The James Webb Space Telescope has no cameras mounted that look at itself. If a malfunction occurred on-board, would any other telescope be able to resolve it with sufficient clarity to provide useful images (assuming that any problem involved one of its large components such as the sun shield or mirror array)?

  • $\begingroup$ Not the mirror array, unless we turn the telescope towards Earth, which is likely to damage the (single) eye of JWST (due to the Sun), IMO. But, considering that L2 will be populated in the future by other probes and/or telescopes (and servicing robots?), embarking a good camera on some of these, to look at each other, seems to be a better idea than to steal operational time from ground telescopes. $\endgroup$
    – Ng Ph
    Jan 9 at 11:20
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    $\begingroup$ other questions here (space.stackexchange.com/questions/57640/…) figure that the JWST will be about magnitude 19.5 seen from Earth orbit. This is quite easily bright enough to be observed by telescopes such as Hubble. However, resolving anything more than a speck of light? It will be less than 1/1200th of a second of a degree is apparent diameter. That's like trying to resolve a football field on Mars, or trying to resolve the Apollo 11 flag on the moon. No chance at all, from Earth. $\endgroup$ Jan 9 at 11:46
  • $\begingroup$ related and potentially helpful, but not a duplicate: Will WFIRST and JWST be able to resolve each other? (no, not quite) and Will the James Webb Space Telescope be visible from earth? $\endgroup$
    – uhoh
    Jan 9 at 18:17
  • $\begingroup$ The JWST will be about 1.5 million kilometers away. The sunshield is about 22 meters across. That means a spatial resolution of about 3 milliarcseconds (mas) is needed to see the JWST as more than a single pixel. Divide that 3 mas by 50 to get an image that is useful. No telescope, Earth-based or otherwise, existing or planned, has anywhere close to that resolution. The best is about 4 mas. The Hubble is about 40 mas. $\endgroup$ Jan 10 at 16:01
  • $\begingroup$ @DavidHammen Feel free to convert your comment into an answer. $\endgroup$
    – DrMcCleod
    Jan 10 at 18:37

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