at 8:53
  • $\begingroup$ @Hobbes theory or fact? I'm still not sure if JWST has AO or not. $\endgroup$
    – uhoh
    Nov 4, 2016 at 9:29

3 Answers 3

11
$\begingroup$

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.

$\endgroup$
7
  • $\begingroup$ 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, 2016 at 10:36
  • 1
    $\begingroup$ 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, 2016 at 10:41
  • 3
    $\begingroup$ The blurring may be due to an overzealous ITAR officer. $\endgroup$
    – Hobbes
    Nov 4, 2016 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, 2016 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, 2016 at 1:59
2
$\begingroup$

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.

$\endgroup$
1
  • $\begingroup$ 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, 2018 at 9:28
1
$\begingroup$

TJW, el espejo primario no está diseñado para corregir las aberraciones ópticas, el movimiento de cada espejo no corresponde a un sistema de óptica adaptativa. Basta ver las fotografías de las estrellas con ocho rayos.

$\endgroup$
3
  • $\begingroup$ Hola Héctor, welcome to Stack Exchange! You are right, the JWST doesn't have adaptive optics. I tried google translate on your answer but it doesn't work well for technical things, so I'm not sure exactly what you've said. Unfortunately most Stack Exchange sites require posts in English, so if you can try to add some English I think others can edit your post and improve on it. We have many members who don't speak English as their first language, so don't worry if it's not perfect. Also, if you can add some links to support your answer, that would be great. Thank you! $\endgroup$
    – uhoh
    Jul 24, 2022 at 6:22
  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Jul 24, 2022 at 12:51
  • $\begingroup$ Atsakymų tikimasi anglų kalba, sorry. For me English is also not born language. $\endgroup$
    – Nightrider
    Jul 24, 2022 at 16:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.