Plastics for use in spaceborne optical sensors

Monolithic space borne optical sensor lens mount, baffle and housing
Conducted by the German firm, ZARM Technik AG, this activity has explored the feasibility of plastics for use in space borne optical sensors.Moreover, the highly satisfactory results obtained from the project are not merely confined to optical sensors - but are applicable to other structural elements found in spacecraft.

The Challenge

Applications for spaceborne optics can essentially be divided into three major categories:
1. navigation cameras,
2. star-trackers,
3. rendezvous and docking sensors.All the above applications share similar requirements. They must :
  • operate in the 400 to 900 nm range and produce high quality, low distortion images;
  • work well over a wide temperature range (typically -40°C to +80°C); and
  • be able to withstand the space environment (UV, radiation, atomic oxygen) without any serious degradation over a lifetime of up to 15 years.
The optical systems required for such applications are generally housed on the exterior of the spacecraft, and subjected to a harsh radiation environment – with exposure to electrons, protons, heavy ions and UV, all being especially high for the outer lens and any other exterior surface, such as the baffle or housing.Furthermore, outgassing of any material - which could condense on a (cooled) detector or other optical surface - could greatly degrade the performance of the optical system, and therefore has to be kept to an absolute minimum.The requirements on the optical barrel and other (lens) mounting structures are driven by: the coefficient of thermal expansion (CTE) of the optics; temperature range; and vibration & shock requirements, which are typically up to 25g RMS and 2000g (0.5ms) respectively.Furthermore, any exterior surface should be electrically dissipative to minimize the risk of surface charging, and any structure close to the field of view or otherwise near the optical path, such as the baffle or the lens mounts, should be black.In addition, the materials should not only have a low density, but also high mechanical strength with good general mechanical and thermal properties. In this way, ultra-light parts can be made, with high mechanical stiffness and a resonance frequency above 140 Hz.To meet these exacting requirements, four different areas of special interest were identified, during the course of this activity, as crucial to successful implementation of a plastic, optical sensor:
  • the transmissive optical elements, such as lenses, diffractive optical elements etc.
  • the lens barrel & mounting structures of optical elements, to replace those made of titanium,
  • other ’optical’ surfaces, such as the baffle, which reduce the amount of stray light,
  • and last but not least, the housing and other mechanical structures.

Achievements

Experimental results in comparing the coefficient of thermal expansion for various polyetheretherketone (PEEK) compounds
The work performed during this activity has demonstrated that plastics are indeed capable of meeting the requirements for structural materials, (a) for space applications in general, and (b) for the specific and stringent requirements for ’space-borne optical sensors’ - at least with respect to the non-optical parts.The greatest challenge was – as expected – the resistance against atomic oxygen (AO), for which at least one suitable way of protection has been demonstrated in the course of this activity.One of the largest benefits in the use of ’plastic’ - or rather, complex polymer & material compounds - lies in their adaptability.Experiments carried out within this LET-SME funded project, have confirmed that it is not only possible to customise the mechanical, thermal, or electrical properties of the material. Results obtained have demonstrated that, with the right mixture of elements – and within the limits imposed by physics – it is possible to adapt the radiation shielding of the housing with respect to the requirements of the sensor or the mission.The benefits which result, with respect to total cost, can be substantial – and even moreso when the reduced costs for assembly, integration and test (AIT) are, in addition, taken into account.
The various zones inside an injection moulding machine
For example, in the case of a lens mount the cost for the production of a typical lens barrel, using current technologies, is around 2000€ per piece,Assuming a typical mission development scenario, typically 4 lens mounts will be required, giving a total of c.8000€.Using plastics such as those explored and assessed in this project, the recurring cost can be reduced by approx. 2 orders of magnitude, yielding a cost-per-barrel of less than 15€.The objective of this work has been geared towards space borne optical sensors. However, the use of ’space grade’ structural plastics is not merely confined to such sensors. They can also be used for many other structural elements typically found on spacecraft.

Project Details

Plastics for use in spaceborne optical sensors
LET-SME contract no.20339

  • Year of award: 2006
  • Amount of funding: €198162
  • Completion: Dec. 2008

SME Prime contractor:
ZARM Technik AG
Bremen, Germany

Cooperating research partner : Universität Bremen: Labor für Mikrozerpanung
Bremen, Germany

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