Comments by "H. de Jong" (@h.dejong2531) on "What does the James Webb Use to See the Universe?" video.

  1. IR sensors are rather different from visible-light sensors. You're comparing apples and oranges.
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  2. There's no mechanism for removing dust. The estimate is that JWST will encounter micrograms per year of dust. These will be high-velocity impacts that make tiny craters in the mirrors. This is noticeable as a tiny reduction in the amount of light the mirrors collect.
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  3. no, the gold is there because it reflects IR better than any other material. The rest of your low opinion will similarly be caused by lack of knowledge.
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  4. There is one sensor more advanced than the H2RG: the H4RG. But it's not a straight swap (IIRC it's larger, which would have required a redesign of the instruments). Also, replacing the detectors would mean repeating loads of tests, adding years to the build process. At some point you have to freeze the design.
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  5. Cellphones don't work in mid-IR. The far longer wavelength means the pixels have to be larger.
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  6. Radiation: won't affect the exposed mirror in the lifetime of the telescope. Dust: on average, JWST will encounter micrograms of dust per year. This will reduce the reflective area of the mirror by a tiny amount, the mirror was designed to still work well enough at the end of JWST's life.
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  7.  @TheJ0RD1E  Yeas, sorry I didn't make that more clear. It spins on its vertical axis, and pitches to/from the sun by 45º. So at any time, it can view a cylinder section of the sky.
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  8. The image projected by the mirrors (the focal plane) is much larger than the detectors. See https://www.youtube.com/watch?v=MzWfUK0yvdY The detectors are spread across the field of view. If you made the field of view smaller, you'd reduce the resolution.
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  9. yes.
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  10. The details on this (actual performance vs expected) haven't been published yet. The big ticket item (diffraction limited imaging) is as expected.
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  11. Have a look at the LUVOIR proposal.
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  12. Teledyne built the H2RG detectors used in JWST. They also offer the H4RG, which is a 4096x4096 pixel instrument with a pixel size of 15 um (H2RG: 2048x2048, 18 um). This seems to be the state of the art for astronomical IR imaging CCDs. The H4RG has been available for at least 5 years now. The problem with a project like JWST is that at some point, you have to freeze the design. If you keep iterating, it'll never get finished. After the design freeze, there's several years of testing (during which you can't replace components, or you'd have to redo all the tests). the pixel count seems low, but you can't compare these to consumer cameras. To detect really faint light, you need larger pixels. To detect IR, you also need larger pixels.
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  13. nope, actual spacecraft.
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  14. You're right, it has about 45º of vertical movement. The maximum wait time is 6 months.
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  15. It uses a solar panel.
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  16. Sure we will. They'll look a bit different from Hubble images, they'll still be cool.
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  17. This is a bit counterintuitive. We're not going to look at the place where the Big Bang took place - there's not a single place we can point to. Instead, we're going to look at galaxies 13.5 bn lightyears away. They will be 13.5 bn years old, so they should be the oldest galaxies in the universe, containing the first generation of stars. We can find these galaxies in any direction.
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  18. Show us the peer-reviewed paper that confirms this.
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  19. No. Space is very empty: JWST will encounter micrograms of dust per year, on average. Asteroids are a potential problem: we haven't found all of them yet. But the chance of being hit by an asteroid is very low. Since the beginning of spaceflight, we haven't lost a spacecraft to an asteroid impact yet.
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  20.  @DelfinoGarza77  10,000 satellites since 1958 and no hits.
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  21. The chance of that happening is very low (less than 1 in a billion).
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  22. Note that Perseverance wasn't a whole lot cheaper than Curiosity. You get some economies of scale from #2, but the production numbers have to be larger to get a real savings. Then there's the infractructure. JWST occupied a giant clean room for about 5 years. Those rooms don't come cheap, so where do you leave the second telescope?
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  23. They all have 8 spikes, but on the dimmer objects some spikes are less bright than others.
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  24. for the mid-infrared instrument, yes. Longer wavelength means you need larger pixels.
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  25. No, they installed COSTAR, which inserted extra optical elements between the secondary mirror and each of the instruments. These optical elements corrected the image. Later servicing missions swapped out the original instruments for new ones that had corrective optics included, so on one of the later servicing missions COSTAR could be removed.
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  26. No. The target list for the 'first light' observations hasn't been published. The first year of regular observations has been published (look for the General Observer program).
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  27. Advancing the state of the art is an unpredictable process. For JWST, the state of the art had to be advanced in 10 areas.
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  28. Musk is close to $10B into his Starship project and hasn't launched to orbit yet. Advancing the state of the art costs money, and JWST had to advance the state of the art in 10 areas.
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  29. To some extent, yes. Some parts of the light are removed entirely, you can't restore those. And depending on the amount of redshift, JWST's filters may not line up with RGB so you may not be able to reproduce the colors correctly.
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  30.  @novadestry  Yes, any processing will be done on Earth. but if information's missing from the incoming light, you won't be able to add it back in. To take a color picture, you basically take 3 images through red, green and blue filters. JWST would have to take 3 pictures using 3 IR filters that have to line up with R, G and B after shifting the light. JWST has a bunch of filters (16, IIRC) so for some redshifts you should be able to do this.
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  31. The diffraction spikes were predicted from the start. They're inevitable in reflector telescopes. Science observations don't require a target star. The mirror alignment will be checked regularly, but doesn't need to be adjusted for individual observations.
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  32. Each of those detectors is 4 MP. JWST has a bunch of them in different instruments.
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  33.  @blue_ish4499  Yes. They're using the Teledyne H2RG, which was developed for JWST. Teledyne sells these for about $150,000...
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  34. Yes. Each image is assigned a visible color. The alignment images were all taken through a single filter (so they're monochrome, and you get one color in the output). NIRcam and MIRI each have multiple filters. They can create color images by taking multiple images through different filters (similar to a consumer camera: this takes an image through red, green and blue filters and combines the output).
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  35.  @MadawaskaObservatory  The problem with making the sensors larger is that everything else gets larger too. They were running out of space with the current design, larger sensors would have made it too large to launch on Ariane 5. NIRCAM has two sets of four H2RG sensors, so that's 2x 16MP already...
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  36.  @MadawaskaObservatory  The choice is between being able to launch JWST now, or having to wait another 5 years for larger rockets to become available. That's not bureaucratic, it's dealing with the limits of what's feasible.
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  37. Gaia did it: https://www.youtube.com/watch?v=mPucRSaFgLI JWST isn't a difficult target: amateurs have spotted it on its way to L2 https://www.youtube.com/watch?v=9PM_ywxQ8OQ
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  38. The universe is expanding. As the light travels through the expanding universe, it expands with it and the wavelength gets longer. Comparable to the Doppler effect.
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  39. 4 million is right (2048x2048, to be exact).
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  40.  @TheEvilmooseofdoom  That's comparing apples to oranges though. For IR, the pixels have to be larger. For very faint targets, the pixels have to be larger again.
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  41. Teledyne built the H2RG detectors used in JWST. They also offer the H4RG, which is a 4096x4096 pixel instrument with a pixel size of 15 um (H2RG: 2048x2048, 18 um). This seems to be the state of the art for astronomical IR imaging CCDs. The H4RG has been available for at least 5 years now. The problem with a project like JWST is that at some point, you have to freeze the design. If you keep iterating, it'll never get finished. After the design freeze, there's several years of testing (during which you can't replace components, or you'd have to redo all the tests).
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  42. This video explains it well: https://www.youtube.com/watch?v=5MxH1sfJLBQ
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  43. JWST can roll on the vertical axis (the line pointing from JWST to Earth). So at any time, it can image a cylinder section of the sky that contains part of the ecliptic. Mars will pass through that cylinder twice a year.
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  44. JWST images are not classified: every observation will be published (via the Mikulski archive for space telescopes). For most observations, there will be a period where the astronomer who requested the observation will have exclusive access to the data. For most observations, the target list is public as well (the General Observer program for the first year has been published already). For the first few hours of science observations at the end of the calibration process, STSci has not published the target list beforehand, you'll see what they targeted when the observations are published.
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  45.  @piconano  And I've dug deeper than the headline. The only thing they're "keeping secret" is the target list for the Fist Light observations, so nobody will know beforehand what the targets are. Those observations will be published immediately. Not much of a secret, is it?
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  46. Teledyne built the H2RG detectors used in JWST. They also offer the H4RG, which is a 4096x4096 pixel instrument with a pixel size of 15 um (H2RG: 2048x2048, 18 um). This seems to be the state of the art for astronomical IR imaging CCDs. The H4RG has been available for at least 5 years now. The problem with a project like JWST is that at some point, you have to freeze the design. If you keep iterating, it'll never get finished. After the design freeze, there's several years of testing (during which you can't replace components, or you'd have to redo all the tests).
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  47. This is a bit counterintuitive. We're not going to look at the place where the Big Bang took place - there's not single place we can point to. Instead, we're going to look at galaxies 13.5 bn lightyears away. They will be 13.5 bn years old, so they should be the oldest galaxies in the universe, containing the first generation of stars. We can find these galaxies in any direction.
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  48. 0:14 HD84406 was the first of about 25 stars that were used during the mirror alignment process. They had to keep switching to fainter stars as the mirror alignment got better. 13:25 this star is 2MASS J17554042+6551277.
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  49. Teledyne built the H2RG detectors used in JWST. They also offer the H4RG, which is a 4096x4096 pixel instrument with a pixel size of 15 um (H2RG: 2048x2048, 18 um). This seems to be the state of the art for astronomical IR imaging CCDs. The H4RG has been available for at least 5 years now. The problem with a project like JWST is that at some point, you have to freeze the design. If you keep iterating, it'll never get finished. After the design freeze, there's several years of testing (during which you can't replace components, or you'd have to redo all the tests).
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  50. Crappy troll.
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  51. The animation is accurate: JWST is orbiting the Sun-Earth L2 point, not Earth itself.
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  52.  @dickJohnsonpeter  This video explains it well: https://www.youtube.com/watch?v=ybn8-_QV8Tg
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  53. for raw images that's correct. But they can rotate the telescope a tiny amount, then take another picture to fill the gaps.
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  54. JWST can't image Earth or the Moon, they would have to expose the cold side of the telescope to direct sunlight, which would overheat the telescope. Luckily we don't need JWST to get high-res images of the Moon. We have the Lunar Reconnaissance Orbiter, which orbits the Moon at an altitude of less than 50 km. LRO images of Apollo landing sites: https://www.youtube.com/watch?v=Pnhnx95LkCc
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  55. If you want to convince people to become Christians, this is not the way to do it. Please stop.
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  56. No.
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  57. that's what they're doing. The NIRCam instrument has 10 image sensors, for example.
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