Here's a cool idea:

Angel, a leading astronomer at the University of Arizona, is proposing an enormous liquid-mirror telescope on the moon that could be hundreds of times more sensitive than the Hubble Space Telescope.

Using a rotating dish of reflective liquid as its primary mirror, Angel's telescope would the largest ever built, and would permit astronomers to study the oldest and most distant objects in the universe, including the very first stars.

One thing I don't get, though. How would you point it? It doesn't seem like it could be angled up very much out of local horizontal without both messing up the surface shape and requiring higher spin rates. That means that it's only going to point at the area of the sky corresponding to the current local vertical at the lunar location (presumably they'd want to be on the far side to avoid light interference from the earth). And even then it would take a month to see the whole sky on a swathe, with no ability to go back and take a second look until twenty-eight days later. But if you had a bunch of them scattered all over the far side, you could have a pretty flexible system. This also means you'd have to have data relay satellites, either a constellation in lunar orbits, or a halo around L-2, if you're going to be able to do earth-based astronomy with it.

As the article notes, this would probably require hand assembly to a large degree, which could provide a lot of motivation for lunar bases and lunar construction workers, and potentially even affordable, as long as you don't let NASA get involved. It's the kind of project that demands a lunar infrastructure, for communications as noted above, and habitats. It would be a great way (and perhaps incentive) to industrialize the moon. NSF should tender some bids, and see what the private sector comes up with.

[Via John Hood]

You point it the same way that the Aricibo radio telescope is pointed -- you move either the secondary mirror, or you move the detectors that are at the focus of the mirror. (If it's a folded telescope, which I would assume it would be, then it's going to have a secondary mirror...and probably others too.)

I thought that worked (sort of) for Arecibo because it's a radio telescope, and a hemispherical antenna rather than parabolic, and not as fussy. In any event, you're still only going to be able to see only half the sky at anyone time, even if you can do that, with a monthly tour.

Telescopes of this general kind have been built on Earth, for example the Hobby-Eberly Telescope (HET) in Texas (note that HET has a conventional mirror and can still rotate in azimuth, though).

I remember reading back in the 1980s about efforts to build large terrestrial mirrors made from rotating tables of liquid mercury. When gently rotated on a turntable, the mercury would form a very good mirror. Like the lunar proposal, the mirror couldn't be directly aimed except perhaps using secondary optics. However, it would sweep a swath of the sky every night. On the moon, a series of very large mirrors at different latitudes on lunar farside could sweep major portions of the sky every month. This could be useful for a variety of purposes, such as NEO searches, whole sky catalogs, and perhaps spectral analysis. Earth-based liquid mercury telescopes of at least 2.5 meter diameter have been built and used in Canada as well as the 6 meter Large Zenith Telescope (LZT). They electronically combine the images. Since the moon only moves about 12.8 degrees per day (as opposed to 15 degrees per hour on the Earth), you could gather a lot of light.

From this article:

Building on this experience (and on some skills I had picked up in my spare time constructing experimental aircraft), in 2005 my coworkers and I fabricated a 6-meter mirror for an instrument we dubbed the Large Zenith Telescope (LZT). The aim of this project was to develop and perfect liquid-mirror technology on a scale competitive with the largest conventional astronomical telescopes. Of course, the inability to look in any direction other than straight up (the zenith) means that such telescopes will never replace their conventional counterparts, which can point to just about any part of the sky.

But this lack of flexibility is not quite so limiting as you might think. For a large number of scientific studies, fixed pointing is not a hindrance. If the aim is to determine the statistical properties of a large number of objects, say distant galaxies, it often does not matter what part of the sky you probe. One finds distant galaxies everywhere, and the zenith is as good a place as anywhere else to look. Indeed, it is the best place to direct your telescope because it offers the least amount of air in the light path and hence the least amount of atmospheric absorption, scattering and image distortion.

For such research programs, liquid-mirror telescopes serve just as well as conventional telescopes but don't cost nearly as much. The LZT was built for less than $1 million—an order of magnitude lower than what one would spend for a conventional telescope of comparable size. So if some of the funds slated for the construction of telescopes were channeled into instruments that use liquid mirrors, astronomers doing this kind of research could get considerably more observing time.

Keeping lunar dust out of those mirrors could be a challenge, though.

You don't point it. You just look straight up, but if you're goal is to study the earliest galaxies formed, this limitation isn't much of a problem. Consider the Hubble Deep Field. They intentionally pointed the thing at a 'blank" portion of the sky and imaged something like 1,500 galaxies.

How will the sun affect it? Will you be offline 50% of the time? Will the heat cause damage to the mirror?

How will the sun affect it? Will you be offline 50% of the time? Will the heat cause damage to the mirror?

The most likely location studied for the lunar LMT would be the South Pole, where it could make very long exposures of a deep field sky. A polar crater would also be shielded from the Sun. You also need to keep spacecraft activities clear of the mirror; otherwise the Moon has no wind to carry dust.

Transit telescopes were discussed in the First Lunar Outpost studies in 1992. We even kicked around the idea of liquid mercury mirrors to make them more portable and easier to construct. UV astronomy was one of the major applications for the Lunar Transit Telescope. There were also some concepts brainstormed for massive numbers of small, cheap antennas to be strewn all over the lunar farside to create a huge radio telescope.

Paul beat me to the punch by ref'ing the HET (now being duplicated in the Southern Hemisphere by SALT, the South African Large Telescope). Use it as a spectroscope rather than an image-creating telescope as such -- but advanced image-processing techniques are beginning to render conventional focusing obsolete; see Pictures Only a Computer Could Love and Photon folding for imaging in nonfocusing telescopes.

Pete Worden talked about this at one or another conference I attended.

Deep (deep, deep, deep) field viewing was the objective as I recall and even if the telescope scope it couldn't be moved, the data collected from looking that deeply into one point of sky would still be extremely useful to astronomers.

After all, if we look "out there" far enough it all curves back to the Big Bang anyways.

(This last sentence may be widely inaccurate however since my being corrected on this point would be eduatcional, I will let it stand.)

L Riofrio wrote:
"...otherwise the Moon has no wind to carry dust."

True but the moon likely has a "terminator storm" (Nasa link) which could be a problem for such telescopes (or anything else) and which they might need protection from (I guess less so if placed at the lunar poles).

Bill - Not wildly off; I believe the limit of visual observation would be encountered at seeing the Universe at an age of 300,000 years (see "Recombination: The Universe Becomes Transparent" at this link.)

Various Others - It takes a surprising amount of dust to significantly degrade the light-gathering power of a telescope mirror. And the effects of the "Terminator Storm" could be confined to the very edges of the mirror by controlling its electrostatic properties.

In general, lots of great comments in this thread.

One tiny flaw: the astronomers don't want it.

Given the immense price, there's much better astronomy that can be accomplished for far less.

Keep in mind that Angel isn't really an astronomer, he's a telescope designer. He wants to build things, not use them.

How will the sun affect it? Will you be offline 50% of the time? Will the heat cause damage to the mirror?

I don't think so. A well focused parabolic mirror reflects the light that strikes perpendicular to the mirror to the prime focus. Sunlight striking at other angles shouldn't reflect into the prime focus. Depending on the mirror's latitude, there could be times when sun intrusion is a factor but that probably wouldn't last for more than a few hours at a time, not anything like 50% of the time.

You also need to keep spacecraft activities clear of the mirror; otherwise the Moon has no wind to carry dust.

You'd also need to keep human activities to a minimum in the area. Humans on the moon tended to kick up a fair amount of dust. Perhaps it would be possible to lay down some form of matting to keep the dust down when humans were in the area performing maintenance, swapping sensors, etc.

I'd think that mercury's not inconsiderable surface tension would be a problem, but maybe it's not only a problem at the edge of the mirror, which is a tiny part of the total surface.