The Billion-Dollar Telescope Race

ThunderCat

nokia69 wrote: »

Its at one of the Lagrangian points

Ah of course. Thanks nokia69

Capt'n Midnight

Also further from interference from earth.

_Tombstone_

The WFIRST - Wide Field Infrared Survey Telescope has officially moved into project phase - it's the one coming after the James Webb, mid 2020s before it'll get up there.

https://www.youtube.com/watch?v=LbJpVHMV1m4

http://qz.com/620446/nasas-new-telescope-will-have-a-view-100-times-bigger-than-hubbles-and-could-solve-key-mysteries-of-the-universe/

http://spaceflightnow.com/2016/02/18/nasa-moves-forward-with-mission-using-spy-satellite-telescope/

_Tombstone_

James Webb Space Telescope's Instruments Removed From Super-Cold Chamber

https://www.youtube.com/watch?v=nfhhkWZrXcQ

_Tombstone_

_Tombstone_ wrote: »

China uproots 9,000 people for huge telescope in search for aliens

Residents within 5km radius of Fast project in Guizhou province will be forced to leave their homes and offered £1,275 in compensation

China finished the world's largest single-aperture telescope

For the past 53 years, Puerto Rico's Arecibo Observatory has been the king of radio telescopes. No more. China has just finished construction of its Five hundred meter Aperture Spherical Telescope (FAST), which is 64-percent larger. That makes it the worlds largest single-aperture telescope -- the world's largest radio telescope is Russia's RATAN-600, which has a sparsely filled aperture.

Nestled in a rural area of Guizhou province, FAST was built in an isolated valley, which is important for radio telescopes, but in order to ensure there will be no magnetic disruptions, some 9,000 people are being removed from their homes and rehoused in a neighboring county. Xinhua News Agency reported displaced families are also being paid 10,000 yuan (roughly $1,500) in compensation, which translates to an average year's salary in the area.

Unlike Arecibo, which has a fixed spherical curvature, FAST is capable of forming a parabolic mirror. That will allow researchers a greater degree of flexibility. Although it's 500 meters wide, FAST effectively offers an 300-meter dish that can be pointed anywhere ±40° from the zenith, with 10 times the sensitivity of Arecibo.

FAST will begin listening to the universe this fall. It will be tasked with surveying neutral hydrogen in the milky way and other galaxies, detecting pulsars and gravitational waves and looking for signs of extra-terrestrial life.

Zubeneschamali

_Tombstone_ wrote: »

China finished the world's largest single-aperture telescope

i see several references to gravitational waves - anyone know how a radio telescope is supposed to detect gravitational waves?

ps200306

Zubeneschamali wrote: »

i see several references to gravitational waves - anyone know how a radio telescope is supposed to detect gravitational waves?

Yes. It's quite a fascinating approach. You know how LIGO works, by detecting the strain induced by gravitational plane waves from distant sources. A passing gravitational wave changes the length of space. The longer the distance (displacement) between two points the greater the change of length (also a displacement) between them caused by the passing wave. The strain (usually denoted h) is the change in displacement divided by the overall displacement:



Since you have metres above and below the line, the strain itself is a dimensionless quantity. Certain astrophysical objects of interest are known to produce strains of . The arms of the LIGO instrument (and therefore the effective displacement between its mirrors) are on the order of thousand kilometre scales, since light is bounced up and down its length several hundred times. So we have:



This is a very small change in displacement we have to measure. One way we could do it is with timed light pulses. We fire a light beam along a length, and count the pulses as they arrive at the other end. If a gravitational wave changes the length, the timing of the pulses changes as they arrive more or less frequently. LIGO uses a laser light beam with a wavelength of about one micron. But when you look at the size of our , it is only a billionth of a single wavelength!

This is why LIGO has to use interferometry. Instead of directly measuring timed light pulses, it measures the relative change in pulse duration in two orthogonal arms which can be done with the required precision. (As you know, it involves interfering the two slightly out-of-phase beams from the two arms to produce a signal).

One of the consequences of using interferometry instead of direct pulse counting is that the LIGO device becomes dependent on the frequency of the gravitational waves. Suppose the wavelength of the gravitational wave was only half the effective length of a LIGO arm. The arm could be partly stretched and partly compressed along its length, resulting in a null signal. Conversely, the wave mustn't be too much longer than the LIGO arm either. We can't measure strain directly using interferometry, only its second time derivative. (If you remember your Doppler effect, it's for the same reason that the siren of the stationary ambulance or ambulance moving at constant speed is at a fixed pitch -- we only measure a changing pitch when it is accelerating, usually due to change of parallax as it whips past us. An unchanging or slowly changing strain is equally no use to us). Fortunately, some parts of the inspiralling black hole mergers that LIGO was designed to detect produce waves of appropriate wavelength.

Even though LIGO has to use interferometry for practical reasons, we could in principle have a detector that works by pulse counting. Imagine you are using a GPS receiver. It works by triangulating timed signals from an array of satellites. Suppose a gravitational plane wave now passes by, changing the length of space in its direction of propagation. Our receiver will detect a change in the timing of the received signals. Real world GPS is not nearly sensitive enough to measure such a change, but if it was it would have a number of advantages over LIGO.

Firstly, we wouldn't have the wavelength dependence of an interferometric LIGO. As long as our time measurement was accurate enough we could measure anything. Secondly, by using a whole array of GPS satellites we would have better directionality. Only the component of each signal in the direction of the gravitational plane wave would be affected. LIGO has a version of this, but it is based on only having two detectors separated by about 3,000 km. An array would allow sources to be better pinpointed.

Ok, here comes the clever bit. Replace your GPS satellites by pulsars. These rapidly rotating objects produce extremely precisely timed radio pulses. A particular class of them -- the millisecond pulsars -- are the most precisely timed of all, with regularity approaching or exceeding that of an atomic clock. Now, enter the new Chinese FAST radio telescope. Firstly it is expected to detect huge numbers of previously unknown pulsars, about five thousand in our own galaxy and perhaps some in other nearby galaxies. About ten per cent of these will be millisecond pulsars. Pulsar rotation rates change over time, but this can be compensated for by observing over a long period of time to calculate the rundown rate. Add to this the fact that a large aperture telescope gives improvements not just in spatial resolution, but in temporal resolution. FAST is expected to yield a pulse timing accuracy of 30 nanoseconds after some years of pulsar observations.

Now when gravitational waves start passing through the space between us and these observed pulsars they change the timing in a way that we can detect. Admittedly, our thirty nanosecond accuracy is extremely rough. Dividing it into the speed of light, we find it corresponds to a change of displacement of ten metres. Compare that to one thousandth of a trillionth of a metre that LIGO has to measure. But our displacement -- the distance to the pulsar -- is vast, so our strain sensitivity can still be reasonable. Of course, a single gravitational wave will have a wavelength much smaller than this. But we'll still be able to measure high strain, very long wavelength gravitational waves. This type is produced by supermassive black hole mergers between colliding galaxies. These may circle each other initially very slowly, producing waves of one cycle in a period of months or even years (on the order of , and wavelengths in light years!). The upside of such long wavelengths and low frequencies is we only have to observe our pulsars to make timing measurements every few weeks or so. The detection rates for supermassive black hole mergers will tell us how (or whether) galaxies were built by these events during the early evolution of the universe.

See the following graph (courtesy of Nan Rendong, chief scientist for FAST) for a comparison of the strain and gravitational wave frequency dependence of LIGO and its planned successors, plus those for FAST with the specified pulsar timing array accuracy (PTA):




Read more here:

• Gravitational Wave, Pulsar and Pulsar Timing Array on Wikipedia
• "The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) Project", Rendong Nan et al., International Journal of Modern Physics D.

Zubeneschamali

Brilliant, thanks!

ps200306

An article by one of the developers of the Thirty Metre Telescope about how its adaptive optics will work. Resolution of the TMT will be around 0.01 arcsecond -- more than ten times better than the Hubble Space Telescope -- which is fairly amazing for a ground-based scope.

http://live.iop-pp01.agh.sleek.net/physicsworld/reader/#!edition/editions_Astro-2016/article/page-16973

ps200306

Another large Chinese telescope in the planning. China currently has nothing bigger than 2.4m and relies on sharing other scopes under international agreements. A proposal to build a new 12m Large Optical/infrared Telescope (LOT) is under way, with participants invited to build their own instruments to attach to it:

http://blog.physicsworld.com/2017/03/20/chinese-astronomers-pin-their-hopes-on-lot/

Rubecula

We have binocular vision for 3D, so how far apart would optical telescopes need to be for a 3 D image of Jupiter? or Proxima for that matter?

Could LOT work in conjunction with Hubble to do this?

ps200306

Rubecula wrote: »

We have binocular vision for 3D, so how far apart would optical telescopes need to be for a 3 D image of Jupiter? or Proxima for that matter?

Could LOT work in conjunction with Hubble to do this?

There isn't a straightforward answer to that (although the short answer is no). The reason is that the degree of depth perception (or stereoscopic acuity) depends on interocular separation. You have to choose your desired depth resolution first before you know how far apart the observing instruments need to be.

Think of it this way: you are looking down from directly above a skinny vertical flagpole. If you had only one central eye the flagpole would appear as a point. But because of your interocular separation you can see both sides of the pole simultaneously. Imagine a triangle formed by the top of the pole and your two eyes. Now make another triangle from the bottom of the pole to your eyes. The difference of the angles at the apex of the triangles is the binocular disparity. Your stereoscopic acuity is the smallest such angular difference at which you can perceive depth. For humans it's about half an arcminute (1/120 degree).

This lets you see depths of about a seventh of a millimetre at a distance of 0.25 metre -- about the depth of the relief in the picture on a euro coin. But because of the geometry of depth perception it decreases with the square of distance. At 10 metres distance you can only perceive a minimum depth of about 25 cm.

Twenty-five centimetres happens to be about the same contour resolution achieved in the stereo images from the HiRISE camera on board the Mars Reconnaissance Orbiter. It has several hundred times the stereo acuity of a human, partly owing to the fact that the entrance pupil of its telescope is several hundred times bigger than a human pupil. Nevertheless, at an average distance of 300 km above the surface, HiRISE needs very large angles between the images in a stereo pair, and it's totally infeasible to have two cameras separated by the required distance. Instead, it uses motion parallax, taking pictures of the same site from different locations in its orbit. The difference in roll angle of the camera for each shot is upward of 15 degrees, which means the spacecraft must travel along a baseline of 80 km or so between shots. In practice, the pair of images might be taken on completely different orbits, which can cause problems if Mars's atmospheric conditions or frost cover on the ground have changed between shots.

Now, if stereo acuity is proportional to the interocular distance, the resolution of the camera and the size of the entrance pupil, but inversely proportion to the square of the distance to the target, it's not difficult to see that large distances will quickly wipe out any gains we can make in bigger telescope size. Recall that we use motion parallax to calculate distances to nearby stars, and this can be done quite easily for something as close as Proxima Centauri, but we use the entire diameter of Earth's orbit around the Sun as our baseline. Plus we are looking at the difference in position of the entire star against background stars, not at some surface feature of the star. Surface features can just about be resolved on some very large nearby stars.

The main problem, of course, is that waiting six months between successive images of a star (or giant gas planet) would be pointless. These are rotating bodies without solid surfaces, so you could never capture a useful stereo image. Jupiter is tens of thousands of times closer than Proxima but a quick B.O.E. calculation suggests to me that the problem is still insurmountable. You would need to be much closer to Jupiter and, as it happens, we do have motion parallax stereo images of Jupiter and Saturn taken by Voyager 2 on its fly-bys in 1979 and 1981. The Galileo and Juno missions have also taken stereo images.

There is another approach and we don't even need two cameras or any baseline. We can use Jupiter's own rotation to give us altered perspectives between shots taken some interval of time apart by a single camera. You can even do it with amateur equipment. Unfortunately, it's fake 3D, as evidenced by the fact that you recreate it even without two separate images.

Capt'n Midnight

The Earth is smoother than a billiard ball is relative to it's size so there's not much to work on. For gas giants with no solid surface even less so.



Back in the early days of radar they used some interesting tricks to map the moon when the radars couldn't focus on it. The first is that the centre of the moon is closer to us so the time delay from reflected pulses can be used to identify which "ring" you are looking at. As the moon orbits / earth rotates there's a Doppler shift in reflections too so you can tell if its the half ahead or behind and if the moon is on the horizon with half obscured then you know which half you are getting reflections from.

So even though the moon only looked like a single pixel to early radar some mapping could be done.