Why not make it out of something that *will* freeze...
Then you can heat it (solar power), spin it, let it freeze and have a pointable 'scope...
Astronomers have revived a decade-old idea that was abandoned by NASA: a 100-metre-wide liquid-mirror telescope on the Moon to study the stars. Specifically, the academics, based at the University of Texas at Austin, propose erecting the instrument – dubbed the Ultimately Large Telescope (ULT) – at a lunar pole to collect …
Inconsistent viscosity might cause imperfections in a frozen surface which may not be as big an issue when it's a liquid in motion. Presuming that the "thicker" parts of the liquid would be held against the outside edge by the spin.
(I'm not an expert by any means, that is just my guess and could very well be wrong)
There are probably huge engineering advantages in the mirror being liquid, but the big problem is you'll have to point the whole moon towards whatever you'd like to look at... Or resign yourself to only studying whatever your telescope accidentally point to, and only for as long as it does.
No specialist, but that seems like a huge limitation to me, especially for the kind of money required to build and run it.
You are kind of right, but a few things:
First you are wrong about the bit with "and only for as long as it does" in the way that being at one of the poles "as long as it does" is forever. Doesn't mute your point though. However it can change direction a bit by moving the detector.
Second, with what they are trying to find out with, it doesn't really matter which way they point it. It has a specific goal. So, yes it is limited, but only to the degree one can say that the Mars rovers also are limited by being confined to a small area on a single planet.
> being at one of the poles "as long as it does" is forever
The moon is orbiting earth which is orbiting the sun, so even if the moon's own rotation is neutralized (not really, objects will either move in small circles or rotate), the sky will still move a lot over the telescope.
> what they are trying to find out with, it doesn't really matter which way they point it
You have a point there.
Puzzled as to how they can achieve a stable mirror with a liquid. Does this behave differently to water that has a flat surface?
A telescope will need a shaped (usually some sort of parabolic form) to focus light on a secondary mirror or a sensor. Can the do clever things with a meniscus?
It will be interesting trying to test it given the gravity difference.
No, it will not move by a lot. The rotational axis of any object is basically stable no matter how many orbits it is part of. Yes, the moon will of course change position during a year by around 300 million kilometres, but the axis will point the same way. When you look to the edge of observable space I don't think that 300 million kilometres would blur the image by much. You are right in that it will rotate though, or move in small circles if it points a little bit off centre. However the moon rotates very slowly.
> However the moon rotates very slowly.
Still, the moon's orbit is not circular, it's axis of rotation is slightly inclined to the plane of its orbit, and the plane of its orbit moves too. They're all small variations, over a period, but I guess they would create issues since absolute precision would be required.
I admit I don't know how much exactly, but Murphy's Law suggests it will be a problem.
Nah, we'll work up from that to the Bloody Enormous Telescope (BET), The Fuck Me That's Seriously Big Telescope (FMTSBT), and on through several more, probably including the Is That A Telescope In Your Space Suit Or Are You Very Pleased Indeed To See Me Telescope (ITATIYSSOAYVPTSMT).
Normally I don’t bother reading the links to sources provided as I’ve come to trust the quality of the reporting here, but today being quiet and having an abundance of spare time I followed the link to the published paper, I read it, twice.
But understood very little of it.
Thanks for summing up and “translating” an interesting idea that mere mortals can understand.
Bravo & have a pint!
I don't get why there are no more missions toward the Moon. Being (relatively) close, it is 'easy' and fast to send a rocket to there. Why is there no mission to test automatic rovers with advanced capacities, to discover what the lava tunnels and caves look like, to experiment 3D printing to built housings, to install telescopes (traditional ones)... Shouldn't be the Moon an experiment field before Mars?
AFAIK, the Moon is closer to Mars environment than the Earth is. For instance, Earth is protected from solar and space radiations, Moon is not, Mars just a few. Earth conditions make cooling easy, it's much more difficult on the Moon with no atmosphere or on Mars with its very tiny one. If something works on the Moon, it will work on Mars.
If the final goal is to colonize Mars, let's start with the Moon. Experiment rovers able to build shelters, or to dig and extract water, let's be more innovative! It's just a 3 days trip, not a 9 months every other year.
Whilst in terms of time it is certainly faster to get to the moon - Mars is actually closer in terms of dV (assuming you use aerobraking)
If/when Starship is flying then we might well do some moon "jaunts" for practice, and with the (frankly absurd) load capacity of the starship (when refuelled in orbit it'll take 150t basically anywhere in the solar system) it will be relatively easy to send out an array of 8m 'diameter' hexagonal mirrors to be assembled on the moon. Not having to put in adaptive optics will be a major benefit in terms of reduced complexity, and the rate at which the starfield slews across the moon's sky is much lower than on the earth as well.
Why don't we railgun all of Earth's shit to terrform the moon? We'd have an abundance of shit, which when added with potatoes and mined moon water, would create an atmosphere we could protect using a magnetic field. We should set it up as a penal colonoscopy I mean colony, until such time the Moon isn't so shitty.
These days, large optical telescope mirrors are made by assembling smaller hexagonal mirrors in an array. I'd have thought doing something like that in space would be far more practical than spinning a large blob of liquid salt.
The surface profile of a telescope mirror has to be accurate to 1/10th the wavelength of light that you're observing. I'm sure they've looked into the practicality of achieving that, and I presume the Moon's gravity would naturally yield the desired shape (something like a parabola). But how would you steer such a thing to look anywhere but straight up?
The spinning fluid can be on top of the nearly formed shape and with the right fluid can be far more accurate than any solid mirror. A structure that approximates the parabola when spun seems to me something that can be constructed with relative ease and a thin layer of the fluid floating on top will provide as near perfect a mirror as you can get. As for what you are looking at I'd imagine the secondary mirror shown in the image would be replaced with a movable 'receiver' that can be moved around to provide a few tens of degrees of potential targets. That's a lot of potential targets.
Spinning liquid mirrors are a thing that is done on Earth already. They are way cheaper than real mirrors and so are mostly built by small universities - you just have to live with the limitations in pointing it.
If you are a national lab building a $bn observatory then the cost of a real 8-10m mirror is a small part of the bill. But if you want to build something on the moon, where the cost of sending onsite engineers to align things is a problem then this isn't quite so daft.
Obviously the whole thing is silly season - but it gets a couple of unknown researchers on the front page of a slow news day.
I don't have an opinion about this particular proposal, but it's clear that optical astronomy on Earth is pretty much doomed. In the next decade or two we'll have about fifty thousand small comsats of the Starlink type in low earth orbit, making almost any kind of optical astronomy a lost cause. It doesn't matter how much antireflective coating they have; that many birds flying around will completely mess up visual observation. They may even make radio astronomy impossible.
That means any and all plans for Earth-based giant telescopes ought to be scrapped now, before any more money is wasted. The only practical places to do optical astronomy will be in orbit (Earth or Lagrange), or on the Moon. Each has its advantages and disadvantages, but the Moon offers a ready source of materials, so it's probably the best bet in the long run.
> The only practical places to do optical astronomy will be in orbit (Earth or Lagrange), or on the Moon
Check the costs of building and running year after year some bigger observatory complex like the Mauna Kea Observatories in Hawaii ( 12+ telescopes, most with laser guided adaptive optics which compensate atmospheric turbulence). Also consider there is a road up there, and accommodations for visiting astronomers (big building). And that's just one observatory complex, there are lots of others (several in the Andes).
Now ponder the cost of abandoning all those perfectly working telescopes, many billions in investment, and trying to rebuild them either in orbit or on the moon (sure, will happen)... Just so some already rich guy can make a quick buck...
> They're suggesting not building any *more*, not abandoning those already operating.
And how do you suggest to keep using those telescopes when the sky is literally covered with sheets of satellites?
Imagine trying to record something as quiet as crickets while a marching band walks by: Yes, of course you can edit the recording and only keep the sound recorded between the band's notes, and after painstakingly filtering out various echos and reverberations, you will certainly end up with a consistent and useful recording of a cricket's song. - Not.
The satellites are only visible when they are lit up by the sun - so for a LEO object only a problem near sunset/sunrise which is useless for most astronomy anyway.
In the middle of the night having dark objects cross the image doesn't make any difference.
A much bigger issue is the moon lighting up the sky for a week..
Now if somebody could arrange to remove that - or at least paint it black - that would really help
"And how do you suggest to keep using those telescopes when the sky is literally covered with sheets of satellites?"
No so long ago, earthbound optical astronomy was thought to be at it's limit due to perturbations in the atmosphere and space telescopes were the only possible future. Then some bright spark, using clever engineering, servos, feedback and massive computational power found they could create earthbound telescope to match or exceed anything we could launch into space. Clearly, someone will do the same to exclude passing objects rendering them "invisible" to the telescopes (assuming they've not already done so at least at a preliminary level). The technique is simple. Automating it and doing it in real time may just need better algorithms and more CPU/GPU. After all, with my first 640x480 digital camera and a tripod, I took multiple images of a road, layered them in GIMP and "scraped through" the bits with cars on till I found tarmac n layers down to end up with a photo of an empty road. Shortly afterwards, I found there was software capable of doing that for me.
> Clearly, someone will do the same
There is a fundamental difference: In the first case all the information is there, it's just scrambled. In the second case, part of the information was blocked, and thus irremediably lost. yes, you could interpolate and guesstimate, but science requires facts.
Now I obviously don't know how much this will affect ongoing science projects, but given the numbers of satellites announced and the fact there will be lots of copycats sending up additional constellations, I'm sure it won't be just a minor annoyance.
I dimly recall a science-fiction story from (I think) the late '50s, perhaps by Arthur C. Clarke, in which a large reflecting telescope is constructed on the moon by filling a large, shallow cylindrical container (like a cake pan) with mercury. The metal is maintained at a temperature above its melting-point while the container is rotated. When it is spinning steadily, the surface of the liquid is made concave by centrifugal force. The apparatus is allowed to cool, so the metal solidifies to form the telescope's objective.
I always wondered whether this would work. Would the concave surface be spherical? If so, there are a good many variations of the Cassegrainian telescope design that place a small secondary lens and/or mirror group on the optical axis to correct spherical aberration.
It had not occurred to me that changing the mercury's phase might alter the geometry of the mirror.
Answering those questions should be straightforward enough. If the answers were affirmative, I think the project would be worth doing. If you did it on one of the lunar poles, a simple altazimuth mount would let you point the instrument at anything in about half of the sky. The task of designing a structure capable of accommodating a 330-foot-diameter mirror and the necessary optics and electronics should be simplified by the much lower gravity of the moon, and the absence of weather. Rotating and tilting it might be about as difficult as aiming the main battery of a World War II battleship.
Has anyone looked for sources of mercury on the moon?
Thanks to physics a spinning liquid naturally forms a parabolic surface.
We already make mirrors like this, currently by melting glass then spinning it while it cools. You still need to polish the surface to make a perfect fraction of a wavelength smoothness, but it's a lot less effort than grinding from a solid block.
Biggest issue with a mirror made from a solid block of mercury is that it would change shape as the temperature changed. The spinning mirrors are made hollow so there is very little mass of glass so it comes to temperature very quickly and the glass is special super-low expansion stuff.
> simplified by the much lower gravity of the moon, and the absence of weather
The big problem is dust. Lunar dust is very aggressive and clingy, and IIRC it flies around on its own during daytime due to electrostatic effects.
A solid mirror would need a means to get rid of it without scratching the surface (so no brush or similar). On the liquid mirror the dust is supposed to simply sink in, although I'm not sure how that would work with the high surface tension of mercury.
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