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I have seen comparisons of the picture quality between Hubble and Webb, and I have read that the size of the pictures Webb is taking is akin to holding a grain of sand at arms length against the sky.

Still, I'm not able to comprehend how far these objects are or have a grasp on how much light is coming through.

I thought then that, hypothetically, if Webb would be pointed at Earth, and would be able to capture at similar range (ignoring all the impossibilities of doing so), what would show up in the pictures? A house? A pixel in a monitor? Molecules?

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    $\begingroup$ Before you had the chance to see anything, you would be assaulted by all the physicists and engineers that worked for decades on how to keep it cool, e.g. by never pointing it toward Earth. $\endgroup$
    – pela
    Jul 17, 2022 at 14:52
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    $\begingroup$ Pointing it to Earth would screw up not just the cooling, but the cameras themselves probably. But as you said, we are ignoring those impossibilities. $\endgroup$
    – Victor Stafusa - BozoNaCadeia
    Jul 18, 2022 at 1:31

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The best angular resolution of JWST is about 0.07 arcseconds and the telescope is 1.5 million km from Earth.

Ignoring any blurring due to the Earth's atmosphere, this translates to about 500 m on the Earth's surface.

The field of view (of the NIRCAM instrument) would cover about 1000 km on the Earth.

Whether you would actually see any details is moot, since the Earth would be in the same direction as, but closer, than the Sun. I guess you would see features in the infrared due to temperature differences.

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    $\begingroup$ so, the resolution is about the same as regular Earth viewing cameras - which are often 2000 times closer (750 km) and get about 1500 times better resolution (30 cm). Disappointing, in some sense. $\endgroup$
    – asdfex
    Jul 17, 2022 at 15:37
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    $\begingroup$ hopefully some of the imaging arrays will still work after being warmed by the Earth's thermal radiation. $\endgroup$
    – uhoh
    Jul 18, 2022 at 4:26
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A blurred fogscape. Even if JW could ever point at such low elongations, motion at (nominally) L2 exceeds the observatory panning specification, and you’d get an image of whatever the relative motion vector was.

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  • $\begingroup$ Fascinating. Can you expand on the angular motion of Earth vs the Webb orbit, and compare it to the panning spec? With a reference? $\endgroup$
    – nealmcb
    Aug 4, 2022 at 2:20
  • $\begingroup$ So, "speed blur" $\endgroup$
    – Criggie
    Nov 6, 2022 at 22:01
  • $\begingroup$ If you want to get pedantic, “speed blur” is what tourists, TikTok teens, and assorted button-pushers call it. In opto the term is “smear” (as opposed to “jitter”). Semantic debate, yes I know. $\endgroup$
    – caInstrument
    Nov 10, 2022 at 13:59

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