Smart Optics
in Space
A selection of Smart Optics development initiatives was introduced in the previous issue of Astronomy and Geophysics. The Smart Optics Faraday Partnership (SOFP) promotes connections between those developing and exploiting this technology. The forum was a chance for all concerned to review the state of play in this area and to review the propspects in space science instrumentation projects.
Smart Optics can be defined as "optical systems, components, and technologies that dynamically adjust". Alan Greenaway, of Heriot-Watt University, introduced this concept, including a brief explanation of how adaptive optics (AO) is used as a near-standard feature of ground-based optical astronomy to improve seeing.
This technique is not likely to be useful in the same way to improve the quality of space-based observations through the atmosphere since any small-scale variations in the refractive medium are associated with the observed object rather than the immediate environment of the observing instrument. However, the technology associated with AO and other smart optics techniques can still be employed both directly and indirectly. Of direct relevance are: the ability to use light-weighted optical components; performance enhancement in space borne optical systems; and the ability to reduce the complexity of support systems, such as thermal control, on the spacecraft. Indirectly, one can anticipate: improvements in the characterisation (i.e. verification) of optical systems and components during preparation; improvements in laser-based manufacturing; and potentially, optical communications techniques for spacecraft communications.
Exemplifying the needs of space engineers, Berend Winter, of UCL's Mullard Space Science Laboratory (MSSL), surveyed the constraints of the space environment, and in particular some study results for the proposed Solar Orbiter mission. This could be regarded as an extreme thermal case, since its trajectory will vary between 0.21 and 1.21 AU and hence the total solar energy received at the spacecraft would vary by at least 18 times; at perihelion, the spacecraft will receive 31 kW/m2. Earth-orbiting satellites, the more usual case, can expect to see temperature variations in their structures between -180 °C and +100 °C, as a result of solar eclipse (periods between 1.5 hours and 30 days). Despite this, typical science instruments require a thermal stability of ±10 C.
To help maintain optical alignment of instruments under these conditions, and to reduce the mass of instrumentation, carbon-fibre based materials are preferred (CTE is ~ 0.5 - 2 ppm/K) in current designs. However, this class of material has drawbacks relative to metals, not least of which is the problem of shrinkage due to moisture loss over time. Optical systems, such as telescopes and spectrometers, in such structures which could adapt accordingly would allow engineers additional design freedom.
Particulate cleanliness, chemical cleanliness (outgassing), radiation exposure, and electromagnetic compatibility concerns at the spacecraft level all flow down to optical component requirements. In general, space engineers are very conservative, and demand proven high reliability components. Spaceflight heritage is vital for any technology. This means that smart optics systems will only be preferred when conventional techniques can't meet the requirements. An interesting discussion arose as to whether a smart optical system should, if for example the control system were to cease operation, degrade gracefully into a usable conventional optical system, or not. There was not widespread agreement on this issue, which evidently requires further understanding.
Resistance to adoption among possible users may deter the development of smart optics technology, and the community should perhaps prioritise those applications where it is absolutely required, or when the performance benefits are overwhelming.