14 votes
Accepted

Can the Hubble constant be measured directly?

The universe doesn't expand on such small scales. In fact there is no expansion even on the scale of the local group of galaxies. It is only when you look at more distant galaxies that you see the ...
James K's user avatar
  • 122k
12 votes
Accepted

Why don't we use amateur astronomers' telescopes to create a huge interferometer?

You can't make an interferometer by combining photographs. You need to combine the light sources so that the light waves can "interfere", which means you need to have not only the intensity ...
James K's user avatar
  • 122k
12 votes
Accepted

Was GRAVITY built to look at one star?

No, the GRAVITY instrument is multi-purpose. This link gives you all the papers that have cited the instrument description paper. The list of papers shows that it has been used for studying: the ...
ProfRob's user avatar
  • 152k
10 votes
Accepted

Is Optical VLBI theoretically feasible? If not why not?

This question amounts to: is optical interferometry possible when detecting photons? The answer to this is yes. Many experiments have been done using interferometers where one photon at a time is ...
ProfRob's user avatar
  • 152k
8 votes
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LIGO: How can laser interferometry (wavelength >$10^{-7}$m) detect length changes of arms <$10^{-18}$ m?

The LIGO interferometer uses a homodyne detection technique. Basically, the light travelling in each arm of the interferometer is derived from the same laser source and is combined in the output ...
ProfRob's user avatar
  • 152k
8 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

With very few exceptions of the biggest and most nearby sources (like Betelgeuze with <~50mas) we cannot resolve the angular diameter of stellar-type sources by direct imaging means. Precision of ...
planetmaker's user avatar
  • 19.4k
7 votes

Why not us interferometry to take a picture of Pluto?

Radio interferometry can combine observations over very large baselines. But optical interferometry cannot. According to a list of interferometry instruments on wikipedia, the largest baseline for ...
BowlOfRed's user avatar
  • 2,085
7 votes
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Can the interferometer called "Gravity" measure "a few centimeters on the Moon"?

A 130m baseline operating at 2 microns gives a theoretical resolution of $2\times 10^{-6}/130$ radians. At a distance of 400,000 km this translates to 6m. My guess is that Genzel is referring to the ...
ProfRob's user avatar
  • 152k
7 votes

Is it possible to overcome the problem of blind spot(s) of current gravitational wave detectors?

The blind spots are caused by the way the current detectors ("L-shaped" interferometers) work. They are sensitive to a gravitational wave (GW) changing the difference in path length along ...
ProfRob's user avatar
  • 152k
7 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

All images of stars (bar one or two of the closest or largest) are effectively those of point sources. The image we see is the convolution of a point with the instumental "point spread function&...
ProfRob's user avatar
  • 152k
6 votes

mm-wavelengths Moon map, scientific case

Millimeter and sub-mm observations (110-300 GHz = 2.7-1.0 mm) are sensitive to the thermal emission and provide a brightness temperature of airless bodies like the Moon or asteroids. The radio ...
astrosnapper's user avatar
  • 8,357
6 votes
Accepted

How are people converting intensities in Janskys to Kelvin?

This is, indeed, a result of how we measure things in radio astronomy. It's not just interferometry, but radio astronomy in general. The thing they're referring to is a concept called "brightness ...
Phiteros's user avatar
  • 3,166
6 votes

Multi-messenger astronomy: what is the potential of simultaneous detection of gravitational waves and neutrinos from a supernova?

This article basically seems to answer the question. They quote from an earlier study: "Although no CCSNe have currently been detected by gravitational-wave detectors, previous studies indicate ...
Steve Linton's user avatar
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6 votes
Accepted

Have interference effects (in space) ever been observed by a single instrument, as opposed to interferometry?

When a star is occulted by a "sharp edge" such as the limb of the Moon or by an asteroid, then diffraction effects are seen. The star doesn't suddenly disappear (or appear); there are a ...
ProfRob's user avatar
  • 152k
5 votes

Converting Jy/beam to Jy?

Actually to convert from Jy/beam to Jy/pixel you need to divide by the beam size. Let's say you have a quantity of 1 Jy/beam, then $\frac{Jy}{beam} \frac{beam}{\Omega}$, then to go from Jy/beam to ...
Miguel Cárcamo's user avatar
5 votes

Why not us interferometry to take a picture of Pluto?

Interferometry at infrared and shorter wavelengths is more difficult than at microwave/radio wavelengths for a number of reasons. Radio signals can basically be recorded on tape (or rather hard drives ...
ProfRob's user avatar
  • 152k
5 votes
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What is the significance of using baseline pairs in radio interferometry?

The basic idea behind interferometry is that of interference, the combination of two waves (in this case the electromagnetic waves from distant sources). Interference inherently implies two signals ...
Eric Jensen's user avatar
  • 4,864
5 votes

rms noise, confusion and dynamic range in radio images

RMS noise: Your understanding is mostly correct. RMS noise is the root mean square of fluctuations. Its square equals the sum of square of mean fluctuation and square of standard deviation of ...
terribleGuitarPlayer's user avatar
4 votes

Could the E.H.T. produce an image of the human artifacts on the moon?

This is a really interesting question! tl;dr: A definite maybe, but you would have to engineer a clever way to focus the transmitted power to a much smaller spot first; possibly several orders of ...
uhoh's user avatar
  • 30.8k
4 votes
Accepted

What are the raisons d'être for the Large Binocular Telescope "binocularity"?

I asked myself the same question and back then, I found the reasoning on the German Wikipedia page pretty comprehensive. I am translating, summarizing and expanding the corresponding section in the ...
B--rian's user avatar
  • 5,616
4 votes
Accepted

Why does the Simbad page "A.A. Michelson's Jovian Galilean-satellite interferometer" show data for Betelgeuse?

The SIMBAD link might be there just because Osterbrock's 2004 AAS presentation about the interferometer mentioned an observation of Betelgeuse. This would be consistent with the policy stated in ...
Mike G's user avatar
  • 18.7k
4 votes
Accepted

What could a cloud of mini radio dishes see?

Partial answer: I imagine this array would have very high resolution due to the large number of large and small baselines and I expect time synchronization would be an issue, but perhaps they could ...
uhoh's user avatar
  • 30.8k
4 votes

Are there any space-based observatories that use interferometry?

As the Wikipedia page on VLBI interferometry points out, there have been a few spacecraft with radio antennae which have formed (along with ground based stations) part of radio interferometer networks....
ProfRob's user avatar
  • 152k
4 votes
Accepted

Why hasn't VLBI been used to try and image giant exoplanets?

I think the radio emissions from the nearby host star would drown out any signal from the planet. To use our Sun and Jupiter as an example, at 10 millimeter wavelength, the flux density of the Sun for ...
Connor Garcia's user avatar
  • 16.3k
4 votes

About angular diameter, parallax and image of the nearest neutron star RX J185635-3754

The angular diameter doesn't come from imaging, but from photometry and spectroscopy. The observers estimated the surface brightness from the spectrum, and then computed how big the angular diameter ...
John Doty's user avatar
  • 1,876
3 votes

Does interstellar cloud obstruct or reduce the visibility?

Yes it is possible that a cloud reduce the visibility because the matter in it will absorb some of the light which is going through. Yes, we cannot see all the universe and so we can see ...
Yoann A.'s user avatar
  • 307
3 votes

Converting Jy/beam to Jy?

So long as you accurately know the beam size, then yes multiplying your Jy/beam measurement (effectively flux density) by the beam size (effectively area of flux) will give you the total Jy (...
zephyr's user avatar
  • 15k
3 votes

Can two detectors for gravitational waves be combined into a very long baseline interferometer?

They do use two or more detectors to triangulate the origin of a source. Most recently the Advanced Virgo detector in Italy was used in conjunction with the two LIGO detectors in the USA. This has ...
Mick's user avatar
  • 1,410
3 votes

Is radar interferometry used, or feasible, for ground based astronomy?

Is radar interferometry used, or feasible, for ground based astronomy? Yes it is! My answer to How can we install a radar on radio telescopes like FAST or GMRT? beings:;; The article is quite ...
uhoh's user avatar
  • 30.8k
3 votes
Accepted

Giant variation of the proposed eLISA mission using reflectors on Earth and the Moon possible?

This would not be a giant version of eLISA, but a small (and complicated) one. The distance between the LISA satellites will be 1 million kilometers, while the moon is on average 380.000km from ...
Alex's user avatar
  • 2,265

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