Imagine if you will, a scientific outpost on the far side of the Moon. No light pollution, no radio interference, nothing to block your magnificent view of the heavens. You need a telescope and you intend to do some serious imaging, so you want it to be as large as possible. How do you get it? You could ship it up from Earth, at a cost of thousands of dollars a pound. If lunar industry is developed enough, one of the near-side factories could gather up a few tons of regolith, separate the glass out and melt and grind a lens for you. Or,build your mirror out of a spinning liquid. On Earth, the largest liquid mirror is the Large Zenith Telescope operated by the University of British Columbia in Canada. It's 6 meters across, 20% larger than the world famous Palomar reflector in California. However the Large Zenith Telescope didn't even cost $1 million to construct - only 1/6 of the cost to build Palomar in 1948 dollars. Today, $1 million is only a few percent of the cost to construct a normal 6 meter telescope.
Another benefit of the liquid mirror is that it's technically simple. It needs only remain horizontal to local gravity, and to spin smoothly to maintain a smooth reflecting surface. On the Earth's surface, the edge of a 4 meter telescope spins at 3 miles per hour. With gravity on the Moon 1/6 that of Earth, the required spin rate would be even lower. The mirror can only point straight up, so no need for heavy and complicated systems for moving the mirror. It's aim can be adjusted by using some of the same techniques as the Arecibo radio telescope in Puerto Rico. If the telescope is place in polar crater that never receives any direct sunlight, you would not need to cool the mirror, which makes it ideal for infra-red astronomy.
From NASA Science News