News from the Liquid Mirror Laboratory
Ermanno F. Borra, Université Laval

Mercury LMs The liquid mirror work has progressed well both in the laboratory and in observatory settings. Mirrors having diameters as large as 3.7 meters have been thoroughly tested (Applied Optics in Press). We understand what it takes to make a high-quality liquid mirror. The NASA 3-m LMT has produced over 100 nights of high-quality astronomical data obtained by Paul Hickson and Mark Mulrooney (NASA). The data has been analyzed and subsets published. Figure 1 shows a cluster of galaxies observed with both the 3-M LMT and the Palomar Schmidt. Additional images can be found at the NODO web site (http://www.sunspot.noao.edu/Nodo/nodo.html).
	Several LMTs have now been built, or are under construction for Astronomy, the Space Sciences, Atmospheric Sciences, industrial applications (e.g. UWO, UBC, UCLA, NASA, Belgium, the International LMT project). Liquid mirrors are now a fact and are used in scientific and industrial applications. The mercury LM project is a success story. At Laval, we have closed the chapter on technological developments of Hg mirrors and have moved on. The demonstration that LMs can be tilted promises to make LMTs far more versatile and interesting. Tiltable LMs Low cost is what makes mercury LMs interesting but they are severely limited by the fact that they cannot be tilted. This restricts them to an astronomical niche (surveys). At Laval, we are now working on a new generation of LMs that can be tilted. Work is done in collaboration with Prof. Anna Ritcey of the Laval department of Chemistry. Our goal is to produce large inexpensive LMs that can be tilted by at least thirty degrees. To understand the principle of tiltable LMs one first must realize that, to work, LMs must be tiltable. Consider the inevitable alignment errors with respect to the vertical: If the mirror cannot be tilted at all, it will not work! Indeed, optical shop tests show that mercury LMs can be tilted by several arcseconds (Borra et al. 1992). A little thought shows that one needs higher viscosity to achieve higher inclinations. Follow an element of liquid on a tilted LM as it rotates. At the high point, it falls towards the axis of rotation; half a turn later it falls away from it.
Figure 1: It shows a sample of the kind of data obtained by the NODO LMT. It compares the same region of sky observed with the NODO LMT (top) and the Palomar 48-inch Schmidt (bottom). The field is 5 arcminutes X 7 arcminutes and centered at 12h 08 m and + 33 degrees (2000.0 coordinates). Courtesy Paul Hickson (UBC) and Mark Mulrooney (NASA).

In the reference frame of the mirror, the element feels a periodic force having the period of rotation of the mirror. If it has not moved significantly within half a period, it never will. I have known this for a long time but could not do anything because of a lack of credible high-viscosity, high-reflectivity liquids. But things have changed because of Metallic Liquid Like Films (MELLFs), a colloidal liquid-like metallic film independently discovered by chemists (Yogev & Efrima 1988, Gordon, McGarvey & Taylor 1989). MELLFs exhibit the optical properties of the metal (e.g. silver) and, at the same time, striking fluid behaviour. The films flow and heal rapidly upon rupture. MELLFs have therefore attracted our attention as excellent candidates for the preparation of liquid mirrors. The approach involves the deposition of metallic colloids (e.g. Ag, Au) on a viscous support liquid.

Figure 2: Interferogram of a MELLF surface	Figure 3: Interferogram of an aluminum coated optical quality glass mirror

Some of our early work has been published in ApJ Letters (Borra, Ritcey & Artigau 1999). The article demonstrated two critical technological steps: a) Optical quality MELLFs b) We have tilted LMs that used silicone oil by 1 degree. A theoretical extrapolation law, confirmed by our experiments, shows that it should be possible to tilt LMs by thirty degrees with a reflective liquid having a few times the viscosity of glycerin. We have continued work on the silver deposition process, obtaining spectacular results. We have produced reflective first surfaces on viscous liquids. Unlike the early MELLFs that were trapped at the interface between two liquids, these MELLFs reside on top of a liquid. An interferogram of such a MELFF is shown in Figure 2. It can be compared to an interferogram of an aluminized glass mirror (Fig.3). Figure 4 shows an interferogram of one of our early MELLFs Letters (Borra, Ritcey & Artigau 1999). Comparison to Fig. 2 shows considerable improvement. We have successfully deposited MELLFs on viscous liquids. Our latest MELLFs are much easier to make and handle than the earlier ones.

Figure 4: Shows an interferogram of one of our early MELLFs. This MELLF is trapped at the interface between two liquids, while the MELLF in Fig. 2 floats on the surface of a liquid, a far better configuration.

At this stage, the project is a success story and we are concentrating on improving the qualities of the colloid and finding ways to scale up the process to handle large surfaces and test liquid mirrors. Our 3 years goals is to produce and thoroughly test in the laboratory a 2.5-m mirror tilted by several degrees. We will produce highly-reflectivity, high-viscosity liquids. We will develop simple and inexpensive techniques to make the liquids that can easily be ported to 10-m class mirrors. We intend to demonstrate a tilted 1.4-m LM to carry out stellar observations on campus.

A tilted LMT will have a large cost advantage over a conventional glass mirror. The cost advantage will not only come from the mirror itself, since MELLFs are very easy to make and use inexpensive chemicals, but also from the mount, since a MELLF mirror will be considerably lighter than a glass mirror. Consider that an optical telescope is seldom used lower than 45 degrees from the zenith. The advent of large inexpensive LMTs that can be tilted by 30 degrees or more will have a major impact on Astronomy.

REFERENCES

Borra, E.F., Ritcey, A.M., & Artigau, E., 1999, ApJ Letters 516, L115

Borra, E.F. et al. 1992, ApJ 393, 829.

ILMT: http://vela.astro.ulg.ac.be/themes/telins/lmt/index_e.html

Ermanno Borra received a physics degree in from the Italian University of Torino, and his Ph.D. in Astronomy from the University of Western Ontario in 1972. For the next two years he served as a Carnegie postdoctoral fellow at the Hale Observatory. Since 1975 Borra has been a professor of physics at Laval University, and he is also a member of the Canadian Network for Space Research. Borra revived and has been the forerunner in LMT research since 1982 when he became interested in cosmology during a sabbatical at the University of Arizona.