Radio Astronomy - General Questions
19 Sep 2017 6:29:54 UTC
Topic 209775
(moderation:
I am not a radio astronomer but I have a few questions that I have not been able to find answers elsewhere.
1. Since LIGO and Arecibo are both in the Northern hemisphere as well as LOFAR do we only scan half of the sky since the earth is round and there are no other radio telescopes scanning the sky in the Southern hemisphere on these open projects, including SETI?
2. If we had to have a 50m diameter radio telescope in low earth orbit (
I know bare with me on this) would it make for better science (pictures with more clarity) as I believe that most of the radio waves apparently get absorbed by the atmosphere? I recall watching a video on Youtube discussing a large radio telescope on the dark side of the moon. They did not really go into the advantages but more about design.
Thanks
LIGO is listening for gravity
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LIGO is listening for gravity waves, not radio waves. I don't think the earth attenuates them enough to be interesting.
RF absorption is very frequency dependent, but the attraction of a science antenna on the side of the moon away from earth (not the dark side) is lower interference from terrestrial noise sources, not lower attenuation.
Big antennas in orbit need to be at least somewhat higher than Low earth orbit to avoid the frequent need to reboost because of drag. There are some up there, but they point down, not up, and they don't belong to radio astronomers.
That are all nice questions,
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That are all nice questions, here are a few thoughts:
LIGO is a gravitational wave detector (actually two sites), it doesn't matter on which hemisphere they are because gravitational waves, unlike radio waves, travel right through the Earth (as these are ripples in space time, not electromagnetic waves like radio waves, X-ray, gamma rays or light waves within the visual spectrum etc).
But even for an EM observatory, it's not true that you can observe only half of the sky from a given hemisphere, unless you are exactly at the poles. The closer you get to the equator, the more of the sky you can observe, and at the equator you can (theoretically) see the entire sky...if your telescope can be pointed to any point of the sky, which is not true for Arecibo as the dish is installed in a fixed position. However the antenna dome above the dish is moveable so Arecibo can look quite a bit "to the sides". And Arecibo is pretty close to the equator. So in one respect it's probably better than you think (close to equator), while in another sense it is more limited (not fully steerable dish).
Also, Einstein@Home (and I guess also SETI) are using or have used other telescopes' data as well: e.g. we used the Australian Parkes dish in the (obviously) southern hemisphere to find new pulsars.
As for space based radio telescopes, there actually is (at least) one in operation: Spektr-R https://en.wikipedia.org/wiki/Spektr-R which has a 10m dish.
The reason to have radio telescopes in space is different from that for other EM telescopes. Earth's atmosphere is actually quite transparent to a wide spectrum of radio waves, e.g. see this cartoon: https://en.wikipedia.org/wiki/Radio_window , so the atmosphere is not that big a problem at all.
Other reasons to do radio astronomy in space:
Radio silence: if you had a dish on the far side of the moon (there is no "dark side of the moon" :-) ), it would be shielded against all terrestrial radio sources!! Perhaps also the absence of the earth's magnetic field and all of it's interactions with solar activity could be useful.
Zero/low gravity: one might think that under lower gravity (and in the absence of e.g. wind loads, earthquakes and other structural hazards), it might be possible to construct larger single dishes, e.g. in a moon crater or even a fully steerable one. The problem is to get the parts into space and assemble them there , the payload for rocket launches is limited by the dimensions of the payload fairings of the rockets (order of 5 meters or so in diameter) and while you can have ingenious mechanism to unfold stuff in space, for a radio dish the whole thing needs to have a shape that is pretty precisely aligned..not trivial for things the size of (say) Arecibo in space.
Most important tho: huge baselines for multiple dish operations => interferometry. The real good reason to get radio telescopes into space is that you can switch together multiple smaller dishes in a clever way so that in some respects, they perform like a dish that has the size of the distance between the individual dishes. This is called radio interferometry https://en.wikipedia.org/wiki/Radio_astronomy#Radio_interferometry . You can do that (and this is actually being done!!) with radio telescopes scattered all over the planet, but you are then limited by ... the size of the planet! . Going into space relaxes this limitation.
Hope this clarified things a bit?
HBE
As Archae86 said, LIGO is a
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As Archae86 said, LIGO is a gravitational wave experiment. However Arecibo is a Radio telescope. And it can only view a portion of the sky because it is located in Puerto Rico.
Also, the Earth's atmosphere does absorb some radio waves, but in general it turns out to be rather transparent to radio frequencies and the visible frequencies (that's why they are visible...) A quick google search pointed me to this link: https://earthobservatory.nasa.gov/Features/RemoteSensing/remote_04.php, which as an image at the bottom showing the atmospheric transmission of various frequencies.
You can see that the visible spectrum is transparent. On the right-hand side, at about 1.0m is the start of the radio spectrum, which is also transparent. If the graph were extended on the right, it would show more of the radio spectrum which is transparent. In between various components of the atmosphere absorb different amounts of IR frequencies. Above the visible spectrum, the ozone layer absorbs much of the UV frequencies.
Interestingly, from an evolutionary standpoint, this is why the rods and cones in our eyes have developed to be sensitive to the visible spectrum. It happens to be a set of frequency of light that are both 1) transparent to the atmosphere and 2) produced by the sun in quantity.
(Also, eyes that are sensitive to the radio spectrum would have to contain lens that are 10+ meters in diameter in order to focus the light. Needless to say that would require a much larger head...)
Einstein@Home Project
Speaking of Arecibo, the
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Speaking of Arecibo, the observatory is currently bracing for the incoming hurricane Maria:
Given its location, Arecibo was designed with hurricanes in mind. But Maria is a real big one, low category 5 to high category 4 (on a scale up to 5), and its predicted path could lead it very close indeed to Arecibo Observatory. Let's keep our fingers crossed, as storm damage is the last thing that this observatory, already under budget pressure, can use :-(. But of course even more severe is the danger this storm poses to the local residents. It's expected to be the strongest hurricane to hit Puerto Rico since the 1930s.
Cheers
HB
Hallo! Please don´t forget
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Hallo!
Please don´t forget the SKA (Square Kilometer Array), planed to become operative in 2023 and situated in Southafrica, Australia and New Zeeland. The positions where choosen because of radio quietness (dessert areas) and distance. The northern hemisphere is too much crowded with humans and their intensiv lifestyle. I´m wondering, why the Argentian Pampa wasn´t choosen. But such decisions are always politcal.
Kind regards and happy crunching
Martin
The SKA headquarters were to
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The SKA headquarters were to be in Italy, at Padova, where Galileo Galilei taught. This site was chosen by a committee of astronomers. But the British Government intervened and the site went to Jodrell Bank, the site of the Bernard Lovell radio telescope near Manchester, UK. I followed this affair on "Nature" magazine, which was very fair.
Tullio
I think I remember that the
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I think I remember that the South Atlantic Anomaly https://en.wikipedia.org/wiki/South_Atlantic_Anomaly was one of the reasons that disfavoured SKA sites in South America.
I often wonder what we see
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I often wonder what we see and don't see because of the atmosphere, and those diagrams showing the spectrum are really good in explaining that, thanks i had not seen them before! It's quite amazing the atmosphere is like a concrete wall to some photons and invisible to others - bit like a suit of armour which some feathers can float through.
Is there somewhere a similar diagram showing the spectrum we would see most, if there was no atmosphere? I know for example there is a microwave background so that frequency must show up as a peak, but are there some frequency ranges where photons are few and others plentiful?
For example the atmosphere blocks very long wavelength radio waves, but would we see much if we had such a telescope in space?
astro-marwil wrote:I´m
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I considered the SKA to have lots of politics in in as there was a debate as to locate it completely in Australia or split between Australia and South Africa. The Australian argument made more sense to me as they had vast empty spaces. I think in the end politics prevailed.
Would the frequency bands now blocked by the atmosphere make for better science - I would guess so.
It seems that Arecibo was
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It seems that Arecibo was severely damaged. This from the News section of messages in SETI@home.
Tullio