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73 votes
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Why are we building larger land-based telescopes instead of launching larger ones into space?

It's cheaper. (1) With adaptive optics you can get 0.1 arc second resolution on the ground (admittedly only on a mountain top with particularly good air flow, but still!). This eliminates one of the ...
Mark Olson's user avatar
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62 votes

Why is Starlink polluting the night sky a big concern if we have space telescopes?

It's a problem because there are still lots and lots and lots of ground-based telescopes. Ground-based telescopes are still (by far) the biggest optical telescopes, and the cost of space telescopes ...
James K'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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35 votes
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Why do satellites appear as streaks in telescope images?

Satellites are moving. They are in orbit around the Earth. Satellites in low Earth orbit are moving at about 7000 m/s relative to the ground. You can work out the orbital speed by $$v=\sqrt{\frac{GM}{...
James K's user avatar
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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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29 votes

Why do satellites appear as streaks in telescope images?

Can you explain me in simple words why the satellite in this telescope image appears as a streak? The exposure time is 1 second. This drawing should explain it: (Note: It could be the other way ...
Ilmari Karonen's user avatar
27 votes

How many pixels could an image of Proxima b taken by James Webb have?

The JWST will have an angular resolution of about 0.1 arcseconds, which is similar to Hubble. The star, Proxima, has an angular size of 0.001 arcseconds, and the planet Proxima b (a super-earth) would ...
James K's user avatar
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26 votes
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Is there enough data in light coming from distant exoplanets for Earth-orbiting telescopes to take a high-resolution photo of it?

No, not with the current or any projected "next-generation" of telescopes. The problem isn't dust, it is distance. To put it in context, you can consider a scale model of the universe. ...
James K's user avatar
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21 votes
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Is the measurement of distance and position of remote celestial bodies accurate?

Large masses can bend light, but space is largely empty. The light from distant stars and galaxies rarely passes close enough to another star or galaxy to have deviated. On the few occasions when it ...
James K's user avatar
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21 votes
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Why use a large separate starshade instead of an occulting disk?

TL;DR: The Sun is well-resolved in small telescopes so a focal plane occulter works well. Other stars are not resolved (except in a few exciting cases using interferometry) so an internal blocking ...
uhoh's user avatar
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21 votes
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Worthwhile to put a telescope on the far side of the Sun?

No, there are currently no plans to put a space telescope, or any other kind of satellite, at the Sun-Earth L3 Lagrange point. The main problem is (as you mention) that it's impossible to communicate ...
PM 2Ring's user avatar
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20 votes

Which (if any) space telescope would have worked longer if it hadn't simply run out of helium?

Past infrared telescopes (IRAS, Spitzer, others I can't remember) in space have generally used expendable liquid He. They either ceased operation or lost a large part of their capability when the He ...
John Doty's user avatar
  • 1,909
20 votes

Is there enough data in light coming from distant exoplanets for Earth-orbiting telescopes to take a high-resolution photo of it?

There is a fundamental limit to resolution called the "diffraction limit" that depends on three things: The angular size of the feature you're trying to resolve, The wavelength of light you'...
hobbs's user avatar
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19 votes
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Is the James Webb telescope going to orbit around the earth?

Your first question - is JWST going to orbit Earth - is a little complicated. It will follow a mission profile that will send it to the Sun-Earth $L_2$ Lagrangian point. It will take the telescope ...
HDE 226868's user avatar
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18 votes

Why are we building larger land-based telescopes instead of launching larger ones into space?

In addition to Mark's great answer ... Why are we building larger land-based telescopes instead of launching larger ones into space? If you had money for two homes, one near work and a 'summer ...
Rob's user avatar
  • 2,656
18 votes

Which (if any) space telescope would have worked longer if it hadn't simply run out of helium?

The Herschel infrared telescope is another recent telescope that stopped working once it ran out of Helium for its coolant.
Rob's user avatar
  • 2,035
15 votes

Why is Starlink polluting the night sky a big concern if we have space telescopes?

To expand on the "space telescopes are expensive" aspect: Space telescopes cannot be maintained or repaired. This applies not just to things like optics and instruments, but also to space-...
Peter Erwin's user avatar
  • 17.1k
15 votes

How many pixels could an image of Proxima b taken by James Webb have?

Any planet around Proxima Centauri (or any other exoplanet around any other star) will be unresolved. That means it is so small and far away, that it would appear as a point source of light to all ...
ProfRob's user avatar
  • 153k
15 votes

Is there enough data in light coming from distant exoplanets for Earth-orbiting telescopes to take a high-resolution photo of it?

The only real problem is lack of angular resolution at optical and near infrared wavelengths. Lack of photons is not an issue. Suppose there were something the size of Jupiter orbiting Alpha Cen A. At ...
ProfRob's user avatar
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15 votes
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What kind of telescope would be needed to image a 10m dim object 1 million km away?

A radar telescope. One million km is close (in space terms that's just over a tad) and well within the range of planetary radars. As an example, the [asteroid 2021 PJ1 was detected by radar using the ...
James K's user avatar
  • 123k
14 votes

Why aren't there any images of Sun-orbiting bodies by Spitzer?

All "big" instruments have observation logs, so does Spitzer. The complete logs are here but there's also a filtered log for solar system observations which shows basically all planets and ...
planetmaker's user avatar
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12 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
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12 votes
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Did this satellite streak past the Hubble Space Telescope so close that it was out of focus? If so, how close was it?

As you note, that particular image has been identified as due an "out of focus" satellite (the other images presumably show more distant satellites in e.g. geosynchronous orbits). The Nature ...
Peter Erwin's user avatar
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12 votes
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At the intersection of engineering and astronomy in its structure as a scientific discipline

Yes, there is an intersection between astronomy and engineering. In my experience, it will mostly be astronomers who have a flair for technology and instrumentation who at some point, either from the ...
pela's user avatar
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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
10 votes
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Which space telescope is the most distant?

Currently New Horizions is temporarily hibernating; it's last activity was two months ago. So I'm going to post a supplementary answer here because it is "operational" in the sense that it ...
uhoh's user avatar
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10 votes
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Questions about convolving/deconvolving with a PSF

Convolution is not a uniquely invertible process in the presence of random noise in your image. Deconvolving a noisy image can give misleading results, even if you have perfect knowledge of the PSF. ...
ProfRob's user avatar
  • 153k
10 votes

Why are we building larger land-based telescopes instead of launching larger ones into space?

Answering your subquestion about building on the moon: This is subject to the same launch costs and restrictions as a space-based 'scope, plus you have to deal with landing and with gravitational sag. ...
Carl Witthoft's user avatar
10 votes
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What is the gravitational lensing focal distance of a white dwarf star?

The gravitational focus you are talking about is actually a minimum value, defined by parallel rays of light from a very distant star just skimming past the Sun as they are bent according to General ...
ProfRob's user avatar
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