Chemical Technology March 2016

The blackest paint that sucks all light by Gavin Chait

T he technical term for it,” declare Terry Pratchett and Neil Gaiman in their astute treatise ‘Good Omens’, “is infrablack. It can be seen quite easily under experi- mental conditions. To perform the experiment simply select a healthy brick wall with a good runup, and, lowering your head, charge. The colour that flashes in bursts behind your eyes, behind the pain, just before you die, is infrablack.” I imagine they were describing Vantablack, developed by Surrey NanoSystems. There is a freaky YouTube clip you can watch if you so desire. A crumpled sheet of aluminium foil is rotated. On one side is an ordinary bit of reflective silver metal. On the other … there’s a hole in the universe. The original Vantablack absorbs 99,96 % of all light that hits it. It has now been improved to a point that its absorption index can no longer be measured since there’s no way to do so. As you watch that clip and contemplate the full haunt- ing horror of it, you’ll want to grab hold of the scientists at Surrey’s Advanced Technology Institute and demand, ‘Why? Why?’ And then you read Michael Vlasov’s blog. Vlasov is an Israeli electro-optical engineer and amateur astronomer. In a deeply detailed ‘how to’, he describes how to improve the light absorption of his Orion telescope. “A Newtonian reflector’s open tube (or any other OTA for this matter) is an attractive target for unwanted stray light, which can come from anywhere: Moon, street lights or even bright stars. This light bounces off telescope’s inner surfaces and eventu- ally enters the focuser and the eyepiece. As a result, the background lightens up and the image contrast is harmed.” The original paint in his very expensive telescope isn’t good enough. So he describes how to take it apart and carefully cover the interior tube of the scope with … paper.

Special black paper, but paper nevertheless. His paper is produced by ProtoStar and is self-adhesive flocking paper and is especially designed for telescopes. The problem is that it’s paper and so can only cover smooth and regular surfaces. Not all surfaces are like that. Gerd Neumann produces a deep-black optical paint which is effectively just chalkboard paint. They suggest – and I’ll quote this – “To improve the effectiveness of the colour, you can add a fixed amount of finely sifted sand, poppy seed or flour to the paint. After this treatment, the coating gets an extra rough finish so that even with a glar- ing reflection of the sun or a halogen lamp the surface remains pitch black. (Please try to figure out the mixture ratio by yourself. The paint should be very, very smooth in its consistency. A good starting point is a volume ratio of 1:1.)” And then there’s the problemwith space flight. The paint has to survive the intense high-frequency oscillations of the launch, not losing bits and pieces that interfere with instru- mentation, and require no maintenance for the lifetime of the orbital instrument. The Hubble Telescope, famously, suffered from a 2,2 micrometre ‘flatness’ in the perimeter of its main mir- ror. This necessitated a major repair job. It was launched in 1990 and first serviced in 1993. It’s been up there for over a quarter of a century. NASA developed a super black coating called — wait for it — ‘super-black’. The basic principle of these coatings is to use carbon nanotubes. Carbon tubes arranged tightly and vertically are not only black, but the nature of the structure means that light is absorbed into the tubes, reflected inter- nally and dissipated as heat. Very little light escapes. The problem is in creating these dense vertical nanotube structures. The process is a familiar

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Chemical Technology • March 2016