Liquid mirror

Liquid mirrors are mirrors made with reflective liquids. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). The container for the liquid is rotating so that the liquid assumes a parabolic shape. A parabolic shape is precisely the shape needed for the primary mirror of a telescope. The rotating liquid assumes the parabolic shape regardless of the container's shape. To reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much cheaper to manufacture.

Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation (the electric motor did not exist yet). The concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of the Dunedin Observatory in New Zealand constructed the first working laboratory liquid mirror telescope.

Another difficulty is that a telescope with a liquid metal mirror can only look straight up, so it is not suitable for investigations where the telescope must remain pointing at the same location of space (a possible exception to this rule may exist for a mercury mirror space telescope, where the effect of Earth's gravity is replaced by artificial gravity, perhaps by rotating the telescope on a very long tether, or propelling it gently forward with rockets).

Currently, the mercury mirror of the Large Zenith Telescope in Canada is the largest liquid metal mirror in operation. It has a diameter of six meters, and rotates at a rate of about six revolutions per minute.

Explanation of the equilibrium

In fluid mechanics, the state when no part of the fluid has motion relative to any other part of the fluid is called 'solid body rotation'. When the mercury mirror has reached a state of solid body rotation, then the dynamic equilibrium can be understood as a balance of two energies: gravitational potential energy, and rotational kinetic energy. When a fluid is in solid body rotation it is the lowest state of energy that is available, because in a state of solid body rotation there is no friction to dissipate any of the energy.

The force of gravity (red), the normal force (green), and the resultant force of those two (blue)

The dynamic equilibrium cannot be understood in terms of an equilibrium of forces, for when the mercury mirror is rotating, there is an unbalanced force acting on the mercury. The force of gravity is acting in vertical direction, the surface of the parabolic dish exerts a normal force on the mercury resting on it. The resultant force of those two provides the required centripetal force.

The following discussion is for the case of the mercury mirror as it is rotating in solid body rotation.

The kinetic energy of a parcel of mercury given by the formula:

In the case of circular motion the relation v = ωr holds, hence

The gravitational potential energy is given by

E_pot. = mgh

where g is the acceleration of gravity and h is the height of the mercury's surface above some arbitrary elevation, for instance, we can set h = 0 to be the lowest mercury surface.

We set the potential energy equal to the kinetic energy to find the mirror's shape:

which is, by definition, a parabola.

Dissipation of energy

To understand the dynamics of energy it is also helpful to consider what happens when the operators of the mercury mirror stop driving the dish, in order to replace the mercury.

Let the rotating dish not be driven anymore, and let a gentle braking force be applied to the rotating dish. Friction between the dish and the mercury will tend to reduce the rotation rate of the mercury. As the mercury sags to the center, gravitational potential energy is converted to rotational kinetic energy. The conversion of potential energy tends to sustain the angular velocity. More precisely: when the mercury is giving in to the centripetal force, the centripetal force is doing work. The total amount of energy that must dissipate is the rotational kinetic energy plus the gravitational potential energy.

Conventional land-based liquid mirror telescopes

These are made of liquid stored in a cylindrical container made of a composite material, such as Kevlar. The cylinder is spun until it reaches a few revolutions per minute. The liquid gradually forms a paraboloid, the shape of a conventional telescopic mirror. The mirror's surface is very precise and small imperfections in the cylinder's shape do not affect it. The amount of mercury used is small, less than a millimeter in thickness.

Moon-based liquid mirror telescopes

Low temperature ionic liquids (below 130 kelvins) have been proposed^[1] as the fluid base for an extremely large diameter spinning liquid mirror telescope to be based on the Earth's moon. Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface.

Space-based ring liquid mirror telescopes

The Rice liquid mirror telescope design is similar to conventional liquid mirror telescopes. It will only work in space; but in orbit, gravity will not distort the mirror's shape into a parabola. The design features a liquid stored in a flat-bottomed ring-shaped container with raised interior edges. The central focal area would be rectangular, but a secondary rectangular-parabolic mirror would gather the light to a focal point. Otherwise the optics are similar to other optical telescopes. The light gathering power of a Rice telescope is equivalent to approximately the width times the diameter of the ring, minus a percentage based on optics, superstructure design, etc.

Advantages and disadvantages

The greatest advantage of a liquid mirror telescopic is its small cost, about 1% of a conventional mirror. This cuts down the cost of the entire telescope at least 95%. The University of British Columbia's 6 meter Large Zenith Telescope cost about a fiftieth as much as a conventional telescope with a glass mirror.^[2] Unfortunately, the mirror can only be pointed straight up. If it tilts from the zenith, it loses its shape. The mirror's view changes as the Earth rotates and objects cannot be physically tracked. An object can be briefly electronically tracked while in the field of view by applying a voltage to the CCD to shift electrons across it at the same speed as the image moves; this tactic is called "drift scanning." However, some types of astronomical research are unaffected by these limitations, such as long-term sky surveys and supernova searches. Since the Universe is believed to be isotropic and homogeneous (this is called the Cosmological Principle), the investigation of its structure by cosmologists can also use liquid mirror telescopes. However, research is underway to develop telescopes that can be tilted.

Since mercury metal and its vapor are both toxic to humans and animals there remains a problem for its use in any telescope where it may affect its users and others in its area. The less toxic metal gallium may be used instead of mercury but has the disadvantage of high cost. Recently Canadian researchers have proposed the substitution of magnetically deformable liquid mirrors composed of a suspension of iron and silver nanoparticles suspended in ethylene glycol. In addition to low toxicity and relatively low cost, such a mirror would have the advantage of being easily and rapidly deformable using variations of magnetic field strength.^{[3] [4]}

Notes

References

See also

• List of telescope parts and construction
• Mercury glass, a variety of reflective products having the element mercury sandwiched between two glass panes
• Mercury silvering, a technique to apply a thin layer of a precious metal to a base metal object
• Rotating furnace, used to make large glass mirrors

External links

• Large Zenith Telescope A 6 meter diameter mercury mirror telescope.