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In Nature News' August 11, 2023 Closing down an icon: will Arecibo Observatory ever do science again? found in The Observatory there is discussion of the Next Generation Arecibo Telescope (NGAT):

Another, more distant hope is for the Next Generation Arecibo Telescope (NGAT) to be built at the site. After the 305-metre dish collapsed, some researchers who had used the site proposed NGAT, an instrument that would combine a 314-metre-wide platform with a swarm of 9-metre dishes on top of it.

What would a swarm of 9-meter dishes on top of a 314-metre-wide platform actually look like. Is the platform just for mechanical support of the "swarm" or is it also a dish reflector? Does this take advanage of the site's natural concavity, or is it something that could be put on flat ground anywhere, especially where the water vapor is lower?

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There are a few diagrams in the original white paper (Anish Roshi et al. 2021; see Fig. 11) and the revised NGAT-130 proposal (Anish Roshi et al. 2023; see Fig. 3), depicting the basic arrangement. Here's Fig. 11 from the first paper, showing the telescope as viewed from the top (left image) and side (right image, shown with only a few dishes installed). The plate looks more like an oval than a circle, but I believe a circular plate would be more representative of the actual design.

Top and side views of the initial NGAT design

Here's an artist's impression from a talk by some of the scientists behind the idea (see 54:30 for a broader set of designs and 55:05 for the then-final design). The blue dome isn't meant to be real, but rather to represent radio frequency interference mitigation techniques:

Artist's impression of NGAT

Some variations (like the one shown above) had the small dishes embedded in the plate, while others had them mounted firmly above it, but the basic principles are the same. Versions of this configuration are already in use on smaller scales; both AMiBA and Pluton have been using it for decades.

Both of these depict the original design of the NGAT: a 314-meter-wide plate with 1,112 9-meter dishes. However, this subsequently evolved into the NGAT-130, a 146-meter-wide plate with 102 13-meter dishes. While smaller, it would be more cost-effective and would still satisfy most of the scientific and technical requirements put forth in the white paper. It could also, potentially, be used as a precursor to a larger instrument like the original NGAT design.

One reason for the array-on-a-plate idea instead of a single dish is that it provides better sky coverage. For atmospheric studies, the NGAT would need to cover zenith angles as large as 45$^{\circ}$, which is very hard to do with a single dish, even with a deformable secondary reflector. There were also issues of limited transmitting power for radar studies. The coverage issue could be circumvented by a tight array of small dishes, but changing orientations of dishes relative to one another in such an arrangement would make it difficult to predict the radiation pattern due to interactions between antennas. It turns out that small dishes mounted on a steerable plate and oriented in the same direction allow for larger zenith angles (up to 80$^{\circ}$ with the NGAT-130!) without the pattern issues.

The NGAT site could be in a variety of places and the design doesn't necessitate that it be built at Arecibo; however, natural RFI shielding from the karst hills around the site and the fact that it is located in a special RFI coordination zone mean that it has a leg up on other options.

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    $\begingroup$ It's actually quite an interesting talk and clearly a lot of work has been put into the idea, thanks! I noticed the "omg planetary radar without Arecibo is severely limited" alarm which seems to have some urgency. It also got me thinking about something else entirely: Do high power radar astronomers try to avoid beaming power at (at least some) artificial satellites as they pass overhead? $\endgroup$
    – uhoh
    Aug 19, 2023 at 0:05
  • $\begingroup$ So in practical terms the important thing is the (trendy buzzword alert:) "swarm" of dishes on a support which would be better flat and placed on a convenient bit of desert. $\endgroup$ Aug 19, 2023 at 6:34
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    $\begingroup$ @MarkMorganLloyd I don't know that a flat design would be better; either you deal with the beam pattern problem by placing them on a tiltable plate or you spread them out like most radio interferometers, but the latter approach would decrease the ability to probe larger angular scales, which is a major advantage of single-dish designs over interferometers. (Or you could come up with another alternative, but I'm not sure any such alternatives have been developed in any detail.) $\endgroup$
    – HDE 226868
    Aug 21, 2023 at 13:15

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