Behind the lens of NASA's self-adapting ISS space telescope
A hunk of space glass this ain't
Analysis Funding cutbacks and an arguably anti-science fiscal policy haven't stopped exciting new projects emerging from NASA's Jet Propulsion Laboratory (JPL) in La Cañada Flintridge, California.
We spoke to Dr Michael Sievers from JPL about one such project, cutely named the Optical Testbed and Integration on ISS eXperiment (OpTIIX). This novel optical telescope will be rocketed up to the International Space Station (ISS) in pieces, assembled by orbiting robots, and attached to the greenback-guzzling space station.
While in its early days, the ISS struggled to justify its enormous cost, experiments such as OpTIIX are now putting it to good use as a testbed for future large astronomical observatories.
OpTIIX is actually a collaboration between the JPL, Goddard Space Flight Center, Johnson Space Center and the Space Telescope Science Institute, with blastoff of the finished components planned for early 2015.
Although it's being billed as a science experiment, OpTIIX could revolutionise the way space telescopes are built and used.
The idea is to create an agile, adaptive telescope that will view the heavens via six "adaptive optics" primary mirror segments. These will focus light onto a secondary mirror that sends the light to two cameras located under the primary mirror. If it works, the self-adapting, morphing telescope will be far more cost-effective than the traditional approach of a single, monolithic-hunk-of-glass mirror.
Dr Sievers told The Reg: "The process of building large single-glass mirrors is very complex and expensive, and moreover requires demanding precision to achieve desired optical performance. By contrast, adaptive optics systems employ less stringently ground mirror segments that are actively controlled to achieve optical performance. Each segment can be tipped, tilted, and moved up and down, collaborating to resolve astronomical objects."
Each mirror can also be locally deformed to achieve the desired wavefront alignment.
The challenges being faced by the OpTIIX team are pretty extreme. The platform upon which the telescope is mounted won't exactly be stable: the ISS zips through low Earth orbit at around 17,500 mph while shaking like a Diamond Jubilee carriage crossing a cattle grid.
The telescope will also be affected by thermal variation, and of course the ISS orbital motion will affect line of sight. To compensate for these variances, Dr Sievers explains, "[t]he telescope control system receives inputs from star trackers and gyros that provide precise position and motion rates; laser metrology which precisely measures the distances between the primary mirror segments and the secondary mirror; and wavefront sensing cameras that measure optical prescription – just like optical prescriptions for eyeglasses."
The control system will be written in C and C++ by a team of 10 developers. The system will run on a high-performance computer with sophisticated flight software that performs the optical and line-of-sight measurements. When a NASA engineer describes a computer as "high performance" or flight software as "sophisticated", you know they mean it.
Many of the tests will be derived directly from the model. The importance of this aspect can't be underestimated, as the operational environment – the speeding, vibrating ISS – is at odds with the telescope's chief requirement: stability.
As Dr Sievers explains: "We have to keep our camera on a planet or star for many minutes to get a good picture. This means that the camera needs to keep a lock on that target to within a fraction of a camera pixel so that we don't get a blurry image. We use star trackers, gyros, and GPS to make sure we are pointing to the right place, and a 'Fine Guidance Camera' measures our position jitter due to vibration. We compare the image centers to each other to see how much jitter there is and then send controls to a small, fast mirror to null the jitter."
With such precise requirements, the unit tests, driven from a SysML design, must prove that the system is entirely bug-free before it's shipped to the edge of space for our robotic overlords to assemble.
The design itself is validated as it's drafted up. The use cases and SysML models are developed hierarchically, and each level of the model is validated with the level above it to make sure it's complete and consistent.
OpTIIX is exciting because if the "adaptive optics" experiment is successful, it'll pave the way for much larger, free-flying telescopes in the future. While it's chiefly an experiment and a precursor to a new age of agile astronomy, OpTIIX will also be put to good use during its service lifetime, performing real science while bolted onto the rumbling ISS.
Dr Sievers added: "We will do real science with this telescope. The goal is to prove that it works well enough that we can do a free-flying and larger version some day." ®