Liquid-mirror telescope

[1] The concept was further developed by Ernesto Capocci (1798–1864) of the Naples Observatory (1850),[2][3] but it was not until 1872 that Henry Skey of Dunedin, New Zealand, constructed the first working laboratory liquid-mirror telescope.

Another difficulty is that a liquid-metal mirror can only be used in zenith telescopes, i.e., that look straight up, so it is not suitable for investigations where the telescope must remain pointing at the same location of inertial space (a possible exception to this rule may exist for a liquid-mirror space telescope, where the effect of Earth's gravity is replaced by artificial gravity, perhaps by propelling it gently forward with rockets).

The red arrow represents the weight of the parcel, caused by gravity and directed vertically downward.

The green arrow shows the buoyancy force exerted on the parcel by the bulk of the liquid.

It is the vector sum of the forces of weight and buoyancy, and acts horizontally toward the axis of rotation.

It is the centripetal force that constantly accelerates the parcel toward the axis, keeping it in circular motion as the liquid rotates.

The buoyancy force (green arrow) has a vertical component, which must equal the weight

Therefore, the green arrow is tilted from the vertical by an angle whose tangent is the quotient of these forces.

The equation of the paraboloid in terms of its focal length (see Parabolic reflector#Theory) can be written as where

for the numerical value of the rotation speed in revolutions per minute (RPM),[9] then on the Earth's surface, where

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.

Such a liquid base would be covered by a thin metallic film that forms the reflective surface.

It will only work in space; but in orbit, gravity will not distort the mirror's shape into a paraboloid.

The design features a liquid stored in a flat-bottomed ring-shaped container with raised interior edges.

The light gathering power of a Rice telescope is equivalent to approximately the width times the diameter of the ring, minus some fraction based on optics, superstructure design, etc.

Therefore, 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 shifting electrons across the CCD at the same speed as the image moves; this tactic is called time delay and integration or drift scanning.

[12] 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 telescopes highly reduced in their direction of view.

Since mercury vapor is 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.

In the Large Zenith Telescope, the mercury mirror and the human operators are housed in separately ventilated rooms.

At its location in the Canadian mountains, the ambient temperature is fairly low, which reduces the rate of evaporation of the mercury.

Recently Canadian researchers have proposed the substitution of magnetically deformable liquid mirrors composed of a suspension of iron and silver nanoparticles in ethylene glycol.

Usually (except if the telescope is located at one of the Earth's poles), the two rotations interact so that, in a frame of reference that is stationary relative to the local surface of the Earth, the mirror experiences a torque about an axis that is perpendicular to both rotation axes, i.e. a horizontal axis aligned east–west.

The point in the sky at which the mirror is aimed is not exactly overhead, but is displaced slightly to the north or south.

The amount of the displacement depends on the latitude, the rotation speeds, and the parameters of the telescope's design.

On the Earth, the displacement is small, typically a few arcseconds, which can, nevertheless, be significant in astronomical observations.

If the telescope were in space, rotating to produce artificial gravity, the displacement could be much larger, possibly many degrees.