
China puts final touches to world's largest telescope - yexponential
http://www.aljazeera.com/news/2016/07/china-puts-final-touches-world-largest-telescope-160703080236077.html
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gjkood
Can someone explain the following to a relative layman to the science of radio
astronomy?

a) How much more can this accomplish in comparison to the well known Arecibo
Radio Telescope?

b) How is a fixed parabolic dish radio telescope different from a radio
telescope array like Karl. G. Jansky Very Large Array? What are the relative
pros and cons of one over the other?

c) How do you 'steer' the telescope to look at different parts of the sky? I
understand the dish is fixed, but the feed horns can be repositioned, but I
don't really understand the physics/math behind it, other than the focus is
changed.

I also assume there may be some massive supercomputers doing the data analysis
of the vast amounts of data collected. Any details of the back end computing
infrastructure dedicated to this effort?

~~~
amluto
> a) How much more can this accomplish in comparison to the well known Arecibo
> Radio Telescope?

It's bigger and therefore collects more light. This lets users see dimmer
targets. It could also have a different field of view, different wavelength
sensitivity, and/or different instrumentation. I don't know details of this
new telescope. FWIW, Arecibo is also a radar (it can transmit pulses and look
for reflections). I don't know whether the new telescope can do that.

> b) How is a fixed parabolic dish radio telescope different from a radio
> telescope array like Karl. G. Jansky Very Large Array? What are the relative
> pros and cons of one over the other?

Very generally, single dishes will have much nicer point spread functions, so
the images are more like camera pictures. Aperture synthesis images can have
weird artifacts. Huge dishes like this are also huge and this have more
collecting area than many small dishes combined, anthough the big arrays, in
contrast, have much, much better angular resolution.

> c) How do you 'steer' the telescope to look at different parts of the sky? I
> understand the dish is fixed, but the feed horns can be repositioned, but I
> don't really understand the physics/math behind it, other than the focus is
> changed.

Imagine a big mirror on a wall. If you stand in a different place relative to
the mirror, you see a different image in the mirror.

~~~
saboot
Could you clarify on the difference between a single radio telescope having a
better PSF, and an array having better angular resolution? What's the
difference between those two qualities?

~~~
amluto
There are a few ways to have good angular resolution with a weird PSF. Even
with just optics, you could build an instrument with a PSF that has a narrow
central peak with a big ring around it. This would be great for separately
resolving nearby objects but bad for resolving a single object against a
background of many nearby objects.

For aperture synthesis, particularly strange things happen. If you take a
quick (no rotational synthesis) exposure with a huge 3-antenna array, for
example, you only get 3 choose 2 = 6 degrees of spatial freedom, but you get
very fine angular resolution. In practice, you do "map making", in which you
try to extract specifically the parameters you care about, but if you try to
make an actual picture, you certainly can't fill in your whole field of view
with tiny pixels, since you can't get past the small number of degrees of
freedom.

My personal favorite example of aperture synthesis is the images of
Saggitarius A*. You can get incredible detail, but the "images" are made under
certain assumptions and don't represent actual individual pixels with
reasonable PSFs.

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Trombone12
Telescope without qualifier implies optical telescope, title should be changed
to clarify that it is a radio telescope.

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ourmandave
_The "Five-hundred-metre Aperture Spherical Telescope", or FAST, is the size
of 30 football fields..._

This thing will look adorable when the Square Kilometre Array comes on line in
2020 and starts pumping many Petabits of data per second.

[https://www.skatelescope.org/signal-
processing/](https://www.skatelescope.org/signal-processing/)

Oh look! They're hiring...

[https://www.skatelescope.org/people-
contacts/vacancies/](https://www.skatelescope.org/people-contacts/vacancies/)

~~~
watersb
Single-dish of this size has great sensitivity: it's a huge photon bucket. It
doesn't need a long time on-source to make an image. So it is "FAST".

A telescope array is basically a huge structure with _lots_ of holes in it. As
the Earth rotates, the elements of the array sweep out arcs. That fills in
some gaps. So even with a relatively bright source, you might need to wait a
while before you fill in enough to be able to discern the details. So a big,
single dish is "FAST".

Array elements are spread over thousands of meters. You get great _resolution_
, like a microscope on the sky. But you also get all these diffraction
patterns. Since you know the shape of your array, you can mostly solve for
this, but it's a pain in the ass.

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copperx
Someone ought to make a shouldwelearnchineseyet.com page.

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wollstonecraft
Please do not call the Trisolarians.

