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)?
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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. – Ng Ph Jan 09 '22 at 11:20
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2other questions here (https://space.stackexchange.com/questions/57640/what-would-jwsts-apparent-magnitude-be) 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. – CuteKItty_pleaseStopBArking Jan 09 '22 at 11:46
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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? – uhoh Jan 09 '22 at 18:17
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1The 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. – David Hammen Jan 10 '22 at 16:01
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@DavidHammen Feel free to convert your comment into an answer. – DrMcCleod Jan 10 '22 at 18:37
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The Virtual Telescope Project has provided a photo of the James Webb Telescope taken from a ground-based telescope. Whether you consider a single bright dot to be 'useful' (in any scientific sense) remains to be seen, but I would guess that this at least allows the observers to confirm that it's where it's supposed to be, rather than in another location and giving off misleading telemetry.
source: AliveUniverse.today James Webb in L2 ripreso dal Virtual Telescope Project
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1@DrMcleod - I'm tempted to ask how big that arrow would need to be to see it with this telescope – Richard Jan 30 '22 at 13:37
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The James Webb Space Telescope has no cameras mounted that look at itself.
But there is a camera! It took this selfie of the objective mirrors:
A selfie taken by a camera onboard the James Webb Space Telescope (NASA)
uhoh
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2You are absolutely right! How exactly did JWST take a "selfie" of its own primary mirror, and what is the real purpose of this capability? Welcome to Space! – uhoh Feb 18 '22 at 16:39
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1Is that a picture taken by the main telescope camera of its own optics, or a picture taken by a secondary camera elsewhere on the space craft? – DrMcCleod Feb 18 '22 at 16:45
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@DrMcCleod It's sort-of a hybrid; It reuses some parts of the astronomical imaging telescope but has additional parts moved into the optical path that are only used in this introspective mode. – uhoh Feb 18 '22 at 23:05