Mercury mirror telescopes: What are they good for?

[ready29003]

I understand they use a rotating mercury mirror telescope to help see space junk. Why? What is the advantage in a mercury mirror telescope over normal mirror telescopes? Cheaper?? More accurate?
Are such mirrors better for small nearby things rather than distant objects?

Is there any limit to how big a mercury mirror could be? Would it be at all possible to make…let’s say..a football size mercury mirror..so we can really see some stars?

[Revtim]

I read an article on this recently, perhaps in Discover magazine. It's *really* expensive to create and polish the mirror of a standard non-mercury telescope; a rotating disk of mercury is much cheaper. That's the big advantage.

The biggest drawback I remember is that they cannot be aimed, and are always pointed straight up. Also, mercury is quite a hazardous and toxic material.

[Squink]

Liquid Mirrors
(http://www.abc.net.au/science/k2/moments/s64751.htm)Scientific American's article describes the Large Zenith Telescope (http://www.phys.psu.edu/~cowen/popular-articles/sciam/1299musserbox6.html), which is to be used to examine space junk.
The link to the LZT project in the SciAm article is no good, but the LZT project page (http://www.astro.ubc.ca/lmt/lzt/) is current.

[bonzer]

From this interesting history of mercury mirrors (http://home.europa.com/~telscope/LMT.txt):


LMTs will not replace glass optics, but they do have some advantages. Costs per unit aperture are far lower. Many complex systems are eliminated, including elaborate mirror support systems, tracking systems, and tube stiffening designs. Seeing conditions are optimal at zenith. LMTs can have excellent quality optical surfaces, very low scatter, very low or very high f ratios, and a precisely controlled variable focus.

In the case of the NASA Orbital Debris Observatory, the fact that you're only looking straight up isn't that much of a disadvantage. Good observational data on space junk is sufficiently hard to get that you can live with that constraint. But according to this page (http://www.nso.edu/sunspot/Nodo/) the telescope was due to have been closed down last year, though with bigger versions to follow on the same site. For a description of a visit to the facility see here (http://www.taas.org/times/nl/dec01nl.html#leadstory).

As for limitations on the size, the obvious one is that rotating anything as big as a football field at the sort of speed required is a big enough problem. A more optical limitation is that, if you want to keep the mirror fairly flat, such a large telescope would have a very long focal length. The secondary mirror might become a non-trivial limitation.