Phys.org's New Indian telescope identifies its first supernova links to the recent arXiv Follow-up strategy of ILMT discovered supernovae. The International Liquid Mirror Telescope is no ordinary spinning puddle of mercury, it's got 21st century improvements!

The introduction begins:

The ILMT is a 4-m diameter zenith-pointing telescope located at Devasthal Observatory (Nainital, India). The first light of the facility was achieved last year (2022 April 29) and presently, it is in the advanced stage of commissioning. Unlike conventional telescopes, the primary mirror of the ILMT is formed by pouring approximately 50 liters of mercury into a recipient, which acts as a reflecting mirror. The effective focal length of the optical system is 9.44 m. The ILMT images are obtained using the Time-Delay Integration (TDI) technique. Given the fixed pointing of the telescope, the stellar objects move in the focal plane along slightly curved trajectories. Therefore, a dedicated five-element optical corrector is being used altogether with the CCD reading the electronic charges in the TDI mode (Gibson and Hickson, 1992; Hickson and Richardson, 1998). A 4k × 4k CCD camera (Spectral Instruments) is mounted at the prime focus of the telescope, which can secure nightly images in g, r, and i spectral bands with a total integration time of approximately 102 sec (in single scan).

Time-delay integration and liquid mirror telescopes are discussed further in:

Question: Instead of a 4 meter diameter, 50 liter spinning pool of dangerous mercury, why didn't the ILMT just use an ionic liquid? Or use liquid gallium?

Ionic liquids are discussed at length in this answer

Wikipedia's Gallium says:

The melting point of gallium (29.77 °C) allows it to melt in the human hand, and then solidify if removed.

and later:

It is also notable for having one of the largest liquid ranges for a metal, and for having (unlike mercury) a low vapor pressure at high temperatures.

It seems like these days, with the understanding of the dangers of mercury poisoning, and that even the vapors from an exposed surface pose a safety risk, most technologies are moving to anything they can to avoid using mercury, elemental or otherwise. cf. Just how dangerous is mercury, anyway?

People tend to associate mercury with its silvery liquid form — as found in old thermometers. But it was also used in electrical switches or relays that were built into machines until the mid-20th century. Later, it was florescent lamps and some early energy saving lamps.

The liquid form of mercury is especially dangerous because it vaporizes at room temperature. And when it vaporizes, it fills the air with tiny, invisible mercury atoms that are both scentless and soluble in oils or fats.

If mercury vapor is inhaled, it is easily absorbed by the body, where it first gets into the lungs and from there into the blood and the brain. It's a nerve poison that can cause sleep disorders, agitation and paralysis.

I'm not saying it can't be contained, we still make nuclear reactors too.

I'm just wondering if there were reasons why a reflective ionic liquid was not used instead. Maybe they're no good to use, chemically unstable, more susceptible to surface ripples induced by vibrations, or even more dangerous than mercury, I don't know. So I'm asking.

  • 2
    $\begingroup$ (+1) Interesting question! Reminded me of the mercury pool and fountain art exhibit: en.m.wikipedia.org/wiki/Mercury_fountain. $\endgroup$
    – Ed V
    Nov 25, 2023 at 13:22
  • $\begingroup$ Getting decent reflectivity may be an issue for one. There is little danger if the vapor is contained in a known volume, with appropriate air monitoring in the human occupied spaces. $\endgroup$
    – Jon Custer
    Nov 25, 2023 at 14:58
  • 3
    $\begingroup$ The wikipedia page mentions that ionic liquids are often highly toxic. $\endgroup$
    – James K
    Nov 25, 2023 at 19:22
  • $\begingroup$ If a sheet of flexible transparent material is fitted on top of the spinning mercury it would keep vapors from escaping. However, it might have wrinkles, etc. which distorted the image. Possibly there could be machnes to remove mercury vapor from the air new the telescope to reduce the mercury air pollution and possibly the control room would airtight. $\endgroup$ Nov 25, 2023 at 19:59

2 Answers 2


Ionic liquids are not reflective.

enter image description here

An ionic liquid being poured, from )source( They have different properties, but may are toxic, note the nitrile gloves, the researcher doesn't want to get any of this stuff on their hands.

To make the mirrors proposed for the lunar telescope, a thin layer of silver would be evaporated onto the surface of the ionic liquid. This is all very difficult to achieve and is done because mercury would freeze solid at the temperatures of the lunar night.

Evaporating silver requires a vacuum, and in Earth's atmosphere, the silver would react with oxygen, and lose its reflective properties. Paper https://arxiv.org/abs/0806.2241 describes the difficulties that the scientist had in forming an acceptable silver coating on an ionic liquid. The silver wrinkled, or diffused into the liquid, or was too thin to reflect light. This is a good area for research, but at the moment the technology for creating a large reflecting rotating pool of ionic liquid doesn't exist.

Gallium, as you note melts at a low temperature. But it isn't liquid at room temperature. This means it would have to be heated (a little), and heating creates convection currents in the air, and so causes poor seeing. However a mixture of gallium, indium and tin is liquid at room temperature and so may be an option. This has been considered There are issues with the formation of an oxide layer so perhaps the only reason "Galinstan" was not used was engineering conservatism.

Now these problems are engineering problems. It is possible that they could be solved, worked around or mitigated. But mercury has the advantage: it is liquid at room temperature and it is itself reflective. The engineering required to make it safe is probably easier than the engineering required to make ionic liquids or gallium a solution to this problem.

Finally ionic liquids are expensive! But this might not be the deciding factor in a telescope design.

  • 4
    $\begingroup$ Well, if anything, it would be rather galinstan than pure gallium, but I doubt it has the mercury's reflectivity. It's wetting properties may be another issue. $\endgroup$
    – Mithoron
    Nov 26, 2023 at 0:18
  • 1
    $\begingroup$ galinstan has good relectivity, perhaps the only reason it wasn't used is that engineers are conservative, and will go with what they know works. $\endgroup$
    – James K
    Nov 26, 2023 at 8:24
  • $\begingroup$ I always imagined (for absolutely no good reason whatsoever it seems) that ionic liquids were shiny with reflective surfaces, like liquids with solvated electrons. Of course that's stupid because what makes (a sufficient density of) free electrons reflective is their mobility (they can oscillate and re-radiate) and being 20,000 to 100,000 times heavier, ions don't do that. This is an excellent and thorough answer, and gets my n-factorial vote [+n!] Kudos and thanks! $\endgroup$
    – uhoh
    Nov 26, 2023 at 22:41

I'll add this to point out that the mercury is managed such that choosing something else might not be worth the bother.

The paper by Brajesh Kumar et al. (Monthly Notices of the Royal Astronomical Society, Volume 476, Issue 2, May 2018, Pages 2075–2085, https://doi.org/10.1093/mnras/sty298) I used in another answer has some supplementary information on the mercury pool that helps alleviate safety concerns. There they note:

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, co-rotating with the mirror, will contain any remaining vapour. This film is required to prevent vortices, produced in the boundary layer above the mirror due to its rotation, from disturbing the liquid surface.

So, the oxide helps contain the liquid mercury, and the mylar keeps any vapor from getting into the larger telescope structure.

These steps reduce the need to find another suitable material.


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