<p>Ionic liquids are not reflective.</p>
<p><a href="https://i.sstatic.net/GLYgY.png" rel="noreferrer"><img src="https://i.sstatic.net/GLYgY.png" alt="enter image description here" /></a></p>
<p><em>An ionic liquid being poured, from )<a href="https://www.chemistryworld.com/podcasts/ionic-liquids/4012785.article" rel="noreferrer">source</a>( 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.</em></p>
<p>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.</p>
<p>Evaporating silver requires a vacuum, and in Earth's atmosphere, the silver would react with oxygen, and lose its reflective properties. Paper <a href="https://arxiv.org/abs/0806.2241" rel="noreferrer">https://arxiv.org/abs/0806.2241</a> 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.</p>
<p>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 <a href="https://www.google.co.uk/books/edition/Minerals_Yearbook_Metals_and_Minerals_20/zyKCNW0BHm8C?hl=en&amp;gbpv=1&amp;pg=SA47-PA26&amp;printsec=frontcover" rel="noreferrer">has been considered</a> There are issues with the formation of an oxide layer so perhaps the only reason &quot;Galinstan&quot; was not used was engineering conservatism.</p>
<p>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.</p>
<p>Finally ionic liquids are expensive!  But this might not be the deciding factor in a telescope design.</p>

CC BY-SA 4.0
2023-11-26T00:18:28.960Z
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.
Mithoron
CC BY-SA 4.0
2023-11-26T08:24:32.647Z
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.
James K
CC BY-SA 4.0
2023-11-26T22:41:57.350Z
I always imagined (for absolutely no good reason whatsoever it seems) that ionic liquids were shiny with reflective surfaces, like liquids with [solvated electrons](https://chemistry.stackexchange.com/q/72264/16035). 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!
uhoh
CC BY-SA 4.0
2023-11-25T23:40:08.017Z
2023-11-26T07:36:27.523Z
2023-11-26T07:36:27.523Z
James K
<p>I'll add this to point out that the mercury is managed such that choosing something else might not be worth the bother.</p>
<p>The paper by <a href="https://academic.oup.com/mnras/article/476/2/2075/4839011" rel="noreferrer">Brajesh Kumar et al.</a> (Monthly Notices of the Royal Astronomical Society, Volume 476, Issue 2, May 2018, Pages 2075–2085, <a href="https://doi.org/10.1093/mnras/sty298" rel="noreferrer">https://doi.org/10.1093/mnras/sty298</a>) I used in another answer has some supplementary information on the mercury pool that helps alleviate safety concerns. There they note:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>So, the oxide helps contain the liquid mercury, and the mylar keeps any vapor from getting into the larger telescope structure.</p>
<p>These steps reduce the need to find another suitable material.</p>

CC BY-SA 4.0
2023-11-28T14:32:21.767Z
2023-11-28T14:32:21.767Z
Jon Custer
<p>Phys.org's <a href="https://phys.org/news/2023-11-indian-telescope-supernova.html" rel="nofollow noreferrer">New Indian telescope identifies its first supernova</a> links to the recent arXiv <a href="https://arxiv.org/abs/2311.05618" rel="nofollow noreferrer">Follow-up strategy of ILMT discovered supernovae</a>. The International Liquid Mirror Telescope is no ordinary spinning puddle of mercury, it's got 21st century improvements!</p>
<p>The introduction begins:</p>
<blockquote>
<p>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. <strong>Unlike conventional telescopes, the primary mirror of the ILMT is formed by pouring <em>approximately 50 liters of mercury</em> into a recipient, which acts as a reflecting mirror.</strong> 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).</p>
</blockquote>
<p>Time-delay integration and liquid mirror telescopes are discussed further in:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/44086/7982">What was the first use of time-delay integration in Astronomy? Are there instances before GAIA?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/41023/7982">Why can&#39;t we build a huge stationary optical telescope inside a depression similar to the FAST radio telescope?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/51963/7982">How would the &quot;Fluidic Telescope (FLUTE)&quot; next generation telescope make and control a smooth, correct concave optical surface figure in microgravity?</a></li>
</ul>
<p><strong>Question:</strong> 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?</p>
<p>Ionic liquids are discussed at length <a href="https://astronomy.stackexchange.com/a/41027/7982">in this answer</a></p>
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Gallium" rel="nofollow noreferrer">Gallium</a> says:</p>
<blockquote>
<p>The melting point of gallium (29.77 °C) allows it to melt in the human hand, and then solidify if removed.</p>
</blockquote>
<p>and later:</p>
<blockquote>
<p>It is also notable for having one of the largest liquid ranges for a metal, <strong>and for having (unlike mercury) a low vapor pressure at high temperatures.</strong></p>
</blockquote>
<p>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. <a href="https://www.dw.com/en/just-how-dangerous-is-mercury-anyway/a-16522491" rel="nofollow noreferrer">Just how dangerous is mercury, anyway?</a></p>
<blockquote>
<p>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.</p>
<p>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.</p>
<p>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.</p>
</blockquote>
<p>I'm not saying it can't be contained, we still make nuclear reactors too.</p>
<p>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.</p>

CC BY-SA 4.0
2023-11-25T09:53:55.137Z
companion question: [Instead of a five-element corrector for arced drift trajectories, why didn't the ILMT just use CCDs fabricated w/ slightly non-cartesian pixel layout?](https://astronomy.stackexchange.com/q/55392/7982)
uhoh
CC BY-SA 4.0
2023-11-25T13:22:47.820Z
(+1) Interesting question! Reminded me of the mercury pool and fountain art exhibit: https://en.m.wikipedia.org/wiki/Mercury_fountain.
Ed V
CC BY-SA 4.0
2023-11-25T14:58:55.543Z
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.
Jon Custer
CC BY-SA 4.0
2023-11-25T19:22:16.197Z
The wikipedia page mentions that ionic liquids are often highly toxic.
James K
CC BY-SA 4.0
2023-11-25T19:59:08.600Z
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.
M. A. Golding
CC BY-SA 4.0
2023-11-25T20:22:25.933Z
According to reports, the tomb of Qin Shih Huang had a map of China with rivers and seas of mercury, his corpse resting in a boat on a mercury river.  Of course he was already dead.  I read that a Muslim ruler of Egypt, probably Ahmed ibn Tulun  (c.835-884) feared assassination ands slept on a boat in a pool of mercury.  Caliph Abd al-Rahman III (890-961) built a new city of Madinat Al-Zahra west of Cordoba, with a room in the palace with a pool of mercury which cast spinning reflections around the room.
M. A. Golding
CC BY-SA 4.0
2023-11-25T20:45:58.173Z
@M.A.Golding ya, like I alluded to in the question, a containment scheme is certainly implementable (with nuclear reactors as an extreme existence proof), but then you need sensors to verify that it's working, alarms when it isn't safety procedures and training or those events, safety officers, chemical safety oversight and bureaucracy, perhaps even regular mercury monitoring for the staff (if there is in fact any way to check humans, it probably doesn't stay in the blood... maybe hair samples?) There's just a lot of effort and challenge that comes together with the risk.
uhoh
CC BY-SA 4.0
2023-11-25T20:51:31.753Z
@M.A.Golding  I'm thinking that the answer might be that there is simply no suitable replacement for mercury, and that spinning ionic liquid mirrors are considered for future Moon-based and space-based telescopes for other reasons. fyi **I've just added liquid gallium as another option to the question.**
uhoh
CC BY-SA 4.0
2023-11-25T20:52:04.187Z
@JonCuster ditto
uhoh
CC BY-SA 4.0
2023-11-25T20:56:02.790Z
@uhoh - what I saw for the ionic liquid telescope on the moon was a need to coat it with silver to get good performance from 500nm into the near infrared. Yes, deposit silver onto the spinning mirror in situ on the moon. Doesn’t make me think the reflectivity of ionic liquids is ver good…
Jon Custer
CC BY-SA 4.0
2023-11-25T21:02:56.243Z
@JamesK So better not use those highly toxic ones! Heavy metals are also "often highly toxic" and yet gallium is pretty safe stuff to handle. Some ≠ all.
uhoh
CC BY-SA 4.0
2023-11-25T21:04:02.750Z
@JonCuster it looks like you're working your way to an answer; great!
uhoh
<mirror><mirror-making><liquid-mirror-telescopes>
Instead of a 4 meter diameter, 50 liter spinning pool of dangerous mercury, why didn't the ILMT just use an ionic liquid? Or gallium?
uhoh
# Instead of a 4 meter diameter, 50 liter spinning pool of dangerous mercury, why didn't the ILMT just use an ionic liquid? Or gallium?

(Asked by: username_0 on 2023-11-25)

Phys.org's [New Indian telescope identifies its first supernova](https://phys.org/news/2023-11-indian-telescope-supernova.html) links to the recent arXiv [Follow-up strategy of ILMT discovered supernovae](https://arxiv.org/abs/2311.05618). 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:

* [What was the first use of time-delay integration in Astronomy? Are there instances before GAIA?](https://astronomy.stackexchange.com/q/44086/7982)
* [Why can't we build a huge stationary optical telescope inside a depression similar to the FAST radio telescope?](https://astronomy.stackexchange.com/q/41023/7982)
* [How would the "Fluidic Telescope (FLUTE)" next generation telescope make and control a smooth, correct concave optical surface figure in microgravity?](https://astronomy.stackexchange.com/q/51963/7982)

**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](https://astronomy.stackexchange.com/a/41027/7982)

Wikipedia's [Gallium](https://en.wikipedia.org/wiki/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?](https://www.dw.com/en/just-how-dangerous-is-mercury-anyway/a-16522491)

> 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.

**Tags:** mirror, mirror-making, liquid-mirror-telescopes
> **Comments:**
> **username_0**: companion question: [Instead of a five-element corrector for arced drift trajectories, why didn't the ILMT just use CCDs fabricated w/ slightly non-cartesian pixel layout?](https://astronomy.stackexchange.com/q/55392/7982)  
> **username_1**: (+1) Interesting question! Reminded me of the mercury pool and fountain art exhibit: https://en.m.wikipedia.org/wiki/Mercury_fountain.  
> **username_2**: 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.  
> **username_3**: The wikipedia page mentions that ionic liquids are often highly toxic.  
> **username_4**: 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.  
> **username_4**: According to reports, the tomb of Qin Shih Huang had a map of China with rivers and seas of mercury, his corpse resting in a boat on a mercury river.  Of course he was already dead.  I read that a Muslim ruler of Egypt, probably Ahmed ibn Tulun  (c.835-884) feared assassination ands slept on a boat in a pool of mercury.  Caliph Abd al-Rahman III (890-961) built a new city of Madinat Al-Zahra west of Cordoba, with a room in the palace with a pool of mercury which cast spinning reflections around the room.  
> **username_0**: @M.A.Golding ya, like I alluded to in the question, a containment scheme is certainly implementable (with nuclear reactors as an extreme existence proof), but then you need sensors to verify that it's working, alarms when it isn't safety procedures and training or those events, safety officers, chemical safety oversight and bureaucracy, perhaps even regular mercury monitoring for the staff (if there is in fact any way to check humans, it probably doesn't stay in the blood... maybe hair samples?) There's just a lot of effort and challenge that comes together with the risk.  
> **username_0**: @M.A.Golding  I'm thinking that the answer might be that there is simply no suitable replacement for mercury, and that spinning ionic liquid mirrors are considered for future Moon-based and space-based telescopes for other reasons. fyi **I've just added liquid gallium as another option to the question.**  
> **username_0**: @JonCuster ditto  
> **username_2**: @uhoh - what I saw for the ionic liquid telescope on the moon was a need to coat it with silver to get good performance from 500nm into the near infrared. Yes, deposit silver onto the spinning mirror in situ on the moon. Doesn’t make me think the reflectivity of ionic liquids is ver good…  
> **username_0**: @JamesK So better not use those highly toxic ones! Heavy metals are also "often highly toxic" and yet gallium is pretty safe stuff to handle. Some ≠ all.  
> **username_0**: @JonCuster it looks like you're working your way to an answer; great!  

---

## 2 answers

### Answer by username_3 on 2023-11-25. Score: 7

Ionic liquids are not reflective.

[![enter image description here](https://i.sstatic.net/GLYgY.png)](https://i.sstatic.net/GLYgY.png)

*An ionic liquid being poured, from )[source](https://www.chemistryworld.com/podcasts/ionic-liquids/4012785.article)( 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](https://www.google.co.uk/books/edition/Minerals_Yearbook_Metals_and_Minerals_20/zyKCNW0BHm8C?hl=en&gbpv=1&pg=SA47-PA26&printsec=frontcover) 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.

> **Comments:**
> **username_5**: 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.  
> **username_3**: 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.  
> **username_0**: I always imagined (for absolutely no good reason whatsoever it seems) that ionic liquids were shiny with reflective surfaces, like liquids with [solvated electrons](https://chemistry.stackexchange.com/q/72264/16035). 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!  

---

### Answer by username_2 on 2023-11-28. Score: 6

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.](https://academic.oup.com/mnras/article/476/2/2075/4839011) (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.


---

