The telescopes I work on all sit on Terra Firma, mapping the cosmic microwave background. This light is a thermal afterglow of the big bang, but it has patterns hidden in it that hint at how the laws of physics work at the highest conceivable temperatures.
However, those telescopes must look through an atmosphere that also glows with thermal radiation, has wind and other sources of turbulence. That turbulence is why distant city lights or stars twinkle, but it's a source of noise in our measurement that we must average down. And that averaging process takes years even at the best observing site in the world (the South Pole)!
Of course, if you could get your telescope above the atmosphere, then you could avoid that nuisance and do your measurement much faster. This is one reason why satellite telescopes like Hubble or JCMT are so useful. But they're expensive and take years to come together. It turns out that you can get a similar experience, albeit for only 2-3 weeks, with a a high altitude balloon, which is the strategy behind SPIDER.
SPIDER is quite literally the balloon version of Keck Array (one of the South Pole experiments I work on), using nearly the same detectors and optics, but optimized for measurements at 130,000 feet (that's 4 times higher than your typical commercial aircraft flies). I've worked on those detectors quite a bit and am rooting for this to go well. Part of our funding for the detectors came from Gordon Moore, of Intel and Moore's law fame, and imitating his genius, we kept the entire sensor design planar so we could print lots of these things the same way Intel does with microchips. Here is picture of one of the 1200 pixels that SPIDER will fly with, and one of the actual sensors at right:
Here's a time-lapsed video of the team assembling the payload over two days (it takes far longer than 2 days; this is just part of the process):
This is currently going on a few miles outside of McMurdo station in the Long Duration Ballooning facility that NSF and NASA co-run. I think the SPIDER team should re-load this to youtube with some sort of 80's montage music. Or as a scientist on a competing project mischievously suggested, perhaps the theme from Benny Hill.
The payload looks like a six-shooter, and each of those barrels holds a camera. Once assembled, they fill the car-sized instrument with liquid helium which will keep it cold enough to operate (more on that later) for the 2-3 week flight. Here's the path that SPIDER's predecessor experiment BOOMERANG took on one of it's flights a decade ago