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Here's a backyard telescope versus Webb: https://twitter.com/AJamesMcCarthy/status/1546941832700932096 https://twitter.com/AJamesMcCarthy/status/154694183270093

by mholt 4y ago

Here's a backyard telescope versus Webb: https://twitter.com/AJamesMcCarthy/status/1546941832700932096 https://twitter.com/AJamesMcCarthy/status/154694183270093209... More comparisons on Twitter, some zoomed in: - https://twitter.com/Batsuto_/status/1546899241880240128 https://twitter.com/Batsuto_/status/1546899241880240128 - https://twitter.com/Batsuto_/status/1546900387931766784 https://twitter.com/Batsuto_/status/1546900387931766784 - https://twitter.com/JBWillcox/status/1546881033597075457 https://twitter.com/JBWillcox/status/1546881033597075457 - https://twitter.com/jason4short/status/1546626672488632321 https://twitter.com/jason4short/status/1546626672488632321 I'm not a physicist, so I've only recently learned about redshift. Hubble's deep field images were very dark red/orange because further objects appear redder (into infrared) before they disappear to the observer. Webb's sensors are more red/infrared-sensitive than Hubble's, so along with extremely fine, super-cooled optics using exotic materials to align and capture every single photon, its red sensitivity allows Webb to peer deeper, further, and dimmer than we've ever been able to before. And I've read that the "spikes" coming off the brighter stars are generally from stars in our own galaxy and they're not lens flares. They're caused by the edges of the telescope. Hubble's stars would have 4 spikes in a cross; Webb has 6 in a snowflake because of the shape of Webb's mirrors having 6 sides. Or something like that.

kadoban 4y ago
They're not lens flares, but they're still an artifact of the system, which is probably what most people mean by "lens flare" anyway, due to lack of a better common term.
- JacobThreeThree 4y ago
  According to the Wikipedia definition, I'd say that Webb's artifacts could be classified as "lens flares". >This happens through light scattered by the imaging mechanism itself, for example through internal reflection and forward scatter from material imperfections in the lens. Lenses with large numbers of elements such as zooms tend to have more lens flare, as they contain a relatively large number of interfaces at which internal scattering may occur. https://en.wikipedia.org/wiki/Lens_flare https://en.wikipedia.org/wiki/Lens_flare
  - Cogito 4y ago
    Diffraction spikes are caused before the lens gets involved; improving the lenses cannot remove the spikes (though there may be structures causing diffraction spikes in-between lenses). The spikes from JWST are primarily caused by the edges of the mirrors and the three support struts. There likely is some amount of lens flare (though I don't know if it is significant, the optics are Very Good), but the dominant artefact is the diffraction spikes.
    - JacobThreeThree 4y ago
      >The spikes from JWST are primarily caused by the edges of the mirrors and the three support struts. That sounds to me like "light scattered by the imaging mechanism".
- m463 4y ago
  Actually I would say the effect mimics a "cross filter" / "star filter" special effect. https://en.wikipedia.org/wiki/Photographic_filter#Cross_screen https://en.wikipedia.org/wiki/Photographic_filter#Cross_scre...
foxhop 4y ago
thank you very informative.
dr_orpheus 4y ago
The spikes are caused by diffraction of light around the struts supporting the secondary mirror. Hubble has 4 supports for the secondary mirror. JWST has 3 support for the secondary mirror, which because...physics (I don't know I'm not an optics guy)...manifests as 6 diffraction lines. https://en.wikipedia.org/wiki/Diffraction_spike https://en.wikipedia.org/wiki/Diffraction_spike EDIT: It may be caused by both the diffraction spikes from the supports struts and the shape of the mirror and aperture. I'm not really sure. The JWST images also seem to have two additional small spikes that look more like the diffraction pattern from a single strut, which could also be a support strut for a stop further down the optics chain.
- WorldMaker 4y ago
  Apparently the Diffraction spikes come from both the primary mirror shapes and the struts holding the secondary mirror. The primary mirrors of the JWST are hexagonal which would explain hexagonal looking effects. The three struts are apparently designed so that two of the struts match the hexagonal mirror angles at all times and are "hidden" inside. (The third strut apparently sometimes causes two much smaller "horizon" spikes for very, very bright objects.) Hank Green on TikTok did a neat, quick demonstration in video form.
  - mintyLemon 4y ago
    Link to that Hank Green video on YouTube: https://m.youtube.com/watch?v=Y7ieVkK-Cz0 https://m.youtube.com/watch?v=Y7ieVkK-Cz0
    - loxias 4y ago
      The hero we need. Thanks!
      - andai 4y ago
        These these two pages have some nice visualizations of the optical phenomena: https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb-team-begins-aligning-the-telescope/ https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb... https://bigthink.com/starts-with-a-bang/james-webb-spikes/ https://bigthink.com/starts-with-a-bang/james-webb-spikes/
deleted 4y ago
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noisy_boy 4y ago
Which begs the question: is there a computational way to "collapse" the spikes in post-process? They are beautiful but also sort of distracting when you are trying to take in the enormous mass of stars in these photos.
- nullc 4y ago
  deconvolution, but it's more art than science because the problem is ill conditioned and blows up without regularization-- particularly if any part of the spike is overexposed (which it usually as you only notice the spikes on extremely bright stars).
  - noisy_boy 4y ago
    Hmm - maybe a use case for (trained?) machine learning? I don't have much idea about ML so not sure if it makes any sense.
    - nullc 4y ago
      There is machine learning to remove stars called starnet, though last I checked it doesn't to an great job with diffraction that severe. Starnet is used by astrophotographers to enhance the contrast of nebula without getting artifacts from stars or to make completely starless nebula images. If someone got starnet to remove the stars in the JWT images it wouldn't then be hard to go overlay stars back in without the diffraction.

by mholt 4y ago

kadoban 4y ago

They're not lens flares, but they're still an artifact of the system, which is probably what most people mean by "lens flare" anyway, due to lack of a better common term.

JacobThreeThree 4y ago
According to the Wikipedia definition, I'd say that Webb's artifacts could be classified as "lens flares". >This happens through light scattered by the imaging mechanism itself, for example through internal reflection and forward scatter from material imperfections in the lens. Lenses with large numbers of elements such as zooms tend to have more lens flare, as they contain a relatively large number of interfaces at which internal scattering may occur. https://en.wikipedia.org/wiki/Lens_flare https://en.wikipedia.org/wiki/Lens_flare
- Cogito 4y ago
  Diffraction spikes are caused before the lens gets involved; improving the lenses cannot remove the spikes (though there may be structures causing diffraction spikes in-between lenses). The spikes from JWST are primarily caused by the edges of the mirrors and the three support struts. There likely is some amount of lens flare (though I don't know if it is significant, the optics are Very Good), but the dominant artefact is the diffraction spikes.
  - JacobThreeThree 4y ago
    >The spikes from JWST are primarily caused by the edges of the mirrors and the three support struts. That sounds to me like "light scattered by the imaging mechanism".
m463 4y ago
Actually I would say the effect mimics a "cross filter" / "star filter" special effect. https://en.wikipedia.org/wiki/Photographic_filter#Cross_screen https://en.wikipedia.org/wiki/Photographic_filter#Cross_scre...

foxhop 4y ago

thank you very informative.

dr_orpheus 4y ago

The spikes are caused by diffraction of light around the struts supporting the secondary mirror. Hubble has 4 supports for the secondary mirror. JWST has 3 support for the secondary mirror, which because...physics (I don't know I'm not an optics guy)...manifests as 6 diffraction lines. https://en.wikipedia.org/wiki/Diffraction_spike https://en.wikipedia.org/wiki/Diffraction_spike EDIT: It may be caused by both the diffraction spikes from the supports struts and the shape of the mirror and aperture. I'm not really sure. The JWST images also seem to have two additional small spikes that look more like the diffraction pattern from a single strut, which could also be a support strut for a stop further down the optics chain.

WorldMaker 4y ago
Apparently the Diffraction spikes come from both the primary mirror shapes and the struts holding the secondary mirror. The primary mirrors of the JWST are hexagonal which would explain hexagonal looking effects. The three struts are apparently designed so that two of the struts match the hexagonal mirror angles at all times and are "hidden" inside. (The third strut apparently sometimes causes two much smaller "horizon" spikes for very, very bright objects.) Hank Green on TikTok did a neat, quick demonstration in video form.
- mintyLemon 4y ago
  Link to that Hank Green video on YouTube: https://m.youtube.com/watch?v=Y7ieVkK-Cz0 https://m.youtube.com/watch?v=Y7ieVkK-Cz0
  - loxias 4y ago
    The hero we need. Thanks!
    - andai 4y ago
      These these two pages have some nice visualizations of the optical phenomena: https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb-team-begins-aligning-the-telescope/ https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb... https://bigthink.com/starts-with-a-bang/james-webb-spikes/ https://bigthink.com/starts-with-a-bang/james-webb-spikes/

deleted 4y ago

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noisy_boy 4y ago

Which begs the question: is there a computational way to "collapse" the spikes in post-process? They are beautiful but also sort of distracting when you are trying to take in the enormous mass of stars in these photos.

nullc 4y ago
deconvolution, but it's more art than science because the problem is ill conditioned and blows up without regularization-- particularly if any part of the spike is overexposed (which it usually as you only notice the spikes on extremely bright stars).
- noisy_boy 4y ago
  Hmm - maybe a use case for (trained?) machine learning? I don't have much idea about ML so not sure if it makes any sense.
  - nullc 4y ago
    There is machine learning to remove stars called starnet, though last I checked it doesn't to an great job with diffraction that severe. Starnet is used by astrophotographers to enhance the contrast of nebula without getting artifacts from stars or to make completely starless nebula images. If someone got starnet to remove the stars in the JWT images it wouldn't then be hard to go overlay stars back in without the diffraction.