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In a story of mine mankind has established a permanent base on the earthward-side of the Moon complete with a beanstalk dangling into Earth-Luna-L1 for easy transport of material off Luna.

Getting off earth is simply a task of getting into LEO at which point craft from Luna take over crew and materials. Returning things to earth is a simple task of slowing containers/ships down sufficiently so they reenter orbit.


After finishing the moon-side constructions for the beanstalk (digging and building foundations, etc...), the construction equipment and teams haven been relocated to start building the Lunar-Radiotelescope (LunaR) similar to the ARECIBO or the FAST.

It can be assumed that active preparations and planning for this massive Lunar-Infrastructure-Project (LIP for short) have been going on for as far back as ~2022.


Considering the limiting factor for this operation being the materials and things that will have to be built on earth and moved up to moon:

What would be the best location to build such a telescope on moon?

Bonus: How long would it approximately take?

The question is asking for a location, not just a vague idea. E.g. the name of a lunar crater or lunar coordinates

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  • $\begingroup$ Why do they want to build a radio telescope on the Moon? The sheer size of a radio telescope - let alone a series of structures needed for radio interferometry - mean that it can be a lot harder to build that an optical telescope, in terms of transporting materials. I can't see the advantage in building on one the Moon, given all the complications that would arise. $\endgroup$ – HDE 226868 May 21 '16 at 15:44
  • $\begingroup$ @HDE226868 well then lets make it an optical. I went with the assumption that a radiotelescope makes more sense as these seem to be of more use for 'looking further' $\endgroup$ – dot_Sp0T May 21 '16 at 18:43
  • $\begingroup$ Given that humans are now cruising around near earth space (and possibly the rest of the solar system) with some regularity, is the purpose of this telescope communication with human vessels/probes in the solar system, detection of human vessels/probes in the solar system (like an air traffic control radar), or is it for SETI or deep space observation. Related question, does it have to be on the moon, why shouldn't it be built in space? $\endgroup$ – kingledion Oct 6 '16 at 11:36
  • $\begingroup$ @kingledion the purpose is mainly for observation, the idea has been that on the dark side of the moon, there will be less interference from earth that has to be filtered out. It is built on the moon because the assembly of such a thing in space without any body to work with has been judged too difficult and overly expensive $\endgroup$ – dot_Sp0T Oct 6 '16 at 12:17
  • $\begingroup$ How does the structure at L1 (200,000 miles away) help crafts in LEO (a few hundred miles up)? $\endgroup$ – JDługosz Oct 7 '16 at 10:39
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Daedalus Crater has been suggested for this purpose.

enter image description here

Located at 5.9°S 179.4°E

https://en.wikipedia.org/wiki/Daedalus_(crater)

Why is it a good choice?

Location

It is near the middle of the far side, furthest from Earth's radio emissions with the bulk of the Moon between it and any terrestrial radio sources. This is the primary benefit of placing a radio telescope on the Moon, the bulk of the Moon's mass would block the majority of Earth's radio output. Placing it just over the horizon or at one of the lunar poles would not block all of the radio waves. The distance the radiation penetrates is based on the Attenuation length and the wavelength of the radiation. This location maximizes the mass of the moon between Earth, offering the most shielding possible.

As an example low frequency AM Radio has a wave length in the 10 kilometer range, so if you want to block 99% of the radiation (for a telescope you would want to block much more) you would need about 5× the wavelength of material or >50 km of material. There are also longer wave frequencies in use on Earth, so more distance of material is better.

Size and Shape

In terms of size it is one of the larger craters on the moon. It is ~93 km in diameter with some central mountains for focal point instrumentation. for comparison Arecibo is only 305 meters, so Daedalus would be about 300× larger. It is also fairly circular and symmetrical (useful for shaping into a spherical reflector) for craters much larger than this the craters stop having the bowl shape needed for a reflector due to the curvature of the surface. Bigger is usually better for a telescope as it allows it to capture and focus from a larger area giving it better resolution. Another feature it has is a central mountain allowing the receiving instruments to be placed without the need for large support towers and very long wire supports or other large support structures, this helps a lot in terms of lowering the construction difficulty.

Shaping it into a better reflector shape would definitely be a major project, somewhat comparable to large open pit mining done on Earth. The largest man made excavation on Earth is only 4 km wide and 1 km deep and has been in production for more than 100 years, so this would be bigger but should only require minimal material removal given the existing crater shape (It's hard to say minimal with a straight face for an amount most likely measured in km³).

Estimates

Likely unifying the central mountain system and siting the instruments would be done first, with sculpting the crater walls into a better reflector shape continuing to improve the telescopes resolution as the project progresses. For improved performance they would also likely want to line the crater with an improved reflector material (Arecibo started with a wire mesh and later upgraded to Aluminum panels).

Making any accurate time estimate for a project this big is going to be impossible without knowing the capabilities of the Lunar construction apparatus, but it could easily be a decades long project just to construct the first instruments with refinements continuing for the foreseeable future, with each upgrade improving the range and resolution of the telescope.

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I imagine it must be built where the Earth is not visible, to eliminate the bulk of manmade noise and interference. Just over the horizon might be sufficient. But if the moon proves to have an ionized atmosphere then more distance will be required.

I expect the half of Luna not visible from Earth will continue to be referred to as the 'dark side' even though it is as illuminated as the visible face. In this context 'dark' refers to the blanking of direct radio communication from Earth.

A high speed railroad should be constructed from the base of the stalk to the LunaR site. This will greatly ease construction, especially in the early stages when the construction site does not yet contribute to its own sustainability and all consumables have to be trucked in.

Unfortunately the railroad will attract other research groups who want to work on the dark side, so a wise administration will build spur lines to keep their settlements away from the LunaR.

One thing to note is that a large ARECIBO style antenna on Earth sweeps the sky at about 360 deg. per day, while a similar construction on Luna will be approx. 28 times slower. OTOH given the technology that built the beanstalk coupled with Luna's famous low gravity, you can expect to build an array of truly enormous steerable dishes.

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  • $\begingroup$ "you can expect to build an array of truly enormous steerable dishes" Not necessarily, though. Compare for example the Very Large Array. $\endgroup$ – a CVn May 20 '16 at 18:25
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    $\begingroup$ @Michael Kjörling: I don't understand this remark at all. Does the existence of a Very Large Array in New Mexico contraindicate the possibility of building a Very Large Array in Rozhdestvenskiy crater? $\endgroup$ – Slartibartfast May 20 '16 at 22:26
  • $\begingroup$ No; quite the opposite. You don't need "truly enormous steerable dishes" to get good sensitivity, you just build an array of smaller dishes, just like the VLA. Building multiple mid-sized antennas is a lot easier than building a single (or a few) gigantic ones. $\endgroup$ – a CVn May 21 '16 at 14:54
  • $\begingroup$ I am no expert in the VLA, but it seems to me like having one large dish would allow you to capture more of the signal, but the large synthetic aperture of the VLA permits better rejection of noise. Thus it seems like large dishes (taking advantage of the low gravity) would be desirable if the noise limits from the receivers are more of an issue than rejecting noise from elsewhere. At least that's what my intuition says -- feel free to correct me with facts =) $\endgroup$ – Cort Ammon Oct 6 '16 at 22:09
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If you put it at one of the poles, you can erect a solar array to keep it powered all the time. If you "find" a sufficiently-sized crater at one of the poles to put it in, or a ridge to hide it behind, you can curtain off most of the radio interference from Earth.

As for time to construct, it's kind of up to you. What's the level of automated construction in your world? Can you send automated or tele-operated bulldozers to carve out the footing trenches? Can waldoes construct the main dish body for you, operating 24x7 until its done? Also, large construction projects like this take a lot of lead time for planning, design, resource deployment, etc., so consider writing hooks into your story a couple of years ahead of groundbreaking that describes project initiation, site selection, etc.

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    $\begingroup$ Thank you for highlighting the planning subject. I'll add a paragraph saying that construction has been planned &prepared with the beanstalk project $\endgroup$ – dot_Sp0T May 20 '16 at 17:18
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How about De Gerlache crater? it's in perpetual darkness so can't be observed from Earth, is near the south pole so can have solar power arrays nearby.

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