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Astronomers want to plant telescopes on the Moon.
The lunar surface offers advantages for infrared and radio astronomy, despite the challenges.
By Ramin Skibba, Inside Science  |  Published: Tuesday, January 19, 2021

Astronomers want to plant telescopes on the Moon | Astronomy.comMoonTelescopes_topNteaser.jpg?mw=1000&mh

 (See the link there to a journal special issue exploring the idea.)

 I speculated about the possibility of detecting exo-civilizations optically  in this Kerbal forum post:

How large a space telescope do we need to see exo-civilizations? - Science & Spaceflight - Kerbal Space Program Forums

 In the discussion in that thread, someone suggested we would need a telescope 1.6 km across to see a visible disk of an Earth-sized planet at the nearest stars. But we might not need to be able to resolve a visible disk to be able to observe illumination of the exoplanet beyond that which would be expected on its nightside.

In any case quite large telescopes could be made on the Moon if you used rotating liquid mirrors. This proposal is for one 100 meters across:

Texans Want to Put a Big Ol' Liquid Mirror Telescope on the Moon
But can they do it? And why even try?
BY TIM CHILDERS
NOV 18, 2020
1-texasastrono-1605719898.jpg?resize=640
https://www.popularmechanics.com/space/moon-mars/a34714863/liquid-mirror-telescope-on-the-moon/

 On Earth there are limits to the size you can make a liquid scope because the rotating mirror surface and containment vessel creates wind currents that distorts the liquid mirror surface. But this would not be a problem on the airless Moon. So that raises a question: is there a limit on the size you can make such a mirror on the Moon?

 Another possibility would be to do the detection through radio telescopes on the Moon. The advantage of radio telescopes is they don't have to have a solid surface but can consist of a set of grid wires, as was done with the Arecibo telescope. And this is the approach taken for one plan for a radio telescope on the Moon:


Apr 7, 2020
Lunar Crater Radio Telescope (LCRT) on the Far-Side of the Moon.
Saptarshi Bandyopadhyay
NASA Jet Propulsion Laboratory
niac2020_bandyopadhyay.jpg
https://www.nasa.gov/directorates/spacetech/niac/2020_Phase_I_Phase_II/lunar_crater_radio_telescope/

So how big would a radio telescope have to be on the Moon to detect Earth-like radio emissions from a near-by star like Alpha Centauri? Note this is a different question than that studied for example by SETI. With SETI they assumed such a civilization was beaming radio emissions directed at us. Such searches have been negative. But in the scenario I'm considering, an advanced civilization is creating omnidirectional radio emissions just as a byproduct of conducting its advanced civilization.

 How large a radio telescope would we need to detect those?

 

     Robert Clark

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Jack was saying this years ago (late 80s, early 90s?). He's a radio guy, and radio makes far more sense than optical on the Moon.

 

(Jack Burns was mentioned in the article, back in the day he was at UNM)

 

Edited by tater
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1 hour ago, JoeSchmuckatelli said:

I like the 'low noise' image.  What's the noise picture like two weeks later? 

?

The noise in radio comes from Earth, or more specifically, US. Far side is always pointed away from pesky humans.

The issue I see with anything much past radio is if humans are spending much time on the Moon (or sending craft), there will be a fair bit of dust hurtling around unless they get way better at prepping landing sites. Look at the dust at Boca Chica and landing on concrete (and it's someplace damp). All that dust goes sideways on the Moon, and at a decent fraction of the engine exhaust velocity.

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2 hours ago, tater said:

?

The noise in radio comes from Earth, or more specifically, US. Far side is always pointed away from pesky humans.

The issue I see with anything much past radio is if humans are spending much time on the Moon (or sending craft), there will be a fair bit of dust hurtling around unless they get way better at prepping landing sites. Look at the dust at Boca Chica and landing on concrete (and it's someplace damp). All that dust goes sideways on the Moon, and at a decent fraction of the engine exhaust velocity.

So pointing straight at the sun isn't noisy? 

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1 minute ago, JoeSchmuckatelli said:

So pointing straight at the sun isn't noisy? 

Well, they'd not point straight at the sun. The radio telescope not far from where (the VLA) I am observes when it's day ;)

The crater type dishes can still point by aiming the feed, and it would be rare for the optical axis to point directly at the sun, anyway.

Way back when (late 80s as I recall), Jack was usually talking about radio interferometry on the Moon (when you do loads of work at the VLA, that's likely right at the top of the toolbox).

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4 hours ago, mikegarrison said:

How long does it take between planting and harvest?

 I get your irony, but with the simplicity of both a liquid mirror optical telescope or grid-wire Arecibo-type radio telescope they might be something constructible by self-assembly.

 

  Robert Clark

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8 minutes ago, Exoscientist said:

 I get your irony, but with the simplicity of both a liquid mirror optical telescope or grid-wire Arecibo-type radio telescope they might be something constructible by self-assembly.

 

  Robert Clark

I tried to imagine a single-ship solution to this, and could not.  Even if you had something large, with 6 rovers that pulled guide wires out from a central lander... with the webbing being deployed by bots that crawled out along the guide wires; with our current lift capacity, I'm not sure we'd get much coverage.

 

Probably better to go for an array-style radio telescope; then you can land a bunch of relatively cheap stuff all over the place.

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While the crater type is fairly easy assembly, it seems like interferometry might be even easier. Land a rover and stand alone telescopes that unfold, and place them.

 

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On 2/19/2021 at 10:42 AM, JoeSchmuckatelli said:

Probably better to go for an array-style radio telescope; then you can land a bunch of relatively cheap stuff all over the place.

On 2/19/2021 at 10:43 AM, tater said:

While the crater type is fairly easy assembly, it seems like interferometry might be even easier. Land a rover and stand alone telescopes that unfold, and place them.

ALMA employs moveable 12 m radio telescopes that each weigh ~115 tonnes. VLA employs 25 m ones that are at least as heavy. None of these would even fit on Starship or New Glenn in one piece.

I'd say throwing a net over a suitable crater is still cheaper. Also gives you a larger maximum photon gathering power. It doesn't really point anywhere much though, that's the only problem (and exacerbated by Moon's slow rotation - 29 days rather than 24 hrs), plus the receiver/collector in the middle might have to be quite beefy if you want transmission capabilities as well. Although having arrays is indeed the next step, hopefully also optical arrays.

Edited by YNM
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6 hours ago, YNM said:

ALMA employs moveable 12 m radio telescopes that each weigh ~115 tonnes. VLA employs 25 m ones that are at least as large. None of these would even fit on Starship or New Glenn in one piece.

Remember the telescopes here need to sit outside. NM weather is mostly sunny, but when storms come by, it sorta looks like a H-bomb going off—dark cloud with a solid shaft coming down, lashing the surface, and with very high winds. On the Moon an antenna can be very minimalist.

 

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3 hours ago, tater said:

Remember the telescopes here need to sit outside. NM weather is mostly sunny, but when storms come by, it sorta looks like a H-bomb going off—dark cloud with a solid shaft coming down, lashing the surface, and with very high winds. On the Moon an antenna can be very minimalist.

 

That is kind of what I was thinking - get SpaceX to carpet bomb a field of repurposed Starlinks withw big antennas across the surface and violla! 

 

Edit - OK, they might need some kind of landing attachment, but you get the picture 

Edited by JoeSchmuckatelli
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13 hours ago, tater said:

On the Moon an antenna can be very minimalist.

You need machinery to move them regardless. Dish structure can be made 'thinner', probably due to lower loads; but the mechanisms, if anything, I'm sure you need more dampers due to lighter mass. Unless you don't want the dishes to be moveable, in which case you're basically doing the same as the static crater thing (in which it loses squarely). We haven't quite done anything like a foldable 12 m or 25 m dish either, maybe those with nets on but that limits your minimum wavelength. Closest we've got on foldable solid thing is JWST and we haven't launched that...

9 hours ago, JoeSchmuckatelli said:

Starlinks with big antennas

How big are you thinking ?

Edited by YNM
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1 minute ago, YNM said:

How big are you thinking

I'm no expert - but given the size, I'd think you could pack something that could spread out from a minimum of 1m to perhaps 3m.  From what I understand of interferometry, several small antennae can be digitally 'stitched' together to form a single big antenna.

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3 hours ago, JoeSchmuckatelli said:

1m to perhaps 3m

That's *very* small. VLA with their 25 m dishes observes wavelengths at .6 to 410 cm, ALMA with their 7 m and 12 m dishes observes .032 to .36 cm. If you want to do the same with such a small dish you'd have terrible resolution and SNR, and correcting that means lots and lots of telescopes (maybe in the million range) and long integration times.

Maybe as a pilot project yes, but for the penultimate mission no.

Edited by YNM
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11 minutes ago, YNM said:

That's *very* small. VLA with their 25 m dishes observes wavelengths at .6 to 410 cm, ALMA with their 7 m and 12 m dishes observes .032 to .36 cm. If you want to do the same with such a small dish you'd have terrible resolution and SNR.

Maybe as a pilot project yes, but for the penultimate mission no.

I kinda figured that out, as soon as you asked the question!  Still - the idea of building a crater antenna given our current tech is really unlikely; interferometry, perhaps better thought out than I put it, sounds like a more successful plan in the short run

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They could do dipole arrays, or even tripole antenna arrays (tripole is a triple dipole, 3 dipoles arranged orthogonally) for ultra long wavelength work (impossible on Earth).

LWA at the VLA is a dipole array telescope:

lwa1.jpg

Obviously what they can build determines what sort of work they can do.

ULW astronomy would be a new branch, since it simply can't be done on Earth, and way back in the day Jack was talking about submilimeter as well as a good candidate. Ideally of course you'd do many different types, including a large single dish. The VLA gives good angular resolution, but it's also a narrow FOV. Usually you'd want the detail, and the big picture.

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Boston Dynamics should send a hundred(s) of robo-dogs.

They should spread around the crater with structural cables in teeth and raise a tent as a dish. A tentenna.

Then by ducking and standing they should adjust its shape and sleep.

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22 minutes ago, kerbiloid said:

Boston Dynamics should send a hundred(s) of robo-dogs.

Train them to think the telescope elements are bones, and they will bury them with no other training.

(of course they will also find dead/crashed spacecraft and roll in the remains)

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3 hours ago, JoeSchmuckatelli said:

Still - the idea of building a crater antenna given our current tech is really unlikely;

What do you call FAST and Arecibo, then ? I'm not talking about building a crater dish over those craters that are visible by telescope from the Earth.

2 hours ago, tater said:

ULW astronomy would be a new branch, since it simply can't be done on Earth, and way back in the day Jack was talking about submilimeter as well as a good candidate.

Yep, for very long wavelengths the atmosphere isn't a good thing (not to mention all the noises from the Earth phenomenons), and since you need massive structures doing them free-standing in space is a challenge. Stretching a cable over the surface of the Moon should be easier...

Edited by YNM
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5 hours ago, YNM said:

What do you call FAST and Arecibo, then ? I'm not talking about building a crater dish over those craters that are visible by telescope from the Earth.

Sorry - I wrote that in relation to my earlier post.  We can certainly build them here - but to build a crater antenna on the Moon?  

 

You would need a fleet of big ships. 

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