Astronomy Buff

Liquid Mirrors: The Future of Large Aperture Telescopes

by Tony on June 25th, 2007

In a paper that will come out in this week’s issue of the journal Nature, researchers proposed a blueprint for constructing a 20 - 100 meter (66 - 328 foot) diameter objective mirror using reflective liquid.

The authors propose using liquid mirrors in a telescope to be built on the moon because:

The major advantages of liquid telescope mirrors include ease of shipping, assembling and maintenance, “which are far easier than for a solid mirror,”

The great thing about using mirrors in telescopes, as opposed to lenses, is that you only need one surface of the glass (the part that holds the reflective coating) to be high quality. The rest of the glass just goes to providing structural support. In large apertures, this is a problem because fused silica and pyrex glass is extremely heavy and you need thick pieces of it to provide support for the main surface to hold its shape.

Glass also flows a little bit at high angles, changing the shape of the reflecting surface.

I don’t know how true this is, but I once heard a story (probably an urban legend, or ‘dome’ legend, heh, heh) about an astronomer firing six shots from a revolver into a primary mirror of a telescope (I think it was MacDonald Observatory at the University of Texas). The glass did not shatter, rather the bullets became embedded in the primary where they reside today.

The moral of that story is that very large pieces of glass behave more like a liquid than a solid.

Large aperture refractors (the kind of telescope that has a lens on the end) are exponentially more expensive to make because you need an extremely large piece of glass completely free of optical defects. This is VERY expensive to produce and so there aren’t any refractors larger than, say, 1 meter. For this reason, large aperture telescopes are always reflectors.

Still, large diameter mirrors are hard to make, in order to focus the light to a point, they need to be of a certain shape. In a newtonian style reflector with a flat secondary mirror, that shape needs to be parabolic - that’s a hard shape to produce in a large mirror, lots of pain-in-the-ass grinding.

For most large reflectors, that shape is usually spherical, a relatively easy shape to make with grinding, but it’s still not trivial with large aperture telescopes.

Just ask Perkin-Elmer how easy it is to make a large primary mirror. They made the 2.4 meter primary for the Hubble Space Telescope and once it was put in orbit, they found it had the most basic of optical defects: spherical abberation, a distortion produced when a spherical mirror does not focus light to a point.

So, what’s so great about liquid mirrors?

• You don’t need to grind them. Spinning a liquid produces the correct shape needed to focus light to a point. All you do is pour and spin, the spinning reflecting surface turns into a parabolic surface.
• You can easily change the focal length of the optical tube assembly. The faster you spin, the deeper the curve, resulting in a shorter focal length and wider field of view. Spin slower and you get a longer focal length and higher magnifications.
• They are easy to transport, just put the reflecting liquid in a can and pour when ready.
• Very large diameters can be produced this way, over 100 meters is possible.
• Depending on the liquid, they are way cheaper than glass mirrors.

What’s not so great about them?

• The reflecting liquid most commonly used right now is mercury, as shown in the above picture. That’s nasty stuff.
• Tilting telescopes with a spinning mirror is tricky. It’s like having a spinning gyroscope at the bottom of your telescope tube. This presents lots of pointing and guiding issues. Most of these designs just point straight up so you’re limited to what’s overhead at the time.
• Developing cheap reflecting liquids that stay liquid in space is still a technological issue. Right now, ‘ionic salts’ are being developed that will stay liquid at cold temps but they aren’t very reflective.

I think the biggest use will be for space-based telescopes because of the transportation advantages. I’ll have to read the paper to learn about the details of what these guys have come up with.

Here’s the NASA press release.
Photo Credit: NASA

Technorati Tags: liquid mirror telescopes, lunar telescope, NASA

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POSTED IN: NASA, telescopes