Posted by Tom Benedict on 08/04/2016
Turns out I’m going to the SPIE Astronomical Telescopes and Instrumentation 2016 conference in Edinburgh, Scotland, from June 26th through July 1st! I’m holding out hope my neck will have healed by then and I can do some kite aerial photography while I’m there. Either way I’m planning on bringing sound gear so I can play in a new environment and come home with some fresh sounds to play with.
I’m still writing the manuscript for my paper, and the poster hasn’t even made it to the sketch pad yet, but the research has been underway for a couple of years. As boring as this may sound, the paper is all about black stuff.
Whether you’re building an optical experiment on a workbench, modifying or building a telescope for your own use, or working at an observatory, at some point you’ll need some way to control stray light. The bulk of stray light control happens in the design: adding baffles and stops, planning light traps, or controlling the output cone of any light source. But inevitably least one of those will involve making some surface black.
The question is: What does “black” mean? And once you define it, what materials fit the definition?
From the standpoint of stray light control, “black” means “doesn’t reflect much”. But what’s “much”? 5%? 1%? 0.01%? And what range of wavelengths do you care about? CCDs can detect light well outside the range of human vision. What about infrared arrays like the H2RG? They can see well past human vision into the near edges of the thermal infrared.
At first glance the obvious answer is that it shouldn’t reflect anything at any wavelength. Unfortunately such as beast doesn’t exist.
The next best thing would be that it shouldn’t reflect much at all (0.01%?) at all the wavelengths we care about. For the sake of making it a tractable problem let’s say from the UV to K-band infrared, or about 250nm out to 2500nm. The problem is that 0.01% is tough to hit. It’s possible to get close using an exotic surface like carbon nanotubes, but such surfaces are fragile and tough to impossible to clean without damaging them. Let’s face it: astronomy is an outdoor sport. We try to close the domes when it rains, but things do get dirty. They do occasionally get wet. Eventually they’ll need to be cleaned. Ultra fragile exotic surfaces simply aren’t practical much of the time.
But maybe we don’t need 0.01% reflectivity. If we’re careful with our design we can make sure that any stray light has to reflect off of at least two surfaces. Let’s say we’re even more careful with our design and make it so light has to reflect off of at least three. That was the design rule I used when I designed the optical baffle for the Megacam Wide Field Corrector, shown here without its outer skin.
If light has to bounce off of at least three surfaces, that means each reflection has the opportunity to absorb light. If each surface reflects a whopping 5% of the light, by the time you’ve bounced off three surfaces you’re down at the 0.01% level. 5% on three surfaces is a heckuvalot easier to deal with than 0.01% on one surface, and it brings our material selection into the realm of common off-the-shelf materials.
Now that we have a good working definition of a practical, workable “black”, that leads us to the second half of the question: What materials fit the bill?
At work I have the good fortune of having a spectrophotometer at my disposal. I’ve used it to scan instrument filters, optical assemblies, even the odd pair of sunglasses. But I’ve also scanned a bunch of stuff that looks black. In the process I’ve run into some surprises. Some years back we made a wrap for one of our instruments in an effort to keep light from LEDs and motor encoders from encroaching on the beam. Out of our own ignorance we used a synthetic black cloth that looks extremely black to the human eye, but turns out to be blindingly reflective above 690nm. We hadn’t fixed the situation at all. We’d made it worse! We replaced the wrap with a different fabric that’s absorptive out past 1100nm (the limit of what a CCD can see), and that solved it.
Over the years our growing catalog of things that are and aren’t black has come in handy. We’ve added paints, papers, tapes, and bulk materials like plastics. We figured it was time to share.
Of course we’re not the only ones doing this. Two years ago a group from Texas A&M published a paper at the SPIE Astronomical Telescopes and Instruments 2014 conference on exactly the same topic. They covered a number of metal treatments – anodizing, black electroless nickel plating, oxide treatments, etc. – in addition to a number of paints and a handful of other materials. But as it turns out our catalogs have almost no overlap, so our paper will be a good follow-up to theirs.
I’m in the process of finishing off the last of our scans over the next few weeks, and finishing the manuscript over the next month. Then it’s on to poster time!
Now I just need to figure out a way to make a visibly engaging poster that’s mostly… black.