6 ms·

Very sad that its next phase is not as a bigger, better observatory. RIP.

oh_sigh 3y ago
Do these massive telescopes have a future, when we could just build smaller ones and use aperture synthesis to pretend like we have a really really big telescope?
- madengr 3y ago
  It had a powerful transmitter for radar work, and there are advantages to having a single, giant aperture when it comes to maintaining low sidelobes, power amplifier coherency, etc.
- mandevil 3y ago
  For receive only, yes, aperture synthesis (like the VLA and the GVLBI) is capable of resolving more precisely than even Arecibo could. But Arecibo also did a bunch of active radar mapping of things within the solar system, which aperture synthesis cannot replace. You can't solve the coherent beam problem without actually having an antenna that really is that big.
  - echelon 3y ago
    > You can't solve the coherent beam problem without actually having an antenna that really is that big. What's the tl;dr on the math here?
    - madengr 3y ago
      You know how Nyquist says you must sample at minimum twice your highest "frequency" or your get aliasing in your spectrum? It's the same with antennas except you must sample at least 1/2 your wavelength or you get grating lobes (the equivalent of aliasing) in you far field pattern; it's all the same math. A sample in time is equivalent to an antenna element sampling space. A big reflector is equivalent to sampling everything at once with no gaps; it's bandwidth independent. You can only put array element so close together, limited by the size of the element (itself is limited to a fraction of the lowest wavelength), electronics, and unwanted inter-element coupling (though there are tricks to take advantage of this coupling and make very wideband arrays). If you leave "gaps" in your array elements, that's not a problem receiving strong, periodic signals. You can physically move the elements to fill in the gaps to remove those lobes; the equivalent of an equivalent time oscilloscope or a synthetic aperture radar. The problem is you can't do that when transmitting since you only have one opportunity to send something (non-periodic). There are other disadvantages with phased arrays. Each amplifier has to be matched in amplitude and phase. I've had to bin lots of amplifiers as some have phase compression and some phase expansion over amplitude. That distorts the pattern over the pulse. On the receive side, each receiver has thermal noise un-correlated with the others. So for N antenna elements, you received signal-to-noise scale proportional to sqrt(N) instead of N like you have on transmit, or with a big reflector. This is why arrays suck for very weak signals. Notice NASA keeps building big dishes for their DSN instead of "cheaper" arrays. Then there's the whole timing and phase noise issue with distributing an oscillator. Tough you can only make a reflector with about 100 dB gain.
  - markhahn 3y ago
    huh? wouldn't all the planes with AESA radar dispute this characterization?
    - mcpackieh 3y ago
      https://en.wikipedia.org/wiki/Thinned-array_curse https://en.wikipedia.org/wiki/Thinned-array_curse > The thinned-array curse (sometimes, sparse-array curse) is a theorem in electromagnetic theory of antennas. It states that a transmitting antenna which is synthesized from a coherent phased array of smaller antenna apertures that are spaced apart will have a smaller minimum beam spot size, but the amount of power that is beamed into this main lobe is reduced by an exactly proportional amount, so that the total power density in the beam is constant So you can transmit with a sparse array, but the power you can deliver with it (for a set synthetic aperture size) decreases with the degree of sparseness. No free lunch.
- adgjlsfhk1 3y ago
  massive telescopes still win in terms of light sensitivity per dollar. apature synthesis/VLBI are the clear way to go for angular resolution but for bigger fainter objects, you care more about area where a big dish does better.
SoftTalker 3y ago
Looking at the photos, the dish itself looks relatively thin. With modern composite materials, I bet it could be rebuilt much lighter than before and not need such substantial support. Still, it would need to be paid for so....

Do these massive telescopes have a future, when we could just build smaller ones and use aperture synthesis to pretend like we have a really really big telescope?

madengr 3y ago
It had a powerful transmitter for radar work, and there are advantages to having a single, giant aperture when it comes to maintaining low sidelobes, power amplifier coherency, etc.
mandevil 3y ago
For receive only, yes, aperture synthesis (like the VLA and the GVLBI) is capable of resolving more precisely than even Arecibo could. But Arecibo also did a bunch of active radar mapping of things within the solar system, which aperture synthesis cannot replace. You can't solve the coherent beam problem without actually having an antenna that really is that big.
- echelon 3y ago
  > You can't solve the coherent beam problem without actually having an antenna that really is that big. What's the tl;dr on the math here?
  - madengr 3y ago
    You know how Nyquist says you must sample at minimum twice your highest "frequency" or your get aliasing in your spectrum? It's the same with antennas except you must sample at least 1/2 your wavelength or you get grating lobes (the equivalent of aliasing) in you far field pattern; it's all the same math. A sample in time is equivalent to an antenna element sampling space. A big reflector is equivalent to sampling everything at once with no gaps; it's bandwidth independent. You can only put array element so close together, limited by the size of the element (itself is limited to a fraction of the lowest wavelength), electronics, and unwanted inter-element coupling (though there are tricks to take advantage of this coupling and make very wideband arrays). If you leave "gaps" in your array elements, that's not a problem receiving strong, periodic signals. You can physically move the elements to fill in the gaps to remove those lobes; the equivalent of an equivalent time oscilloscope or a synthetic aperture radar. The problem is you can't do that when transmitting since you only have one opportunity to send something (non-periodic). There are other disadvantages with phased arrays. Each amplifier has to be matched in amplitude and phase. I've had to bin lots of amplifiers as some have phase compression and some phase expansion over amplitude. That distorts the pattern over the pulse. On the receive side, each receiver has thermal noise un-correlated with the others. So for N antenna elements, you received signal-to-noise scale proportional to sqrt(N) instead of N like you have on transmit, or with a big reflector. This is why arrays suck for very weak signals. Notice NASA keeps building big dishes for their DSN instead of "cheaper" arrays. Then there's the whole timing and phase noise issue with distributing an oscillator. Tough you can only make a reflector with about 100 dB gain.
- markhahn 3y ago
  huh? wouldn't all the planes with AESA radar dispute this characterization?
  - mcpackieh 3y ago
    https://en.wikipedia.org/wiki/Thinned-array_curse https://en.wikipedia.org/wiki/Thinned-array_curse > The thinned-array curse (sometimes, sparse-array curse) is a theorem in electromagnetic theory of antennas. It states that a transmitting antenna which is synthesized from a coherent phased array of smaller antenna apertures that are spaced apart will have a smaller minimum beam spot size, but the amount of power that is beamed into this main lobe is reduced by an exactly proportional amount, so that the total power density in the beam is constant So you can transmit with a sparse array, but the power you can deliver with it (for a set synthetic aperture size) decreases with the degree of sparseness. No free lunch.
adgjlsfhk1 3y ago
massive telescopes still win in terms of light sensitivity per dollar. apature synthesis/VLBI are the clear way to go for angular resolution but for bigger fainter objects, you care more about area where a big dish does better.

SoftTalker 3y ago

Looking at the photos, the dish itself looks relatively thin. With modern composite materials, I bet it could be rebuilt much lighter than before and not need such substantial support. Still, it would need to be paid for so....