Structure of Liquid MirrorsAbout mercury...
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 withErmanno 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:
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.
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.
Safety: When handling mercury, or cleaning the mirror, it is essential to
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
The principal results are the following:
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
75.8% at 400nm, 77.2% at 600nm, 77.6% at 800nm
mN/m (at 20 deg C, in air)
Last updated: 1999/11/13
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: