The effects of realistic surface properties on low temperature space observatories

Blake, Robert (1998). The effects of realistic surface properties on low temperature space observatories. PhD thesis The Open University.



I investigate the effects of modelling radiative heat transfer correctly on equilibrium temperatures of passively cooled telescopes. I compare the traditional diffuse approximation with a surface exhibiting the directional properties of gold, both with an emissivity of 0.03. I also modeled surfaces with much lower, more realistic emissivities.

Directional surface properties are derived from values of optical constants at room temperature since there exists a dearth of reliable data at cryogenic temperatures. The effect of using optical constants at different wavelengths is also investigated.

My simulations show the diffuse assumption produces higher temperatures than the directional surface. Both surfaces produce higher temperatures than the low emissivity cases when conductivity is low, whilst the reverse is true for high conductivity. The directional surface always outperformed the diffuse surface

For radiative exchange, the diffuse approximation simply produces pessimistic results. However, incorrect modelling of radiative exchange can have knock-on effects. The higher emissivity, diffuse approximation predicts much greater improvements due to tilted sunshields than would realistically occur. This may recommend unnecessary changes to telescope designs in the attempt to improve cooling performance.

The lack of reliable directional radiative data for candidate passive cooling materials will require a major measurement program before a mission is launched. In this thesis I describe apparatus to help in such measurements.

The bi-directional reflectometer quickly and accurately builds up a picture of directional surface properties, performing very well when combined with a reliable laser and measuring device, the few component parts being relatively cheap. Thus, the many directional measurements required for accurate modelling could be made cheaply and quickly.

The multiple reflection cell was designed to make high accuracy measurements at one angle only. This was not as effective as hoped due to laser problems, but should be able to make very good measurements. Further work is recommended.

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