# Tag Info

68

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 major advantages of space until you get above several meters mirror diameter. (2) Rocket fairings are the shrouds which protect payloads during the supersonic ...

19

Hmmm no, it wouldn't be cluttered with debris, and yes, it's a good idea to park the JWST (James Webb Space Telescope) at the Sun-Earth L2 point. The five Lagrange points are unstable, for one because of the gravitational anomalies of the two massive bodies of the Lagrange system, eccentric orbits, and there are many other factors to their instability. At ...

19

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 about three months to achieve its orbit in $L_2$. Now, $L_2$ is unstable, and so some station-keeping - essentially, course corrections by thrusters - will be ...

14

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 cottage' in the woods, how would you divide your budget? This question is a follow-up to Do bigger telescopes equal better results? Yes, and I'm not a fan of ...

11

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 the cosmic microwave background (CMB), are located here because Earth is a hundred times brighter than the CMB in this wavelength region. Herschel observes in ...

11

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 the 1940s. It kind of worked, but just barely, for the 6 meter mirror on Caucasus in Russia in the 1970s. It did work, but that was a major achievement, for the ...

10

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 still works and will be used again, even though it is not "active" at the moment. The most recent and farthest-from-earth telescopic observations that I know of are from ...

9

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 would waste a lot of space within the cubesat. Also, traditional designs tend to be much longer than they are wider; they don't fit well in a cube; it is very ...

9

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. In general, when you are fitting models to data, it is far better to compare the models and data in the observational space of the data, where the uncertainties ...

9

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. So the first thing you need is a functioning moon base that can manufacture all components from local raw materials. Once that's in place (insert large ...

8

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 kilometers above the surface) is above almost all of the atmosphere, but there's still enough residual air to cause some slight drag. It's not higher because it was ...

8

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 Relativity. The general formula for such lensing is that light is bent through an angle (in radians) of $$\alpha = \frac{4 GM}{c^2 r},$$ where $M$ is the mass ...

7

It's very unlikely that large optical telescopes will ever be built on the Moon, because the Moon is almost the worst possible place to build them. (The surfaces any of the planets other than Earth are worse.) It has no particular advantages over orbit and costs a lot more to build there. The Moon looked like a good location when observatory technology ...

7

Gravitational lensing works from anywhere beyond the focus, so in that sense, we could use any star as a gravitational lens. The problem is that the field of view is tiny. We only get any useful information from alpha centauri as a gravitational lens if the target object is almost exactly behind alpha centauri from our point of view. To look in a slightly ...

6

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 0.7au. Both are at a distance of 4.37 light years. 1au at 4.37 light years, subtends an angle of 0.74 arcseconds. If working at blue wavelengths (the aim appears ...

6

The James Webb Telescope is the next one on the launchpad that you might be familiar with. Although there are a few differences that one ought to be aware of. NASA has an entire program of telescopes to observe the universe, and many of them are designed for different wavelengths of light. The James Webb is primarily designed for the infrared part of the ...

6

The IRAS Point Source Catalog, Version 2.0, is a catalog of some 250,000 well-confirmed infrared point sources observed by the Infrared Astronomical Satellite (IRAS), i.e., sources with angular extents less than approximately 0.5, 0.5, 1.0, and 2.0 arcminutes in the in-scan direction at 12, 25, 60, and 100 microns (um), respectively. This includes some ...

6

The James Webb Telescope will not be orbiting around the Earth, but the Sun, at a distance of 1.5 million kilometers or 1 million miles from the Earth. A benefit to sending it further away from the Earth is that there's less of the interference of light pollution from the Earth. The JWST's mirror is 21 feet wide, though, so its sensitivity to this will be ...

6

According to Rydberg et al. (2013) "Detection of isolated Population III stars with the James Webb Space Telescope", the answer for non-lensed population III stars appears to be no. For lensed stars, the detection would be difficult but the expected number of detections is very small: To detect even one 60 M⊙ Population III star when pointing JWST through ...

5

If the starshade was to stay in place over the ground telescope, it would have to be in a geosynchronous orbit, which puts it almost 36,000 km from Earth's surface. To cover an angular distance of 1 arcsecond (1/3600 of a degree), it would have a size of around 10km. Certainly not impossible, but technologically impractical at the moment. Also, the starshade ...

5

So most of the costs should end up as salaries to engineers. Even in a project that is solely people-based, most of the costs do not end up as salaries to engineers. Salary is typically a bit less than a half of the cost. Those engineers get benefits. Health care in the US isn't cheap. They get holidays, vacation, and sick leave. They receive matching funds ...

5

These two uses of PSFs are applicable to different situations. Sometimes you know your PSF well enough to deconvolve your image and get something reasonable out of it, but most of the time you have a lot of assumptions in this process and you're not going to get something perfect, so you might want to go the other way and convolve your model with the best ...

5

I can't speak for the JWST, but I do work at a company that designs telescopes for spacecraft and I can tell you that a lot of the structure of the telescope is to protect the instruments during the initial ascent to space where the launch vibrations and g-loading will be the worst. Plus mirrors for telescopes that big have really precise alignment ...

5

I'm an amateur telescope maker. Single vane designs do exist. Their main problem is lack of stability. You would have to use a very thick vane to hold the secondary in place in a stable way. That would block off a significant amount of light and may impact the performance of the instrument. Active systems, while theoretically possible, would be very ...

5

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-linearity problems in the detectors. The first light images look quite similar to simulated pre-flight images (e.g. Hoyer et al. 2018), so I don't think there are ...

4

A systems cost breakdown (as of 2011) is given here: The "% spent" refers to the status of spending at the time. That's not completely helpful, because that's only broken down by instrument sections. For example, "Northrop-Grumman" doesn't tell you the salary breakdown, as you mentioned.

4

This can best be described by two slides from this presentation. "Forward-facing" implies looking towards Earth in the spacecraft's orbit, in the direction of the spacecraft's velocity vector: "Backward-facing" implies looking in the opposite direction, away from Earth and in the opposite direction to that of the spacecraft's velocity vector: These are ...

4

The mindboggling GAIA spacecraft-telscope sort of does what you suggest! That's kind of the closest concept to what you describe. Just as explained in JamesS answer, if you were that far away you wouldn't be in orbit around the Earth. But, GAIA's L2-like orbit is kind of the closest thing to what you say. Explanation: https://en.wikipedia.org/wiki/Gaia_(...

4

Its a nice idea, but it wouldn't work. To stay in the Earth's shadow a telescope would need to orbit the Earth in the same time the earth orbited the Sun so that it always stayed on the opposite side. For its orbit to be this long it would need to be a long way out, remember geosynchronous orbit (one orbit in 23h 56min 4sec) is at a distance of 42,164 km. ...

4

The table you linked to gives limiting magnitudes for direct observations through a telescope with the human eye, so it's definitely not what you want to use. The quoted number for HST is an empirical one, determined from the actual "Extreme Deep Field" data (total exposure time ~ 2 million seconds) after the fact; the Illingworth et al. PDF you linked to ...

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