Telescope with a Mercury Mirror - Sixty Symbols 

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.

She’s an excellent self-interviewer.

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.

@@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.

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?

​@@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

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

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

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

@@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.

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

Ultimaterererer. Next!

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

Labels are limits indeed.

​@@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.

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

@@luipaardprint like most mirrors :-)

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

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

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.

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

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".

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

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.

@@pixxel5392 got'em!

We should use this

I like the way you think

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.

But now you got motion blur

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

Vibrations are going to make this a nightmare

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.

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

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

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.

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

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.

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

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

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).

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

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

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

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?

@@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.

@@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.

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.

Unlikely. You get shrinkages that will warp different parts.

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.

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.

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

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.

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.

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

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

negligible

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.

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.

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

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.

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

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

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

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)

I’ve read that a parabola has a point of focus, not a line or area of focus, so this wouldn’t work. Oops!

​@@nicksamek12cheers for both seeking out the answer for yourself and also for sharing it 🙂

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..😉

Not if you have to ship it into space.

@@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.