The International Liquid Mirror Telescope Project (ILMT) involves the use of a thin layer (0.5 mm to 1 mm) of mercury on the surface of a spinning parabaloidal bowl, a similar technique to the Large liquid mirror telescope. The surface of a spinning liquid takes the shape of a paraboloid that can be used as the primary mirror of a telescope. Following the suggestion that modern technology (Borra 1982) gives us tracking techniques that render liquid mirrors useful to astronomy, research and development programs were begun to assess the feasibility of the concept. Mirrors up to 2.5 m diameter were extensively tested and showed the high surface quality of such mirrors (Borra & al. 1992; Borra, Content & Girard, 1993; Ninane & Jamar, 1996; Girard & Borra, 1997). For a well tuned mirror, the root mean square values of the deviations from a perfect parabola are typically l/20. It must be noted that liquid mirrors have been tested by two independent labs (CSL and Laval). Liquid mirror telescopes cannot be tilted and hence cannot track like conventional telescopes do. To track with imagery, narrow-band filter spectrophotometry or slitless spectroscopy, one can use a technique, called time delayed integration (TDI) also known as drift scan , that uses a CCD detector that tracks by electronically stepping its pixels. The information is stored on disk and the night observations can be coadded with a computer to give long integration times. The technique has been demonstrated (Hickson & al. 1994) with a 2.7-m diameter liquid mirror telescope.

Why are we interested in liquid mirror telescopes, considering their limitations? The main reason comes from the size and cost advantages. The low cost (2 orders of magnitude less than an equivalent classical telescope) makes it possible for a small team of astronomers to have their own large telescope working full-time on a specific project. This is in practice not realistic with expensive classical telescopes. Some research projects (e.g. time consuming surveys, long term photometric monitoring programs) simply cannot be envisioned with classical telescopes but are possible with LMTs. This is particularly true for the types of research where the region of sky observed is not particularly important (e.g. cosmology).

Over 65 nights of observations have now been obtained by Hickson & Mulrooney (1997, Ap.J. Suppl., in press, paper astro-ph/9710044 available on the WWW SISSA server); see also paper by Cabanac, Borra & Beachemin (paper astro-ph/9804267 available on the WWW SISSA server) analyzing the observations obtained with the NASA 3-m telescope, demonstrating that LMTs are sufficiently robust for high quality astronomical observations.

External Links

International Liquid Mirror Telescope Project (ILMT) on the internet