Telescope with a Mercury Mirror - Sixty Symbols

Professor Meghan Gray discusses the 4m ​International ​Liquid Mirror ​Telescope in India - and does a demo. More links below ↓ ↓ ↓
More at: www.aries.res.in/facilities/a...
Or: www.ilmt.ulg.ac.be/home/
Professor Gray at the University of Nottingham: www.nottingham.ac.uk/physics/...
With thanks to Paul Munday for setting up the demo.
More videos with Professor Gray: • Meghan Gray - Sixty Sy...
Brady's telescope tours on Deep Sky Videos: • Telescope Tours
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This project features scientists from The University of Nottingham
bit.ly/NottsPhysics
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Sixty Symbols videos by Brady Haran
Additional filming and editing by James Hennessy
www.bradyharanblog.com
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[@sixtysymbols]

More videos with Professor Gray: kzbin.info/aero/PLcUY9vudNKBORQN625NgO3c4T82M6w2st Brady's telescope tours on Deep Sky Videos: kzbin.info/aero/PLFDDC58C2516AE284

[@phonkey]

Brady didn't have to do anything in this video. The professor provided both the questions and the answers. 😅

[@ryanjohnson4565]

She’s an excellent self-interviewer.

[@MichaelEdelman1954]

According to Peter Manly’s 1991 book, “Unusual Telescopes,” physicist R.W. Wood built a 50cm rotating mercury telescope in 1908!

[@KennethAGrimm]

My parents, both of whom had taken classes from Professor Wood at Johns Hopkins University, inherited an autographed copy of Wood's autobiography from my grandfather. As a preteen in the 50's, I read this through several times. I was particularly fascinated by his description of the mercury telescope which he invented and built in the hand-dug water well of the rural farm which the university owned and used as the astronomical and physics laboratory annex. The mercury telescope had a 36" mirror bowl; the primary challenge was the vibration-free rotation drive. The farm location of the telescope is now the site of the Johns Hopkins Applied Physics Laboratory, a classified government contract facility.

[@francoislacombe9071]

Fun fact. Today, when making large glass mirrors, the molds are often spun while the glass is melted and allowed to slowly cool back into a solid. This puts an initial parabolic curve on the mirror that makes it much easier to polish to the desired high accuracy optical surface.

[@markholm7050]

So far as I am aware, only one mirror production facility, the Mirror Lab at the University of Arizona, uses this technique. Under the direction of Roger Angel, this lab developed the technique and has used it for several large mirrors over the last three decades. Most recently, they have been making the primary mirrors for the Large Magellan Telescope which is under construction in Chile.

[@red.aries1444]

@@markholm7050 The Giant Magellan Telescope with its seven round mirrors might be the last big telescope which make use of combining several parabolic mirrors together. Most other newly build or planned telescopes are composed from hexagons, like the JWT or the Extremely Large Telescope. So spinning the whole mold together with the melted glass isn't necessary anymore.

[@soberhippie]

Is it possible to keep molten glass on top of molten metal while spinning the two, just like they do to make normal window glass (except for the spinning, of course), to produce thin concave mirrors?

[@jasonklassen3972]

​@@soberhippieI'm sure it's possible, but you would have the problem of the two different densities of material creating two different parabolic shapes. The molten metal would have its own parabola and impart it to the bottom of the glass. The top of the glass would follow a different parabola and you would have all sorts of diffraction issues through the glass

[@babilon6097]

Yes. There was a video on DeepSkyVideos on this topic. Maybe SixtySymbols could get some collaboration going with them?

[@wati52]

I could listen to Meghan for hours, she is a great communicator.

[@JavSusLar]

The main problem of mercury that ionic liquids solve is not melting point, but vapour pressure. Mercury would quickly evaporate on the moon, but the effect of vacuum on ionic liquids is negligible.

[@fonkbadonk5370]

Naming a telescope "ultimate" is a bad idea. We couldn't ever make a larger one after that!

[@dan110024]

Nah we still have the pro and max keywords to combine. Plenty of iterations.

[@stan-bi3hl]

Ultimaterererer. Next!

[@maxtroy]

They called King Charles the “ultimate” royal on the radio here in the Uk yesterday. I had the same to thought!😊

[@user-tw7vl7kt9e]

Labels are limits indeed.

[@tim57243]

​@@stan-bi3hlThere is a game Super Hexagon that has levels with difficulties of harderest, hardererester, etc. I don't know if the difficulty descriptions are procedurally generated.

[@DarkMatterBurrito]

Great to see Dr Gray. I haven't seen her in several years on here.

[@SaquibFaisal]

Rotation can help adjust the focus as well which is not possible in rigid mirror counterparts.

[@fep_ptcp883]

But there is adaptive optics. Hundreds or micro motors bend the surface of a mirror to compensate for atmosphere distortion. It works like that at the VLT

[@crackedemerald4930]

​@@fep_ptcp883yeah but for a liquid Mercury telescope you just spin it differently. granted, there's probably some downtime between changing foci. but still.

[@outputcoupler7819]

@@crackedemerald4930 Adaptive optics are a fundamentally different thing. What you're talking about is adjusting the speed of the mirror. Adaptive optics is when you warp the mirror from the theoretical perfect shape to compensate for atmospheric effects, the speed of the mirror is unchanged.

[@skarrambo1]

Great memories of that second year lab - big up Paul Munday, the biggest legend of the department!

[@Jobobn1998]

I'm genuinely curious how they keep the mercury from oxidizing. I know that mercury normally develops an oxide layer after a period of air exposure, and with such a huge surface area as a mirror, managing that has got to be a practical consideration. My guess is there's possibly a liquid on top of the mercury, such as an oil, that protects it from oxidation, but that's got to introduce more optical challenges.

[@digitalplayland]

Absolutely amazing!!!

[@wktodd]

Surely, a frozen metal mirror is exactly what is required on the moon,. Once frozen ,in the correct shape onto a suitable frame, it could be steered like a normal telescope

[@luipaardprint]

Once frozen it’s not a smooth surface anymore, and would need polishing.

[@wktodd]

@@luipaardprint like most mirrors :-)

[@wktodd]

I wonder if there's an electro-chemical way to polish frozen mercury.

[@Macieks300]

@@wktodd you don't need to polish liquid mirrors

[@red.aries1444]

You don't necessarily to steer a telescope on the moon into the right direction. You just have to remember that a dark night on the moon last less then 14 days on earth. Yes, when it's dark it last much longer than 24 hours, but most of the time the sunlight will be to bright for the telescope to see other stars. So it would be better to have a telescope on the moon without all the complicated steering mechanism. You could then only look a small part of the sky, but it might be easier to set up a few more telescope with liquid mirrors on the moon pointing in different directions as one with a normal mirror. @luipaardprint It's an important point, that a liquid mirror doesn't need polishing. All the big mirrors of normal telescopes need a cleaning and recoating evey few years. That would be very difficult to do on the moon.

[@ELYESSS]

Hear me out, what if you double spin your telescope. You first spin the mercury to make the parabola, then spin the whole telescope in a circular track, you can point it sideways while ignoring gravity. You can image a whole circle in the sky that way.

[@luipaardprint]

I like the way you think

[@ElectricGears]

You could, but you'd have a worse problem than it pointing at only one spot. Now it's focus would be constantly moving as you would have to keep it rotating on the circular path because any change in that motion would disturb the surface.

[@S1nwar]

But now you got motion blur

[@puneetbajaj786]

​@@ElectricGears also as the earth is moving, we will have to change the track radius with time

[@jurjenbos228]

Vibrations are going to make this a nightmare

[@BengalBoy16]

I wonder how they deal with Mercury vapour at this Telescope, as it's in a warm place of the planet and its just a bowl of liquid vapour, potentially evaporating its contents into the area.

[@thirstyCactus]

I had the same thought. Perhaps health & safety regulations are less stringent in that area of the world.

[@18booma]

A quick google search reveals "Although mercury vapour is harmful, it is greatly suppressed by a thin transparent layer of oxide that forms soon after emplacement. Moreover, a thin film of mylar,4 co-rotating with the mirror, will contain any remaining vapour." @thirstyCactus, maybe look into the science instead of assuming anything about "that area of the world".

[@pixxel5392]

​@@thirstyCactushow are mall shootings and school shootings going on in your part of the world😂😂

[@Muonium1]

The tops of mountains where telescopes go are generally pretty cold places at night when the structure is open and mercury has an extremely low vapor pressure. At 25C its vapor pressure is only about 2 millimeters of ....well... mercury. You might need to wear a filtering respirator in the building with the telescope like they did at the LZT in Canada, but outside the building it's not going to be a concern. When closed during the day, the building could have a vapor recovery system preventing release to the environment. The LZT only contained around 400kg of mercury, a tiny fraction of which would ever evaporate away. For comparison China alone releases several hundred TONS of Hg into the atmosphere every year.

[@thirstyCactus]

@@pixxel5392 got'em!

[@crunchyfrog555]

Several years ago I toyed with the idea of making my own telescope. After reading up about what a royal pain in the arse it is to lap your own mirror, I thought then about wouldn't it be cool and probably relatively easy to pour mercury onto a flat plate and spin it to achieve the correct focal length. Of course I never looked into it any further, but it's nice to see that I wasn't far off.

[@darkpalidin]

This is crazy - I remember reading about the theory for this in a science book when I was in grade school about 25 years ago.

[@edsmith2562]

Thanks for the UT shout out. "Hook'em Horns"

[@jorgenskyt]

Instead of using liquid Mercury the same effect can be uptained using a low melting alloy like Galinstan (Gallium-Indium-Tin). This alloy is more expensive than Mercury, but is non-toxic compared to Mercury. It won't expell toxic fumes and any spillage will not harm the environment or the humans working with it. It's easy to make yourself. There are though two challenges: 1) Galinstan must be kept away from some metals, like Aluminum, as both the Gallium and the Indium content will alloy with metal and introduce fatal brittlenes, and 2) Galinstan will "wet" any surface it encounters, even glass. This is very anoying, but in the case of glass this can be accounted for by covering the glass with Indium Tin Oxide (the conductive coating used on the inside of LCD-screens and on airplane windows).

[@pmhgod318]

We should use this

[@euromicelli5970]

First time I ever saw this concept was in the 1934 science fiction story “Old Faithful” by Raymond Z. Gallun, as collected by Asimov in his “Before the Golden Age”. I read it as a kid and I remember thinking it was a brilliant idea and wondering why nobody did it for real. Of course, I was too young to realize the rotation would need to produce the right shape for it to be workable and that wasn’t a given - fortunately it does. Also, at the time I had no way to find out that it had in fact been attempted to a limited extent.

[@stephenweber9141]

In 1971 I had a summer job at the US Naval Observatory. One of my responsibilities was to come in in the evening and set up the photographic zenith tube. This had a flat mercury mirror and a traditional lens, not the same thing as described here. The light went through the lens, was reflected by the mirror half a focal length below, and was focused near the underside of the lens. I wiped oxide off the surface of the mirror. Safety did not seem to be an issue back then. I loaded film into the camera. It was programmed to open the shutter when a star was expected to pass overhead, tilt the film to track for the duration of the exposure, and close the shutter. There were multiple exposures over the night since none of the stars was exactly overhead. It was used to measure variation of the rotation of the earth. The horizontal mirror guaranteed that the telescope looked directly overhead.

[@TAP7a]

Ah, vector calculus and vector fields. Honestly, learning these maths tools was one of my favourite parts of my undergrad, alongside numerical methods for differential equation solutions. Really fun stuff until it gets quantised

[@rachh9731]

Fascinating, Would have been nice to hear a bit about what you could do with the 100m mirror that you couldn't do with other projects like JWST

[@judychurley6623]

Tremendously more light-gathering ability. 100m diameter (7854m^2) v 6.5m (33m^2) .

[@ChoChan776]

It would be interesting to know what other liquids have been considered for moon application. I wonder if they'd have to trade some "shinyness" for the lower melting point.

[@40KTTR]

Is there any good literature on the matter regarding the fluid viscosity, rotational speed and shape of the parabola?

[@TrebleTS20]

How do you keep it clean from contamination like dust insects ect

[@ehfik]

Fascinating. Please, continue.

[@sylak2112]

Cool! just avoid licking it. (Not directly related but in november 2023 I visited the Cerro tololo 4m scope en ESO VLT in chile! amazing. Could not lick them, either, but would probably not be sick from doing it haha)

[@CompanionCube]

5:36 wtf do you mean „it doesn‘t look real“? bro never stirred a cup of hot chocolate in his life

[@lucidmoses]

Could you pick a different angle if the secondary mirror was distorted in the opposite way that the mercury would cause?

[@wily_rites]

Nice idea, a mirror on the moon, just add some rotational thrusters for full sky viewing, we'd even be able to shed some light upon the dark side of the moon, well at a time when we could see it. On a more serious note, these telescopes would likely make very good clocks for telling barycentric time.

[@babilon6097]

What about dust that is a problem for big mirrors? Mercury is heavy. I imagine all the dirt will flow on the surface.

[@goz4659]

It's probably easy to swab/remove dirt if it settles on the surface, or have a process to exchange the mercury periodically to purify it.

[@thirstyCactus]

It looked like there was dirt / oxides around the perimeter of the mirror. Perhaps centrifugal force keeps that stuff on the rim.

[@Muonium1]

It's not the dust that's the problem, it's the oxide scum that constantly forms at the surface. You can see a whole crapload of it on the mercury surface at the end of the video here. The LZT in BC constantly needed to be stopped and the surface skimmed with paper towels to clean it. The dust accumulation on a normal telescope, even in the worst cases, surprisingly only accounts for a loss of a few percent of light.

[@babilon6097]

@@thirstyCactus I think you missed the demonstration in this video where they put something on the surface of mercury and it stays in place.

[@MichaelEdelman1954]

One technique that’s been used is a thin layer of liquid floating on the mercury that is continuously recirculated and filtered.

[@TheTablet314]

Can you do the float glas process with these parabolical liquid surfaces? That way you could actually make large parabolic mirrors very easily. Ofc, glass has such a high melting point but if the application can handle a larger thermal expansion coefficient, then you could also fo that with a thermoplastic.

[@SPotter1973]

8:30 might you be able to offset the tilt with magnitism? A grid of electromagnets along with a processor etc.

[@worldbridger9]

So based on gravity or constant accelaration, like a probe constantly accelerating towards a target... so can a light sail going to a new planet not only be stabilized by self rotation but but produce a mirror focusing on its target throughout the way?

[@JPBelanger]

Could you have a large array of smaller versions of this telescope?

[@jorgenskyt]

One of the challenges with liquid spinning telescopes is the gyroscopic effect on the spinning mass. As the planet rotates it also rotates the spinning liquid which will try to correct for the rotation of the planet, trying to keep its rotational axes static. This will result in a slight "wave" effect - or a deviation from the perfect parabula. This means there are limits to the image quality you can uptain with this method.

[@Kwauhn.]

Couldn't you correct for this type of thing? Or does the waving effect translate to a loss of information?

[@vberz4778]

If it's not chaotic motion, then it can be fixed with digital data post-processing.

[@Corvaire]

A magnetic band under a thin pool of mercury is out of the question seeing as it's electron stability (negatively) is very low. However, Galinstan could possibly be manipulated in such a way as to parallel a rotation once spun up. Could this be a safer/cheaper solution? Just asking! ;O)-

[@severeon]

Could you have a set of pools of mercury all spinning attached to a much larger spinning chamber. The pools could, in theory, be tilted gradually as the centrifuge speeds up. You could then position a camera at the focal point in the center and take timed images... Does that work?

[@RFC3514]

Even a single pool can be tilted "as the centrifuge speeds up", but gravity will always introduce a deformation unless the focus is straight up.

[@KanalFrump]

that drift scanning trick is amazing. Neat!

[@serta5727]

Quite a smart and innovative construction 😎

[@Dreddip]

NASA has one in New Mexico to catalog space debris and NEOs for years.

[@pillington1338]

Not sure if this was mentioned, but how is the mercury kept from evaporating?

[@petercollin5670]

I think of how the Indian Himalayas are seismically active. Would a tremor make you have to throw out an entire night's data? Or can you delete the observations made when the mirror got distorted?

[@rutger4131]

How do they prevent the mercury from evaporating?

[@nodakamakadon]

That was a comprehensive spiel.

[@babilon6097]

Another advantage of mercury is that it is self healing from small, surface-level damage.

[@factzilla1868]

another advantage is gradual exposure to mercury vapors motivates the research assistant to finish their thesis quickly

[@thirstyCactus]

That advantage would be excellent for a moon-based mirror, where there's likely space randomness hitting the surface more often than on Earth's surface.

[@JavSusLar]

That property is not mercury's. It is any liquid's property.

[@thirstyCactus]

@@JavSusLar OP was referring to mercury as opposed to solid mirrors, not mercury as opposed to other liquids.

[@factzilla1868]

@@thirstyCactus even better for lunar applications is that mercury corrodes aluminium spacesuits and spacecraft

[@NomadUniverse]

I imagine the mercury on that telescope would be vulnerable to dust. Would any floaties be pushed to the edge? Does it oxidise? Does it corrode? Do they need to do anything to maintain the reflective properties?

[@scowell]

On the Moon the transit is a lot slower... 30x or so... amazing to think of what might be found with a 90m transit telescope on the Moon.

[@diraziz396]

Oh Fascinating stuff. How about use it to "Spin Molding" -use Glass or proper material, Rotate it Faster, then Cure it by heat or cold or Ray, and you get good base.

[@UncleKennysPlace]

It's been done exactly that way for decades. I believe the University of Arizona has a lab tod do this.

[@diraziz396]

@@UncleKennysPlace (-: Call that a late bloomer... Cheers

[@edwardjrenegaud1040]

I wonder if using a magnetic field can help with rotating the mercury instead of a large rotating platform.

[@romado59]

Talking about Newton, his conclusion was that an absolute system of reference. Maybe the frame of reference could be the quantum fluctuation or maybe the CMB.

[@stoatystoat174]

Could you make it from Galinstan? (Extra melty Gallium alloy) [Not a moon telescope 🔭 🌝, im thinking one for the garden 🏡]

[@RFC3514]

That description of CCD sensors was kind of messy, and that process can be done in software with any type of sensor. The key reason for using CCD sensors was / is their low noise compared to other types. CCD was the type of sensor used by virtually _every_ high-end video camera until CMOS sensors (currently more common) became so dense and so cheap that they effectively overtook them (they still have worse quality "per pixel", but you can make them with a lot more pixels for the same money, so you end up with more detail across the image as a whole, as long as you don't mind the noise when you zoom down to individual pixels).

[@jasonharrison25]

How do you prevent the mercury from oxidizing?

[@vinigretzky97]

Couldn't you use electric currents in the mercury in an external magnetic field to create a spinning vortex?

[@paulpinecone2464]

Mercury being solid on the moon started me thinking-- You could make a reflector mirror by spinning a liquid metal and then lowering the temperature until it freezes? If it's not reflective as a solid, deposit a thin layer of something like gold. Keeping the temperature just below freezing point would minimize warping from contraction. For mercury that's around -40°C. To avoid needing so much refrigeration, gallium freezes at 30°C. Would that make up for it being 10x the cost? And that you can cut it with a knife?

[@CAPSLOCKPUNDIT]

What about Earth's rotation? How much distortion from paraboloid convex surface would be induced by coriolis force?

[@pef1960]

The problem with liquid mirrors on the moon would be lunar dust, which is incredibly fine, abrasive and electrically charged.

[@Alasdair-Morrison]

Wouldn't a round cylindrical shape with a reflective material stretched over either end and a vacuum underneath form a the same shape with out the moving/spinning parts? Be a Vacuumscope and adjusting the vacuum will adjust the shape.

[@ronakchhatbar]

If you can put it on a gyroscope, then image creation can be done with projective geometry 7:53

[@TheCosmicGuy0111]

Nice

[@MusikCassette]

could you use gallium for that?

[@digitalplayland]

Can you spin it and freeze it?

[@bibibo743]

Could they start with a rotating reflective liquid, cool or cure it while its moving so it retains its shape, and then use that as a normal reflective element in a traditional telescope?

[@jurjenbos228]

Yes, that's the first step in making a glass based mirror.

[@michaelogden5958]

I wonder what the frequency/wavelength constraints are for a mercury reflector. Any wider or more narrow than 'glass'?

[@RFC3514]

Glass is generally not used as a reflector itself, due to being transparent and all. It's just a substrate on which a thin metallic layer is deposited (i.e., it's not like bathroom mirrors where the metal is typically _behind_ the glass, on reflector telescopes it's usually in _front_ of the glass, so you don't have to deal with interference from the glass itself).

[@michaelogden5958]

@@RFC3514 Silly me. Brain fart. I knew that! Thanks for the reminder!

[@wandamaddox7824]

It's a better demonstration of Mach's principle than it is a telescope.

[@babilon6097]

Couldn't you steer this telescope the way they steered Arecibo? By moving the camera around. You will not get 180° but a decent portion of the sky.

[@glodino88]

Arecibo was different shape (spheric compared to parabolic)> This allowed it to have a line of focus instead of just a single point

[@Muonium1]

@@glodino88 that's right. Arecibo was deliberately chosen to have a spherical shape so that multiple receivers could be simultaneously listening at the line of focus and so a secondary reflector could be steerable. FAST in Guizhou, which is now the world's largest uses the same design. The shape is also easier and faster to build due to the more symmetric nature of the pieces. Further still, the spherical aberration of the primary can be corrected for with a Gregorian secondary and tertiary reflector assembly at the focus, which was eventually exactly what was done on Arecibo in the 80s.

[@SeanRhoadesChristopher]

Doesn’t the light get distorted by the mirror’s movement?

[@antivanti]

Newton described the phenomenon of rotating liquid describing a paraboloid and invented the reflecting telescope. Newton heavy episode 😊

[@Amonimus]

Can't we like, freeze the mercury or shove it behind another glass, so the whole thing can tilt?

[@robertolson7304]

So its a anti-bubble. Liquid metal inside. Thin film of electrons. We already have that in wires. Then there is bubbles. Metal on the outside. Electrons in the middle. Bubbles will sink. Anti bubbles will float. Gravity has to share some common link to be a median. That is a curve. So a straight line is needed. To act like adjustable leverage.

[@rjhrjh3]

One way to use mercury on the moon would be to put it on the base of the mirror, heat the base until the mercury melts, spin the base, allow the mercury to freeze. Then you can stop the spinning. Then you have a large mirror you can point anywhere. Or use silver instead of mercury as that is reflective as a solid. If the mirror gets damaged from small impacts then melt the mercury, spin it up and the damage is repaired.

[@tomholroyd7519]

If you spun a molten metal that is warmer than room temp, then let it freeze as it's spinning, would you get a smooth and solid shape?

[@luipaardprint]

I would imagine controlling its shape as accurately as needed would be very difficult. Also a big advantage of the Mercury mirror is that it’s does not need polishing, which a solidified metal would need.

[@mytube001]

Unlikely. You get shrinkages that will warp different parts.

[@ElectricGears]

Yes, although you will have distortion as it cools. Some glass mirrors are made like this, then ground with CNC machines into the precise shape. The centrifugal casting drastically reduces the amount of grinding and stress releaving that would need to be done if they started with a solid block. I think some amateur astronomers have done this with plaster to create the pre-forms or physical templates for tracing attachments used to grinding their own mirrors. When we didn't have CNC machines accurate to nanometer distances, that's one way of generating these mathematically-defined surfaces.

[@jasonbouvette1077]

Is there a way to use this in space? I think that without gravity, it could be turned in any direction. But without the effects of gravity would the same shape be possible?

[@gasdive]

No, it won't work in space, but in space most of the problems of large glass mirrors go away. No need for active optics because there's no gravity sag to counter. No atmospheric distortion to counter. No dust. No corrosion. You could have a 1000m optical telescope if you wanted.

[@jasonbouvette1077]

​@@gasdiveMy plans for a giant mercury death ray thwarted again! 😂

[@ezion67]

Some questions popping up: 1) Could Gallium be a non toxic Mercury replacement. 2) What happens if you meld a metal and cool it down while spinning, would the contraction from the cooling deform the parabola or is there a way to let it keep its shape?

[@tolkienfan1972]

On the moon, use the mercury: heat it and spin it, then while spinning, let it cool. A solid mercury paraboloid mirror.

[@RFC3514]

I'm sure there are no problems with that at all. Like, it'll remain perfectly shiny when solid and won't require any polishing, it'll never heat up and deform, etc..😉

[@gmtom19]

For the who "you can only point straight up" part, this might be a lite bit crazy but could it be possible to put the the entire spinning plate on the edge of a second, larger plate and thus use the centrifugal forces generated to angle the mirror?

[@RFC3514]

Not really (gravity would always introduce a deformation). But how would that help you keep it pointed at a fixed part of the sky, anyway?

[@gmtom19]

@@RFC3514 Gravity would be consistent throughout and the vectors of the apparent forces would combine to make one stable force at an angle to the earths surface allowing you to point the telescope at an angle.

[@RFC3514]

@@gmtom19 - And if you try to keep it pointed at a fixed point of the sky as the Earth rotates (instead of changing direction even _faster_ than it does if you just keep it pointing up), the result won't be a parabola.

[@darthrainbows]

This sounds like it might be a way to construct absolutely gigantic mirrors in space. Launch a whole bunch of raw metal into space, melt it, put it in the dish to spin up the paraboloid shape, and very slowly cool it down to freeze, and you have your mirror. Whether the shape could be maintained during the cooling process or not is questionable, but seems like something worth testing.

[@Merto6]

That moon telescope must have really long exposures. 1 month for a full circle.

[@Jcewazhere]

I wanted Cody from Cody's lab to DIY a mercury scope with all that mercury he's got. Shouldn't be too hard for him. Neat video.

[@JaredBrewerAerospace]

@10:45 What's a checker board?!

[@rianfelis3156]

One thing is that you can get a certain distance off of zenith by moving your detector. This is how they aimed Arecibo and similar very large radio telescopes. Range is definitely limited, but it's not zero.

[@D1ndo]

False. Parabolic mirrors have only one focal point and it's pretty tight. Arecioe was using a spherical dish where the focal plane is large and you can steer by moving the collector (or a secondary)

[@PhilBoswell]

OK, at 7:40 I now want to know what kind of surface you *do* get if you tilt that rotating container 🤔

[@Iconoclasher]

Why couldn't this principle be adapted to a similar system that a solar observatory works. A tracking scope to follow the object and it redirects the light downwards to the mirror.

[@DANGJOS]

Put it at one of the poles haha. Would probably freeze though

[@xja85mac]

Is the mercury mirror a sealed container? or would they need to top it up from time to time? What are the implications for polluting the environment?

[@S1nwar]

negligible

[@stefangadshijew1682]

The vapour pressure of mercury is pretty low for a liquid (but high for a metal), less then 0,2 Pascal at ambient temperature. The vapour pressure of water is about 1000 times that. (On the flipside, a molecule of water weights about 1/10 that of an atom of mercury, and when water evaporates, it cools down a lot more, inhibiting further evaporation.) So just to get a feel for it, mercury stays where it is much more willingly then water. As long as the mecury is kept cool and does not experience a permanent air flow, it will only leak tiny amounts. Now some perspective, the yearly mercury emissions due to anthropogenic sources are estimated to be about 2000 tons. (metric, obviously) About 1000 tons of mercury are released from "artisanal (small scale) gold mining and refining", which makes up about 12 % to 15 % of total yearly gold production. About 400 tons of mercury are released from burning coal. That means that any losses of mercury from a mercury mirror are, globally, a drop in the bucket and absolutely meaningless. I bet that the effective mercury release due to the production of electronics in their systems and from the energy consumption to keep it running are higher then from evaporation. The environmental impact of mercury mirrors are meaningsless, they might just be a minor, controllable health risk for the employees, but I'm sure they use proper safety measures. It might even be that the production of conventional mirrors emits more mercury due to electronics and burning coal due to the higher energy- and labour requirements. All in all, I would say that there is close to zero environmental damage from using a mercury mirror over a different mirror.

[@ElectricGears]

If we ignore spills (that could happen any time mercury is used) evaporation would be a concern. Generally people wouldn't be allowed in the room when the mirror was uncovered. Although they wouldn't be allowed anyway to reduce vibrations. The building would also need a slight negative pressure to reduce air from leaving the opening. Nothing basic air quality management systems couldn't handle.

[@hakimal-hakim8890]

Isn't possible to freeze the mercury telescope after having the paraboloid shape? Or maybe using it as a mold to make a solid one with glass mirror?

[@ConsciousAtoms]

You don't need a mold for a glass mirror - in fact, large parabolic mirrors are made by spinning molten glass and letting it cool down very slowly.

[@bjornmu]

It would have to freeze while spinning and I imagine it might not freeze evenly. The whole apparatus would have to be kept at -39C which probably costs more energy than spinning it, and everything has to tolerate that temperature. And we already know how to make a solid one with glass mirror.

[@vernacularbarnarchitecture]

❤

[@booodelooplamphead8746]

I thought this was techno based off the thumbnail :')

[@TheRealInscrutable]

If mercury is solid at lunar temperatures could you not take advantage of that and have a heated platform and once the desired shape has been achieved then switch off the heat and let it freeze in the correct shape? Once frozen you could even stop spinning it!

[@Skeke]

Lets do a liquid version of JWST

[@free_spirit1]

Wouldn't the paraboloid get all warped due to the rotation of the earth? It's like a giant gyroscope.

[@rudyvaldez]

Why would we want to make a telescope out of a Liquid Metal?; To potentially calculate distances.

[@kiatsommart]

If we build enough of this around the surface of the moon, then we'll have a giant 360 camera that provide constant feed of the universe 😁

[@koolguy728]

Why is it a parabola that you want and not a spherical dome? Wouldn't a spherical dome have a focal point?

[@kuronosan]

13:23 Or you could.. heat the dish holding the Mercury.

[@uwezimmermann5427]

but if you limit yourself to a zenith looking telescope - also all the arguments why handling and mounting the traditional solid mirror is so difficult disappear.

[@Bunnysinger]

Not if you have to ship it into space.

[@uwezimmermann5427]

@@Bunnysinger admittedly, but as was also pointed out later in the video there are other obstacles, like the low temperatures and the low pressure, with together with the high vapor pressure of mercury will lead to quick evaporation. There are other alloys, but for the dark-side of the moon with -180°C the choice even of ionic liquids should be limited, balancing between freezing and immediate evaporation. Also still the containment vessel and motor assembly would need to be transported to space/the moon. JWST has shown that a quite thin, free-standing membrane can work as mirror surface, which sounds much more attractive.

[@carnsoaks1]

Why not join me in Australia, make 10 Small 12m Galistan Dish Telescopes and use interferometry and the CCD Software to make the Supersized Sibling Spinning Soup Scope.

[@jmalmsten]

The idea of using highly toxic liquid metal for a large mirror seems almost like an intrusive thought an engineer just couldn't resist. Also. In ancient asia, there were talks about palaces having mercury pools and rivers. Probably just to show off wealth. But. An entrepreneurial mind might make the mental leap to a sci fi scenario that they were in fact giant ancient telescopes.

[@keithbromley6070]

I wonder if they could use galinstan instead. Not toxic and it freezes at 11C so they could spin and freeze then stop spinning for the duration of the observation.

[@RFC3514]

It's not "highly toxic" as long as it remains a liquid. People keep confusing metallic mercury with mercury vapour and organic mercury salts.