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58 votes

Why is one picture of this star blue with red, and the other red with blue?

They are two pictures of the same object. The Southern Ring Nebula. They look different because we are looking at different wavelengths. The picture on the left is near infrared (about the range 0.7 - ...
Prallax's user avatar
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56 votes

Why does the alignment evaluation image from JWST look like this?

A quick check by pasting the image into PowerPoint and rotating a line shows that the spikes have threefold symmetry; they're at -30°, 30° and 90°. This is exactly what you would see from diffraction ...
uhoh's user avatar
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48 votes
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Why does the alignment evaluation image from JWST look like this?

Any telescope will have diffraction of the light due to the edges between mirror and non-mirror. This sets a fundamental limit to the telescope's resolution, given by the size of its primary mirror. ...
pela's user avatar
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39 votes

Has NASA released any JWST images of Trappist-1? If not, why?

The Trappist-1 system consists of an ultracool red dwarf at a distance of 12.5 pc from the Sun, surrounded by a system of planets with orbital semi-major axes ranging from 0.01 to 0.06 au. As seen ...
ProfRob's user avatar
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37 votes
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How many light seconds away is the JWST?

Almost 5.1 seconds, plus or minus around 1 second. Here's a daily plot (at midnight) of the light travel time from the JWST to the centre of the Earth, courtesy of JPL Horizons, using a script derived ...
PM 2Ring's user avatar
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36 votes
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How much larger will the "observable by us" universe be when JWST becomes operational?

tl;dr Conservatively 10%, realistically 25%, optimistically 60%. I assume that by the "observable-by-us Universe", you mean not the theoretically observable Universe, which is given by the ...
pela's user avatar
  • 38.7k
35 votes
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How does JWST position itself to see and resolve an exact target?

It's true that James Webb carries fuel, and you're right that it is not used for positioning, at least not directly (see below). Fuel is used for maintaining its orbit around L2, and was also used ...
pela's user avatar
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27 votes
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What is the probability of a photon from one of these galaxies hitting the James Webb detector?

Probability without lensing The probability of any one photon of an ensemble of isotropically emitted photons is indeed proportional to ratio between the area of the detector, and the area of the &...
pela's user avatar
  • 38.7k
26 votes

How many light seconds away is the JWST?

According to NASA, the JWST is 1.5 million kilometers away, which is about 5 light seconds. For comparison, the distance to the Moon is about 1.3 light seconds, Mars ranges from about 3 to 22 minutes, ...
Zamicol's user avatar
  • 1,005
26 votes
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Why don't we just unshift the IR photos from Webb?

The infrared data cannot be unshifted to produce visually pleasing images simply by linearly scaling observed wavelengths because JWST does not work like the human eye nor like the camera in your ...
doubleunary's user avatar
22 votes
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Why will HD 84406 be chosen as the first target for testing JWST?

The most important selection criterion is that the star should be available for observation for a prolonged time. Because James Webb observes in the infrared, it must hide the Sun (and Earth and the ...
pela's user avatar
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22 votes
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What is the most common activity of astronomers in the JWST?

JWST operates in a mode where groups of astonomers make detailed proposals to observe particular obects with particular instruments. There are no random pointings, although there may well be ...
ProfRob's user avatar
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19 votes

Will JWST be as durable as the hubble telescope?

Hubble was in low earth orbit, and was always intended to be serviceable. In fact, the original plan for Hubble was to have the space shuttle carry it down from orbit and take it back up, but they ...
Sean Lake's user avatar
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19 votes

Has the small meteorite that hit Webb done a lot of damage?

Has the small meteorite that hit Webb done a lot of damage? The TL;DR answer is an emphatic no. Details follow. How seriously must we take this? The TL;DR answer is an emphatic not at all, at least ...
David Hammen's user avatar
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19 votes
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What are the black spots/blobs in JWST Jupiter's images?

The black blob comes from a region of dead pixels on one of the NIRCam detectors, A1 (as someone on Reddit correctly identified). STScI has a display of flat field images for a number of segments; a ...
HDE 226868's user avatar
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19 votes
Accepted

As of now how much larger is our practically observable universe compared to just prior to JWST becoming operational

The current redshift record is set by the JADES galaxies. The highest-redshift one at $z = 13.20$ isn't too convincing tbh, but there's one at 12.6 that looks better, and the one at $z = 11.6$ should ...
pela's user avatar
  • 38.7k
17 votes
Accepted

What is this bright blue object taken by the Webb telescope?

That is Triton, Neptune's moon. The colours are false, since the JWST is an infrared telescope. Triton is an unusual object and probably didn't form with Neptune. Instead, it is likely a captured ...
James K's user avatar
  • 125k
17 votes

What would evidence of stars and galaxies significantly older than 13.8 billion years old look like? In what parts of space has it not been found?

Assuming the cosmological principle still holds, then we might expect that your own galaxy and those around us should contain evidence of stars/objects older than 13.8 billion years old. These might ...
ProfRob's user avatar
  • 155k
13 votes
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Why not build a swarm of space telescopes?

There are a mixture of factors here. Firstly the telescopes used to photograph the black hole were radio telescopes. Radio-waves are at a low enough frequency that we can process them directly as ...
James K's user avatar
  • 125k
13 votes

What parameters of the Big Bang model will have to be adjusted to account for JWST's observations of highly redshifted galaxies?

The big-bang model does not have much to say about the redshift-dependence of the galaxy mass function (NB It is really the luminosity function that is being probed), other than that at some high ...
ProfRob's user avatar
  • 155k
13 votes
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How does the James Webb Space Telescope achieve a focal length of 131.4m?

The JWST has three mirrors(*), the main mirror (6.5m in diameter) A secondary mirror, held on three struts in front of the main mirror and a tertiary mirror that reflects the light into the various ...
James K's user avatar
  • 125k
12 votes
Accepted

Will the James Webb Space Telescope be visible from earth?

At 1.5 million km JWST will be roughly 4,000 times farther than the ISS when it's overhead, and very roughly a tenth of the cross-section. So if the ISS can be as bright as -4 magnitude, then JWST ...
uhoh's user avatar
  • 30.7k
12 votes

Why is one picture of this star blue with red, and the other red with blue?

The JWST operates in the infrared spectrum, which is invisible to the human eye. Infrared has longer wavelengths than visible light. To make visible images, we map infrared date into the visible ...
Connor Garcia's user avatar
  • 16.3k
11 votes

Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?

There are approved proposals for Cycle 1 to point the JWST at The Jovian system Jupiter's great red spot Mars Saturn and its moons and rings In those PDFs, they describe exactly what instruments ...
Gavin S. Yancey's user avatar
11 votes
Accepted

What is the numerical aperture of the James Webb Telescope?

Background Imagine you are a pixel sitting at the primary focus, looking up at the incoming light. What do you see? If the image of a star or something bright is falling on you, you'll see a big ...
uhoh's user avatar
  • 30.7k
11 votes

Difference in field of view between JWST and Euclid

The exact field of view (FOV) of a telescope is often not a unique number, but depends on the instrument in use. For JWST, the FOV is a typically few arcmin (1/60 of a degree). Here you can see the ...
pela's user avatar
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10 votes
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Question: By design, JWST cannot observe in the anti-sun direction. Is this due to Gegenschein backscatter?

I'm going to say no. Gegenschein is, as you note, reflected sunlight, and is the brightest part of the (reflected component of the) zodiacal light. It's not very bright in the optical (there are no ...
Peter Erwin's user avatar
  • 17.2k
10 votes

How to fix James Webb space telescope?

There are no plans in place to fix JWST. It has to work.
ProfRob's user avatar
  • 155k
10 votes
Accepted

What percentage of visible stars will JWST be able to survey over the next 10 years?

Not Very Many - It Isn't a Survey Scope The JWST is designed as a narrow field of view telescope for examining very small targets. It is not suitable for sky surveys. But let's go through some ...
Dan Hanson's user avatar
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