The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for April, 2016

SPIE 2016

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.

Megacam Baffle Internals - Rendered

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.

Tom

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Mistakes Were Made

Posted by Tom Benedict on 01/04/2016

If I ever write an autobiography I’m calling it “Mistakes Were Made”. It’s an accurate statement that can be interpreted with a straight face or with a smile, and it does a good job of summing up the parts of my life that make decent stories. Let’s face it: It’s fun to read about other people’s mistakes!

Last Friday a storm system rolled through that brought with it real thunder and lightning. As strange as it sounds the weather in Hawaii doesn’t lend itself to thunderstorms, so I knew this was a rare and wonderful event, and a unique chance for me to record thunder rolling across the sky. As broken as I was (and am!) as the day darkened I grabbed my SASS and stand, slung my sound bag over my shoulder, shoved two trash bags in my pocket, and headed out.

I live across the highway from a ranch, so getting away from intruding sounds is as straightforward as crossing the highway and walking until I can’t hear anything any more. Time was of the essence, so I walked as quickly as my neck and back would let me. All the while the thunder was coming from every part of the sky, rolling from horizon to horizon, and stirring up echoes from the nearby mountains. It was perfect! When I deemed I’d gone far enough I set up the stand, took off my backpack and found…

You know when you see a school kid with their book bag, and it’s unzipped and stuff is hanging out and you tell them, “Zip up your bag! You’ll lose something!” and they, one way or another, flip you off?

I found that my backpack was unzipped, and had been from the moment I left home. In the dark I hadn’t checked, and hadn’t seen. By then I had crossed a stream, walked at least a mile through tall grass, and stepped around countless cow patties. With a sinking heart I checked to see what was missing. To my intense relief the only things unaccounted for were my three contact mics – stuff I’d built myself. I was lucky! But I still kicked myself for losing gear.

I had to set those thoughts aside and get busy if I wanted to record thunder, though. So I pulled out my recorder and cables, and started hooking everything up. Just as I finished plugging everything in, the first of the rain hit.

What I’d assumed was a lighter patch of cloud upwind of me turned out to be a rain line. I pulled the cables back out of my recorder, zipped everything up in my bag, and pulled a trash bag over it and over the SASS. I’ll just wait this out, I thought, It can’t be that much rain!

It was that much rain, and it just kept getting harder. I couldn’t even hear the thunder any more because of how loud the rain was against the grass and rocks. Without having hit the record button even once, I picked up my gear and started the long, slow, wet slog back home.

Good news is I found one of my contact mics along the way! The next day I went back and found a second. The only one missing is the one I made with an alligator clip for clipping onto fences and the like, which only took me an hour or so to make. As dumb as my mistake was, the cost in the end wasn’t all that high. Lesson learned.

But I still wish I’d recorded some of that thunder!

Tom

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