Talk:Liquid-mirror telescope

For the history section:

• Olsson-Steel, D. (1986). "A Note on the History of the Liquid Mirror Telescope". Journal of the Royal Astronomical Society of Canada. 80 (3): 128.
• Gibson, B. K. (1991). "Liquid Mirror Telescopes - History". Journal of the Royal Astronomical Society of Canada. 85: 158.--Stone 08:40, 30 July 2007 (UTC)

On merging with Mercury Mirror, I would suggest that this article is more general and any mercury mirror entry be merged into this entry. Any liquid mirror has the same characteristics, and having two entries is kind of silly. Increasingly, people are using non-mercury substances (for instance, liquid epoxy) that achieve the overall purpose of being flowable, but it has the added benefit of solidifying and then being pointable. JustAnyone (talk) 20:38, 26 December 2007 (UTC)

Why only straight up?

It seems pretty obvious that you could simply freeze the liquid. Selection criteria for the material would be...

• does not form big nasty crystals as it cools
• thermal expansion can be well-matched to a container, and not too extreme
• melting point is reasonable
• cheap
• takes and keeps a shine, or can be plated to do so

Plain old lead solder hits most of that pretty well. I don't know if you can coat it with something (aluminum, rhodium, silver...) that has high reflectivity.

72.40.45.79 (talk) 03:43, 18 August 2008 (UTC)

There are a variety of methods that can be used to create a large, reflective paraboloid. The problem with using them for a telescope, however, is that it can't be just approximately paraboloidal, it has to be paraboloidal to within optical flatness. The liquid mirror solves this problem because the paraboloid is its neutral energy surface, so any small imperfection is automatically self-correcting (with a caveat about dust floating on mercury). As soon as you freeze it, you lose that property. If the freezing process causes any distortions at all -- even on the scale of wavelengths of light -- your mirror is now no better than one made by a variety of cheap processes. Even if the freezing process causes no distortions, the frame (which formerly only needed to hold the bottom of the liquid pool with an accuracy of a mm or so) now needs to hold the surface as rigidly as the supports of a glass mirror, or once again, it loses its excellent optical properties. In short, it is the quality of being liquid which enables this method to produce an optically perfect large paraboloid cheaply. If it was solid, it would be as expensive as a glass paraboloidal mirror. -- Securiger (talk) 03:10, 9 November 2008 (UTC)

Aiming Mirror

What is the problem of combining a flat refective mirror (which has its point of rotation on the optical axis of the liquid mirror)that can be pointed in almost all directions, to reflect the light onto the liquid mirror? I would imagine creating a flat mirro is quite cheap and easy compared to curved shapeLife is short, but the years are long! (talk) 15:06, 18 March 2011 (UTC)

This is such an obvious idea and yet I couldn't see it anywhere in the article. Producing an optically flat surface is much easier than making a mirror. and would allow the telescope to be aimed anywhere. Stub Mandrel (talk) 14:07, 3 April 2015 (UTC)

I too wondered why this is not done. In fact, coming to this article, I presumed an aiming mirror would part of it. It would seem essential for the telescope to be of any use at all. Only aiming up? Really?--23.119.204.117 (talk) 05:31, 19 February 2016 (UTC)

Adding a large, exactly plane, steerable mirror to the telescope would add greatly to its complexity and cost. It wouldn't be much cheaper than a conventional telescope. DOwenWilliams (talk) 15:31, 19 February 2016 (UTC)

Merge suggestion

Mercury mirror is clearly a subset of this article, itself a merge of an earlier version with liquid mirror telescope. The three topics are practically identical, but liquid mirror is both the most general, and the most complete. However, it currently lacks the theory description found in mercury mirror. -- Securiger (talk) 03:02, 9 November 2008 (UTC)

the derivation of the parabolic shape of homogeneous rotating liquid body is not satisfying. shouldn't one start to derive the shape by letting the hydrostatic force acting on a small piece of liquid equal the centripetal force. I obtained the same result using this method. I'm however not familiar with the wikipedia math syntax.

Consider an infinitissimale small cube of liquid with dA as the surface parallel to the rotational motion. dr is the side of the cube being perpendicular to the rotational motion. r = distance from centre of rotation, rho = density of liquid, g = acceleration constant, h = is the heigth of the liquid above the motion-parallel surface of the cube which is closest to the rotation centre, dh is the is the small heigth change towards the other motion-parallel surface of the cube, Omega is rotational speed.

rho * g * dh * dA = rho * dr * dA * r * Omega^2

dh / dr = r * Omega^2 / g

integration for a series of little cubes at the bottom of the container yields:

h - h0 = r^2 * Omega^2 /(2 * g) —Preceding unsigned comment added by 193.190.253.144 (talk) 07:44, 1 October 2010 (UTC)

Base materials

One method to reduce the amount of mercury or gallium required is to fill the cylinder with a plastic resin then spin it until the resin sets. The liquid metal can then be the same thickness all across the top of the plastic parabolic shape. Alternatively, the surface of the plastic can be finished to optical flatness and a perfect parabola then metalized. Bizzybody (talk) 08:31, 7 October 2010 (UTC)

Rice mirror?

There is reference to a Rice liquid mirror, and a link but there is no mention of why it is called a Rice mirror in either article. Perhaps a Rice University project or someone else named Rice? Jascal (talk) 07:13, 13 October 2010 (UTC)

De-merge suggestion!

There is a problem with the redirect of mercury mirror to this article, so I propose a disambiguation article. There is an antique mercury mirror in which 75% tin and 25% mercury were used to give the mirror its shine and a blue tinge. This is seen, for example, at the Hall of Mirrors (Palace of Versailles). See the website for restorer of the Versailles mirrors, http://www.miroir-ancien-vincent-guerre.com/en/antique-mirrors/antique-mirror-restoration.htm and you will see what I mean.

Perhaps a new article is in order about the antique version. I like to saw logs! (talk) 06:59, 14 January 2011 (UTC)

Presumably mercury was widely used as a backing - as in the poem:

Little Willie, from his mirror,
Licked the mercury right off,
Thinking in his childish error -
It would cure his whooping-cough.

At his funeral his mother,
Said rightly to Mrs Brown;
'Twas a chilly day for Willie -
When the mercury went down.

I forget the author, but it was in Faber's Verse and Worse I believe. Rich Farmbrough, 21:45, 5 March 2011 (UTC).

Centripetal vs. centrifugal force

There is a common misconception concerning mechanics in the section "Explanation of the equilibrium". The diagram shows that rotating disc with reflective fluid froms a parabolic concave due to centripetal force conterbalancing the gravity. Centripetal force is a common misconception (I was tought that in the first year of Hyrostatics ad fluid dynamics and rigid body mechanics classes on my University) - it simply doesn't make any sense, even the diagram is false - the vectors don't cancel themselves out. If centripetal force was true and it existed in relity the liquid should actually from a blob in the middle.

I've changed 'centripetal' to 'centrifugal' and I hope someone will correct the diagram. — Preceding unsigned comment added by LechuCzechu (talk • contribs) 20:45, 21 January 2012 (UTC)

Decomposition of the gravity force applying to mercury in a liquid mirror. gravity -> centripetal + compression => flotation counterbalances compression. --A Pirard (talk) 12:17, 14 October 2013 (UTC)

The centrifugal force itself is generally a misconception, especially ever since Sir Isaac Newton explained to the world that the Moon is revolving around the Earth due to a gravity force directed towards the Earth (centripetal) and not towards the outer Universe (centrifugal). There is a centrifugal force in that system, but it's applied to the vacillating, wobbling Earth. Same for someone falling out of a roundabout not because of a centrifugal force but because of a lack of centripetal force to keep him turning on it. He falls in a straight course, tangentially and not radially, watch the ticket man jumping out.

The misleading aspect in the diagram is that the gravity does not compose with the "normal force" but that it decomposes into centripetal and what I call compression force. Flotation ("normal") and compression forces are counterbalancing forces.

Otherwise said, that compression force is finally transmitted to the wall of the bowl and produces an opposed distension, canceling force in that bowl, with null sum intervening nowhere.

See diagram on this right.
--A Pirard (talk) 14:22, 14 October 2013 (UTC)--

The diagram does not represent a static situation. The liquid is rotating around a vertical axis which is in the plane of the diagram. The centripetal force keeps the parcel of liquid accelerating toward the axis, as is required to keep it moving in a circular path. Since the parcel is accelerating, of course the forces on it do not cancel out.

The original author of this piece decided to think in terms of a non-rotating frame of reference. Traditionally, most people think in a rotating frame in which the material is stationary (and the rest of the universe whirls around in circles). In the rotating frame, there is a centrifugal force which would be represented by a blue arrow pointing to the right in the diagram. The three vectors then cancel out. I agree that, for many people, this would be an easier representation than the one that the original author used. However, physicists prefer inertial frames of reference, and scientific purists follow the same preference.

I find it highly implausible that you were taught in university that centripetal force does not exist. I suggest you dig out your old textbooks and check this. Far more likely, you'll find they say that centrifugal force does not exist. I've seen many books that say this, and it's true in a non-rotating frame of reference. If you swing a weight around on the end of a string, then apart from gravity, the only force acting on the weight is the tension in the string, which is centripetal. The idea that there's a centrifugal force comes from the mistaken notion that there must be something to balance the string tension. But in fact there's no such thing. The tension is not balanced, and therefore it continuously accelerates the weight toward the centre of its circular motion.

In a truly inertial frame of reference, gravity also does not exist. An orbiting space station behaves inertially, so the astronauts in it experience "zero gravity", even though they are still quite close to the earth.

"It's all relative."

DOwenWilliams (talk) 02:23, 22 January 2012 (UTC)

Article title in singular

I think the title of the article should be Liquid mirror telescope in singular, see WP:PLURAL. Ulflund (talk) 11:12, 4 July 2012 (UTC)

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Requested move 17 December 2019

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this discussion.

The result of the move request was: Moved. (non-admin closure) – Ammarpad (talk) 16:22, 24 December 2019 (UTC)

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Liquid mirror telescope → Liquid-mirror telescope – MOS:HYPHEN. Mikhail Ryazanov (talk) 01:02, 17 December 2019 (UTC)

This is a contested technical request (permalink). — Martin (MSGJ · talk) 04:58, 17 December 2019 (UTC)

Both the hyphenated and hyphenated versions both seem to be used by reliable sources — Martin (MSGJ · talk) 04:58, 17 December 2019 (UTC)

I guess, you meant "hyphenated and non-hyphenated"? Since MOS:HYPHEN tells us that compound modifiers should be hyphenated, we should use the variant that conforms to the Wikipedia guidelines instead of the one that contradicts them. — Mikhail Ryazanov (talk) 05:21, 17 December 2019 (UTC)

• Support per MOS. This is an issue for style guides, so let's follow ours. Dekimasuよ! 03:50, 24 December 2019 (UTC)
• Support: this is obviously a case of a compound modifier. HandsomeFella (talk) 07:51, 24 December 2019 (UTC)

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The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.