Toxic Telescope Makes You Mad As A Hatter

[Hank Green] posted an interesting video about the first liquid mirror telescope from back in the 1850s. At the time, scientists were not impressed. But, these days, people are revisiting the idea. The big problem with the early telescope is that it used mercury. Mercury is really bad for people and the environment.

The good thing about a liquid scope is that you can pretty easily make a large mirror. You just need a shallow pool of liquid and a way to spin it. However, there are downsides. You need to isolate the liquid from vibrations and dust. Another downside is that since gravity makes the shape of the mirror, these telescopes only go one way — straight up.

Modern liquid telescopes have high-tech ways to combat all of the problems except the straight-up view. While it can be expensive to safely handle a huge amount of mercury and isolate it from vibrations, it can still be less expensive than polishing mirrors of similar size. The 6-meter Large Zenith Telescope at the University of British Columbia, for example, claims to cost about 2% of what a similar instrument made with glass would cost. In addition to rotating to maintain their parabolic shape, Earth-bound liquid mirrors also rotate around the Earth’s axis, which causes the mirror to point ever so slightly off the point directly overhead, something that precise observations have to take into account.

The first actual practical liquid mirror telescope had a 50cm mirror. Modern examples are mostly in the 3-6 meter range. Gallium is another possible liquid, but that raises the cost significantly. In addition, research is underway to find safer liquids and solve the problems associated with tilting the mirror.

The classic way to make a mirror is to grind it from a glass blank. If you make your own telescope, you can play with the look of it in interesting ways.

50 thoughts on “Toxic Telescope Makes You Mad As A Hatter

    1. Viscosity, density, and temperature. Spinning that much mass, which flows at about the rate of molasses in February, while maintaining a temperature above 1400 C for a period long enough for it to take on the correct shape, then keeping it spinning long enough to cool down would be incredibly difficult, and very, very expensive.

      Then there’s the problem of finding bearings that could last that long in those conditions.

      1. Yeah if you’re going to build all of that stuff you might as well just build a glass grinding machine and save a ton of headache and expense. Wonder if you could do something like it with epoxy.. Probably wouldn’t stay optically true.

        1. I did exactly that in the 1980’s after reading about liquid mirrors. My two attempts involved a record turn table set to 16 RPMs and a 12 inch baking pan. I used fiber glass resin from a boat shop. The results of the first run were very bad, the resin generated a lot of heat warping the surface into a bubbly consistency. On the second attempt, I used 1/3 the hardener called for on the bottle. This one came out much better with only a little distortion on the edges. I coated the surface aluminum foil and polished it well.

          Telescope? no, Light stuff on fire? hell yes!

    2. They already do this to at least get the rough shape, but it still has to be finish-ground

      IIRC Arizona State has a world-class facility in Phoenix that makes large mirrors this way

        1. Seems like that would introduce distortion and issues from dual-surface reflection if it were thick enough to block oxygen effectively. But hey, if they’ve tried it and succeeded, that will ALWAYS trump my speculation.

  1. I go to UBC and have lived in Vancouver all my life, I’ve always thought that it’s super cool that one of the largest telescopes ever built was a couple hours by car away by a team from my university! Unfortunately, the light pollution and cloud cover made it infeasible to operate, but it was an excellent technology demonstrator for liquid-mirror observatories. Hopefully a new, less-toxic liquid metal can be found to replace the mercury in LMTs.

  2. For some rotating Mercury mirrors they first spin up some plastic resin to form the shape. It’s kept spinning until the resin sets. Then it needs much less mercury to cover the resin.

  3. Mid ’70s, UK secondary school, physics lessons, we were allowed to play with mercury in our hands as an end of term “special treat”.

    Then I would go home and clean out my rabbit hutch I had made by sawing up asbestos sheets in the garage.

  4. @treacheroustoast It’s sad that for a short while light pollution was a topic the urban planners and various authorities such became aware of, so then they actually started to use streetlights that were designed to only light downwards and to reduce unneeded light and things like that, then a few years later it was all forgotten again and no effort is done whatsoever anymore, and in fact when I look out at night during a cloudy night I see more light reflected than ever before.

    And I see the same constant tendency to attempt to forget with carbon emissions. on the one had they spend billions and make big laws, but then another department forgets and happily does stuff 180 degrees counter the goal. And Germany opens a coal mine…

    1. Politicians do as their voters demand.

      The Energiewende was a big boondoggle to fund the wind and solar power industry with public money with the help of “little green men”, who knew very well that building a power grid on highly intermittent supply makes it dependent on Russian gas.

    2. Old style sodium lamps used for streetlights emit at specific wavelengths and are (kinda) easily filtered out with a notch filter for amateur telescope operation. Unfortunately modern LED street lights are not, and to your point because of that light pollution is much worse for the amateur astronomer.

  5. The main issue with making mirrors by melting a material, rotating it, and cooling, is that a telescope mirror must have a surface that is accurately paraboloidal within less than one-tenth of a micrometer. (Today’s amateurs strive for accuracy rather tighter than that.) And it must be incredibly smooth as well, with tolerances near the molecular level.

    There was an account in “Amateur Telescope Making” about making mirrors with two solutions that were mixed, poured into a mold, then rotated until hardened. That simply removed most of the work of rough and (some) fine grinding; the finished mirror still had to go through some fine grinding, then the time-consuming work of polishing and “figuring” it to a paraboloid.

    There aren’t many materials that will go from liquid to solid without either expanding or contracting, not to mention the strains that occur in many materials (unless annealed…and that probably changes the dimensions slightly as well).

    But…that’s why research is being done!

  6. Can I ask a question? I get, obviously, why it’s good to have a perfectly-shaped mirror. However, Hubble didn’t, and was successfully “repaired”. With all the computing power and disk space available, can’t we just get a mirror to “good enough”, measure the cr4p out of it (we’re good at doing lots of high-accuracy distance measurements), and make a “map” of the mirror imperfections, and let the computers do the rest?

    1. By that logic do we need a mirror at all? Or would a really high resolution camera and some good software suffice? That kind of reminds me of SDR. I guess it is since light vs radio are just a difference of frequency.

      As I think that through more though I think it would take a ridiculously high resolution camera. It would take something that could get as many pixels per degree in it’s field of view as the with-lens does however that FOV is a lot more degrees without a lens to magnify a smaller part of the sky.

      Maybe that is also why they aren’t doing what you suggest with the imperfections. Imperfections would mean some areas in the FOV are pinched, thus the light of those areas isn’t spread out between as many pixels and so they would lose resolution.

      1. The mirror provides light-gathering power. All the light striking the mirror goes into the sensor (human or electronic). Far-away objects are incredibly dim. The more light that can be gathered, the more detail that can be seen. Same idea of long exposures; increases the amount of light that goes into the image.

    2. > However, Hubble didn’t, and was successfully “repaired”.

      Hubble was repairable *because* it had a very precisely shaped mirror. It’s just that, due to an error in a measurement tool, it was very precisely made to the wrong “prescription”.

      Fortunately, it was a type of optical error that could be corrected by a secondary lens.

    3. I think the answer is yes and no. There are some kinds of issues that you can correct for, and some that you can’t. The problem is that when light gets combined, you can’t just uncombine it without having additional data, and in many cases you just don’t have that. For instance, if you take a series of images over time, each with a slightly different viewpoint, you can look at how the light moved around (due to the imperfections), and if you can figure out how the objects being imaged were moving (with respect to the image), then you can calculate where their light went for the various images, and make corrections. However, if the objects didn’t behave predictably (or they changed in brightness between images), then your assumptions may be off, and you may correct incorrectly. Or they might move in such a way that you don’t get enough information to make corrections.

      With ground-based telescopes, corrections already have to be made to account for atmospheric conditions, for instance. Adding more sources of error makes the problem that much harder.

  7. I have made experimental mirrors from metalized mylar “rescue blankets” stretched over a section of 12″ PVC pipe. The mirror film seals the front side and the back is sealed by gluing a 1/4″ of acrylic or polycarbonate with silicone. I attach a fitting for a vacuum hose and valve, draw out some of the air (suction with the mouth is adequate) and close the valve. I have used that type of mirror for solar heaters and may one day try a crude telescope. Of course, any out-of-roundness of the pipe alters the shape of the mirror, it vibrates from wind and even loud noises, so it will never be great, but might provide fleeting glimpses of things otherwise unseeable with the eye alone, and it’s cheap to make even very large mirrors with tunable focal length.

  8. I have made experimental front surface mirrors using metalized polyester “rescue blanket” stretched over a circular frame made from a short section of 12″ PVC pipe. The film seals the front surface and the back is sealed by gluing a piece of acrylic or polycarbonate using silicone caulk. One could also use a larger ring of acrylic or polycarbonate. Add a pneumatic hose fitting and valve, and draw a little air out or the mirror by sucking on the hose and close the valve. My 12″ mirrors will burn wood easily. It’s not even close to a glass mirror, but it’s cheap and easy to make in very large sizes, light weight, and tunable by varying the vacuum inside the chamber. Some day I’d like to try making a crude telescope using the technique. Wind and loud noises will cause the mirror to vibrate, and the metallization isn’t a terribly efficient reflector, so it’s not close to ideal, but it would be interesting to play with it.

    1. FWIW in order to get anything close to an optical surface the pipe must be accurately round with a fairly sharp edge (actually the edge would need to be round) and extremely flat. But for burning stuff it’d work a treat!

  9. LMTs don’t have to look straight up. (2) flat mirrors angled at 45 degrees could perform the steering to allow a LMT to view a much larger portion of the sky. Flat mirrors are easier to make than parabolas. Carefull steering of the flat mirrors would allow for tracking a source in a vary large portion of the sky.

    1. I don’t think OPTICALLY flat mirrors of this size are any easier to make than parabolas. From my amateur-telescope-making days, I think flats were actually technically HARDER to make than spheres and parabolas, although that probably doesn’t generalize to modern techniques. In any event, the required flats would be even LARGER than the primary.

      You’d probably have better luck adjusting the position and angle of the secondary/tertiary mirrors (which are smaller), and introducing an adaptive element to compensate for the off-axis aberrations that result — but I’m way out of my depth here, so I should probably shut up. :)

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