72 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 ...
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  • 7,390
63 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 ...
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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}{...
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34 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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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20 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 ...
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  • 34.1k
17 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 ...
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  • 2,527
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-...
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  • 14.8k
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 ...
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  • 117k
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 ...
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13 votes
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Theoretically, what is the biggest optical telescope that may exist?

It's complicated. Until late-20th century, we've tried to make bigger and bigger monolithic telescopes. That worked pretty well up to the 5 meter parabolic mirror on Mount Palomar in California in ...
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11 votes
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Why does a mirror bent 'like a potato chip' allow space telescopes to be smaller and have a wider field of view?

Freeform optics are a response to the specific challenge of cramming a telescope in a very limited space. A traditional instrument would have all optics symmetrical and aligned on the same axis. It ...
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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 ...
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10 votes
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Are space telescopes completely out of the earth's atmosphere?

A handful of space telescopes are located in Langrange point L2, 1.5 million km from Earth. This is much farther away than the Moon, and far outside Earth's atmosphere. WMAP and Planck, which measure ...
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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 ...
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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 ...
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  • 117k
9 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. ...
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  • 117k
9 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. ...
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9 votes

JWST First Image With Star 18x - Which image is from which mirror?

How do they tell which image corresponds to which mirror? The JWST team did this commanding each mirror segment to reorient itself by a bit, one segment at a time. The changes in the captured imagery ...
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8 votes
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How could a 20 inch space telescope "be able to make out Earth-size planets" orbiting Alpha Centauri?

I'm not familiar with the design of the ProjectBlue telescope, but I think you have answered your own question. The habitable zones for Alpha Cen A and B, are approximately centred at 1.25au and 0....
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  • 117k
8 votes

Are space telescopes completely out of the earth's atmosphere?

This article contains a list of space telescopes. It's likely to be nearly complete. The extent of the Earth's atmosphere is not very well defined. The altitude at which Hubble orbits (about 550 ...
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8 votes

Does this CHEOPS first light image imply bad astronomy?

As the article you reference makes clear, the defocusing is deliberate. It spreads the light of bright stars (the main targets for CHEOPS) over more pixels and hence mitigates saturation and non-...
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  • 117k
8 votes
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Why didn't we see those campfires on the Sun until now?

tl;dr they're a bit too small for SOHO and STEREO and only visible in EUV The "campfires" are described as a few hundred km across, The smallest of those campfires are about the size of a ...
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8 votes

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

In addition to the target list linked to by @planetmaker in their answer, there are two recently published review articles (from Nature Astronomy) summarizing the many different aspects of Solar ...
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8 votes
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What is "Charged time" in JWST ERS proposals?

"Charged time" includes all of the overhead associated with carrying out the observing program, rather than just the open shutter exposure time. The overheads include slewing time, ...
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