The news about Arecibo got me looking into big telescope installations recently. I’m curious, why are there two super huge radio dishes (Arecibo and FAST), but nothing that size for optical or IR or something?
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These dishes act as mirrors that focus light onto one point. This is fairly easy to build for radio waves because simple things like concrete to nearly perfect reflections of radio waves. As others have mentioned, radio’s long wavelength allows a lot of error for “good enough” focusing.
But yea, turns out building a perfect mirror 300 meters across is incredibly difficult. Typically what we do instead is build individual hexagonal mirror sections then assemble these hexagons on site to create the telescope itself.
And there are a few giant visible light telescopes in the works. None are as big as Arecibo, Arecibo is gigantic still incredibly difficult to do with visible light, but we have the aptly named Extremely Large Telescope in Chile and Thirty Meter Telescope in Hawaii.
A little further from earth we are also getting the James Webb Space Telescope launching soon hopefully.
In order to make a good reflector you need to make sure that any flaws/imperfections in your reflector are smaller than the wavelength of the radiation you’re trying to reflect, otherwise the flaws in your reflector will screw up your image.
Radio waves have wavelengths that are measured in meters. Building something to that degree of precision is fairly easy to achieve on a REALLY large scale.
Visible light has a wavelength measured in nanometers, which ends up limiting the practical size of visible light optics to maybe a few meters.
There is also a secondary concern – how much does the atmosphere mess up your observations? You mention IR in your post. A lot of the IR wavelengths are absorbed by earths atmosphere, and may of the ones that are not tend to be wavelengths that are emitted by the earths atmosphere.
1. ~~Building optical telescopes on Earth generally doesn’t get you very good results; the atmosphere distorts the image and isn’t fully transparent (hence why the Hubble is on a satellite)~~ My information was true in the past, but not so much these days. See replies below ↓
2. It’s simply not feasible or practical to build a lens that’s 100m+ across. The largest is 10m (https://www.spaceanswers.com/astronomy/whats-the-largest-optical-telescope-in-the-world/).
3. The reason we have such large dishes is to resolve the much lower frequencies of radio waves – generally, when building a parabolic reflector, the larger the aperture, the lower the frequencies you can resolve. Light is electromagnetic radiation in the order of THz (i.e. trillions of cycles per second), but radio waves are KHz (thousands) or MHz (millions).
I watched a YT video of some scientist talking about this. He said the hard part is not melting down all the glass into a big puck, it’s the cooling and tempering of such a monstrosity. Molten glass is exceedingly delicate when cooling; if you cast a cup and leave it on an open table to cool, it will burst. You have to put the glass in a kiln and bring it down slowly, over the course of hours. The biggest castings in the world for the biggest telescopes in the world take several hours to melt, but almost a year to cool! Then after that, you have to be able to lift and support the thing, which requires feats of engineering, and the more mirror you have, the more work it takes to grind and polish the whole thing; I heard that the error on one side of these mirrors to the other is less than 1/4 the width of a human hair, and it has to be, or the mirror is unusable. And then you have to move one of the most expensive and delicate objects ever made by man across the planet and up a mountain to the observatory.
Think about that. Even if we could cast a single piece of glass the size of Arecibo, *HOW WOULD YOU MOVE IT INTO PLACE*?
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