What is an LMT?

Liquid Mirrors

UBC/Laval 2.7m LMT

Table of Contents

History

The The idea of using a rotating liquid to create a perfect paraboloid was originally proposed by Sir Isaac Newton but the very stringent requirements, in particular on the speed of rotation and leveling, prevented any serious attempt to build a prototype before the second part of the nineteenth century. The first published account of a working LMT was provided by Skey (1872), who constructed a 35cm telescope, and made the first detailed calculations of the focal length in terms of angular velocity.

By 1909 an optical physicist, Robert Wood, published a series of papers describing his success using a 51cm LMT. He carefully analyzed the main sources of vibrations, and was able to obtain photographic trails of stars. He could even resolve double stars having separations as small as 2.3 arcsec. In order to suppress the ripples on the surface of the mirror, he experimented with the effect of oil layers. In spite of his success, Wood decided to abandon the LMT because he felt that its restriction to zenith observations made the astronomical applications too limited.

The current era of LMT research began with Ermanno Borra's landmark paper (Borra 1982). He reassessed the details of the theory and practical limitations of LMTs as true astronomical tools in light of technological advances since Wood's time. Over several years, the technology was developed successfully to produce a 1.5m diffraction-limited LMT. Then, a fruitful colaboration began between Paul Hickson at UBC, and Borra. Hickson designed the 3m-class LMs, and built several 3m-class LMs for UBC, NASA, and UCLA. Using this design Luc Girard (Laval) built a 2.5m diffraction-limited LM. Hickson and collaborators are now building the 6m LZT located at the UBC Liquid-Mirror Observatory .

Originally developed for astronomical research, LMs soon proved to be useful in other fields of science, such as LIDAR science, optical testing and search for space debris. Certainly, LMTs do not replace the classical instruments of astronomical research, but they are cost-effective and they could avantageously find a niche in dedicated surveys.

For more history, see the excellent review article and related publications by Brad Gibson.

Structure of Liquid Mirrors

The technology of LMs is relatively simple. Three components are required:

- a dish containing a liquid reflecting metal
- an air bearing on which the LM sits
- a drive system.

The dish

In order to reduce the amount of mercury required, the surface of the dish matches the required parabolic shape as closely as possible. The structure of the dish depends on the diameter of the mirror. For mirrors smaller than 2m in diameter, a simple dish spincast with epoxy or other polymer-resin has sufficient stiffness. For many reasons, including a large coefficient of thermal expansion and large mass density, epoxy cannot be used for mirrors larger than 2m. The figure below shows the design for mirrors in the 2m to 4m class.

Structure of the 2.7m mirror (Hickson et al. 1993)

It's made of a foam core sandwich, stiffened by a Kevlar skin. A central tube sits on a bottom plate both made of Al. The upper surface of the mirror is spincast with an elastic resin about 1cm thick. The final deviation from a paraboloid is less than 0.1mm.
For the next generation of LMs, i.e. mirrors larger than 4m a stiffer and lighter design is being developed at UBC. The new design will provide a higher stiffness-to-weight ratio and better long-term stability for such large mirrors.

The air-bearing

The optical quality of the reflective surface depends mainly on three parameters: the Hg-air interface, vibrations and the vertical alignment of the rotation axis. The third parameter implies that wobbling should be controlled accurately. A search of available technology (late 80's), showed that only air-bearings provided an angular stiffness, low friction and a precision compatible with LMTs requirements. The photo shows the bottom of the 2.7m LMT with a close view of the air-bearing.

The drive system

This component of the mirror has undergone a major evolution in its design since the first LMTs. The drive system has to be regular (10E-6) and must not transmit vibrations to the mirror that would disturb the surface.The first designs used a synchronous motor linked to a precise oscillator. The mirror was driven by a belt over a pulley attached to its base. This design was sufficient for a laboratory LMT but it was not adapted for night observing conditions. The system suffered from moisture and temperature variations.

In the 2.7m UBC/Laval LM, the belt drive has now been replaced with a direct drive based on a similar system developed by NASA for their 3-m LM. The motor stator is mounted directly on the air-bearing base, and the rotor is attached to the rotating spindle. An optical encoder senses the angular velocity. This new design is simpler and more reliable.

For more information on the structure of LMs:

Hickson, P., Gibson, B.K. & Hogg, D.W. (1993). "Large Astronomical Liquid Mirrors", Publ. Astron. Soc. Pacific.,105, p. 501-508.

Hickson, P., Borra, E.F., Cabanac, R., Content, R., Gibson, B.K. & Walker, G.A.H. (1994), UBC/Laval 2.7m Liquid Mirror Telescope, Astrophys. J. , 436, p. L201-L204.

About mercury

Like all other heavy metals, mercury is potentially harmful for health. Although, metallic mercury itself has not been proven to be hazardous, comprehensive information can be found in medical publications on the toxicity of mercury vapours and metabolized mercury oxides. In particular, mercury oxides are thought to have an effect on the nervous system on long time-scale. After years of exposition to mercury vapours a series of symptoms can appear, from severe neurologic troubles, insanity, to Parkison disease. In addition to hazardous effects, pure mercury can be easily contaminated when mixed with other chemical elements. It forms amagalms with almost all metals, and consequently looses its reflective properties. For all these reasons, one has to address the problems of mercury storage and handling.

Some data on mercury

Atomic weight	200.59
Density	13.691 kg/l (temp. var. = 13.595 x (1 + 1.8144e-4 x T))
Melting point/ Boiling point	-38.84 deg C/ 356.73 deg C
Electrical Resistivity	94e-8 Ohm/m
Reflectivity	75.8% at 400nm, 77.2% at 600nm, 77.6% at 800nm
Surface Tension	435.5 mN/m (at 20 deg C, in air)

Storage: Even moderately pure mercury still has good reflectivity. Therefore we don't need special glass containers for mercury storage. We use plastic laboratory bottles of 500 ml. capacity. Mercury must be stored in double or even triple containers, in order to contain a spill if a bottle leaks or breaks. The plastic bottles containing the mercury are stored in large plastic bins which are themselves located inside a sealed spill container.

Safety: When handling mercury, or cleaning the mirror, it is essential to avoid direct contact with mercury. Respiratory masks approved for mercury vapour, polymer gloves, Tyvek lab suits and boots are used. The mercury vapour levels must be monitored at all times.

A study was made of the mercury vapour concentations at the UBC/Laval 2.7-m Liquid-Mirror Observatory under various conditions. The principal results are the following:

Vapour concentrations inside the observatory depend strongly on the degree of ventilation. Even a small amount of ventilation, is effective at reducing the vapours to acceptable levels. (the Canadian legal norm is 0.05 mg/m3 on the basis of 8h/day, 5d/week exposition without protection)
During normal operation of the observatory, in which the roof is open, vapour concentrations are below 0.1mg/m3 for all but the first hour after implacement of the mercury on the mirror.
The mercury is stabilized by a thin surface layer of oxide which forms naturally. Although pure mercury doesn't oxide alone, oxidation is catalysed by water vapour and impurities on time-scale of a few hours. This surface layer reduces the rate of evaporation of the mercury by more than five orders of magnitude.
After the oxide layer is established, vapour concentrations within the observatory are inversely proportional to the ventilation fan capacity. The width of the oxide layer increases constantly with time, and after two weeks of operation, the evaporation rate is negligible.
During startup and the first few hours of operation ot the mirror, vapour levels in the observatory can be minimized by preventing ventilation. High vapour concentrations are thereby restricted to a thin layer of air above the mirror resulting in a low evaporation rate.

Hickson, P., Cabanac, R., Watson, S.E.M., A study of Mercury Vapour Concentrations... (priv. com.)

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Last updated: 1999/11/13