Liquid Mirror Telescope on Mars

A large telescope using a spinning bowl of mercury as the mirror. (Read the full article)

"Ok, there are two reasons why the description of the 'leaned over' liquid mirror telescope need not be viewed as a mistake. Firstly it the most obvous one. The web of supports has some other form of optics in it that bend the angled light straight down upon the rotating liquid mirror. I think the correctional optics used in Hubble are a reasonable example of this although they are applied to abberations rather than dirction. Second and more interesting, what if the Martian had exactly what was described, a way for the bowl to be spinning, at an angle to level and still generate a reasonable parabola. This could be done by taking into account some forms of surface tension and adhesion as well as the use of optics above the mirror (perhaps active ones) that would correct for the abberations that a non-perpendicular to gravity spinning might generate. I don't know that I could design it but I'll bet that it COULD be done. I just thought of one other possibility. The spinning liquid is very viscous and the turntable is, at first, horizontal, it spins up to speed, forms the mirror and then the liquid is frozen and the mirror is tilted into place still spinning. When finished the mirror is put back to horizontal and the substraight allowed to become liquid again so that it can be sanitized and cleaned and such for use later."
(Victor Nazarian 2/8/2006 2:04:46 PM )
"Amazing comments Victor. You are really creative :-) It was similar insights that made me visit this site to leave a comment. A few ideas I had beyond all the ones left by Victor (I had thought of the freezing one and the reflection one) is that Mercury being a metal, will respond to a magnetic field. Having an "adaptive magnetic field" that corrects for abberations caused by a tilted light (you don't have to worry about tilting - just let the tilted light fall on a flat spinning platform) could work. Another option, with an appropriate adaptive magnetic field, we could change the direction of the net force on the liquid to point at the approprate angle, and then spin it. Thirdly, just take the distorted image formed from reflected light coming in at an angle to this flat mirror and un-distort it by using a complementary small mirror ground to the right shape to correct it. Note this second mirror can now be very small since the distorted image would be in a tiny area. "
(Gunjan 2/8/2006 5:08:52 PM )
"Great comments! I just wanted to respond to Victor and his idea of a "frozen" mirror. One of the ideas for liquid mirrors that I think went unused was the idea of using some sort of polymer, or even regular glass in molten liquid form. You spin it up to speed and then let it harden at speed. It would then retain its parabolic shape when cooled; you might have to polish it a bit, but you'd be that much closer at the start. If you've ever tried making a telescope mirror from a flat blank, you'd realize this would be easier. I think that the effort failed because of stresses in the glass. BTW, there are some good comments on liquid mirror telescopes in the Lunar LMT article."
(Bill Christensen 2/8/2006 6:04:13 PM )
"These are some imaginative ideas, and I thought of some of them myself. Unfortunately, as far as I can tell, they are all flawed in some way. The whole point of an LMT is to make a large mirror cheaply and in a form that's easy to transport to the lunar surface. Several gallons of mercury is easy to transport. Another approach, besides a large glass mirror, is to create a large mirror out of many smaller hexagonal mirrors that are adjusted under computer control. This is done in several very large telescopes today, and is less expensive than creating a large mirror out of a single piece of glass. The point of the LMT is that it's even cheaper and easier to do than this. The idea of reflecting the light down to the vertically oriented LMT was indeed the most obvious and at first to me seemed to be the most practical. I too thought of the comparison to the Hubble corrective optics, which use a small corrective mirror placed at the focal point of the large mirror. However, there is a hidden flaw in this plan/comparison. The Hubble adaptive optics mirror is placed into the light path AFTER the light is collected and focused by the large mirror, so the corrective mirror can be small. The mirror that would redirect the light down to the liquid mirror is, by necessity, placed into the light path BEFORE the light hits the large mirror. If you use a small mirror to reflect the light down to the larger mirror, then you are only collecting the amount of the light corresponding to the aperture of the smaller mirror, and the large mirror will receive no more light than what is reflected by the small mirror, making the large mirror useless as a light-gathering bucket. If you instead make the flat light-deflecting mirror as large as the liquid primary mirror, to correct the aperture issue, then you're back to having a large mirror that's expensive to make and transport, and that defeats the purpose of the LMT. You might as well make the primary out of glass at that point, since any technique you apply to the redirecting mirror can be applied to the primary. That makes the redirecting method a no-go. The frozen liquid idea seemed okay until I thought about it some more. The problem with this is two-fold. First, the freezing process does not leave the surface of the mirror unaltered. It changes the surface and in so doing, the perfect reflective qualities of the liquid are not maintained. Second, if it were that easy, they'd make mirrors that way now. Think about it. Mirrors blanks are made by taking molten glass (a liquid), spinning it in a parabola-shaped mold, and then letting it harden, or "freeze", exactly like you're describing for the LMT. Unfortunately, when the glass hardens, the surface is no longer perfect, and the blanks have to be polished to get a usable, high-quality surface. In the scenarios described, I see no provision (nor any practical way) to polish this spun-up, frozen mirror on-the-fly. And once it hardens and is polished, you've essentially made a permanent mirror that never needs to be returned to its liquidj state, again obviating the whole idea of the LMT. Also, corrective optics are expensive, difficult to make, and must be custom-made as a one-of-a-kind mirror that corrects the primary's flaws. This defeats the purpose of making the LMT cheap and easy to build, and dispenses with the idea of using corrective optics to cancel out the major flaws in an unpolished mirror. Also, the LMT's mirror isn't simply distorted when you tilt it. If you tilt by much more than a few degrees, the centrifugal force no longer counteracts enough of the gravity to hold the liquid in place and the liquid simply pools to the side of the bowl (which is now serving as the bottom) and sloshes around down there like water in the bottom of rotating drum. So corrective optics won't fix this, either. Surface tension also is not promising, as the surface tension is going to be the same regardless of its place on the mirror, and to adjust the optics you're going to need different forces in different places on the tilted mirror. The only promising idea that I read about in these posts seems be the idea of using a magnetic field to adjust the shape of a liquid mercury mirror. However, if we're doing that, then spinning the mirror is simply counterproductive. Better to simply use the magnetic field to shape the mirror than have to take rotational forces into account and having to modify a moving target. There are only two problems I see with this idea. First, we don't have anything even approaching the technology to manipulate magnetic fields on the small scale necessary to hold the liquid mercury in such a precise shape, let alone the technology to maintain that finely focused magnetic field over such a large area. Second, this would require massive amounts of energy to run, not to mention massive amounts of computing power. The bright spot here is that hundreds of years into the future, technology may reach a point where this is possible, feasible, and inexpensive, and as far as I can tell there are no fundamental obstacles that keep this from being done, as there are with the other options. However, I wonder if such an advanced civilization will even need an LMT. They'll probably be able to cheaply make large, solid mirrors on-site or have even better ways of achieving the same objective, I think. It's kind of like a medieval person speculating about computerized, automatic reloading crossbows, and not releasing that while that will be possible in the future, gunpowder is going to make that idea obsolete."
(Joe Brooks 2/9/2006 12:57:49 PM )
"Um...someone should point out that mercury is not magnetic! The only magnetic elements are iron, cobalt, and nickel. Only a metal that contains one of these (like steel, for example) can be magnetic. Try sticking a refrigerator magnet to a lead fishing weight or an aluminum window sill. (I don't recommend experementing with mercury itself because it's pretty toxic.)"
(Dan 2/12/2006 7:52:55 PM )
"Some questions for those of the right knowledge: 1) Anyone here ever tried spinning a bowl of mercury up ? 2) If you spin a bowl of liquid up to a good speed, would it not still spin when its placed 90 degrees from it original starting point (eg. start it whilst perpendicular to the ground, then keep it going when parallel to the ground)? I'm guessing that to spin mercury up is going to need some heavy-duty friction (otherwise the bowl will spin whilst the mercury sits there) and to put friction into the game will cause horrendous ripples on the surface. Not worth bothering with IMHO. I haven't tried point two out. I'm guessing that once spun up to the correct speed, you can face the liquid anywhere you like. Cheers, ausdrac [at] gmail [dot] com ."
(AusDrac 2/13/2006 12:50:54 AM )
"This is a very interesting thread and all have good points. The easiest solution is not to tilt the LMT at all but to leave it perfectly level and move the imaging unit above the spinning mercury mirror (like the tremendous radio telescope located in Puerto Rico) and incorporate distortable lensatic adaptive optics in the light path between the mirror and the imaging unit. There are two choices for this adaptive lens: one that is made of a distortable matrix or one made of two or more solid lenses that can be shifted against each other to correct for the off-axis distortion caused by the parabola of the liquid mirror being off center. Aiming of the telescope in right ascention would not necessarily have to be far in any direction as planned observing sessions would make use of the moon's slow rotation to aim the telescope. The only drawback is that if one is to take in the entire heavens, several telescopes would need to me set up along the meridian as the practical north-south movement of the imaging unit would be limited to something in the area of 20 degrees from zenith. Energy consumption of the entire aparatus would be far less than that if it were installed on earth due to the lower gravity of the moon (1/8 that of earth). The less gravity, the fewer RPMs required to form the mirror. Just a few bones to chew on :) Jeff the Great"
(Jeff the Great 3/4/2006 9:10:19 PM )
"Jeff has a good point about simply making several telescopes and placing them at a variety of locations on the moon, for different views. Or collapse & move one telescope on a monorail system or lunar "road". Presumably, some form of nano-tech will be used to construct such facilities from lunar materials. Concerning slight tilting adjustments of the mirrors, use software to correct the image after it is captured. 180-degree camera systems do that now. A hemispherical lens is used on the camera, and then software removes the distortion to properly view any portion of the image. A scanning system could constanly monitor the lens for distortions to be corrected. Research for Star Wars beam weapons has resulted in a great deal of progress in the field of adaptive optics, and the computing methods needed to implement it. At the current rate of growth in computing power, this should not be difficult to do, within a few years. For that matter, it may become practical and economical to make considerably less-perfect solid mirrors, and just use software to correct for the known imperfections. "
(Rick O. 7/31/2006 8:25:00 PM )

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