Blackening the mechanical housing surfaces in an optical system is one of the primary methods of controlling stray light. Examples of such surfaces include the inner diameters of cylinders in which lenses and mirrors are mounted, struts to hold mirrors (usually the secondary mirror) in centrally-obscured systems, baffles, vanes, sunshades, cold shields, stops, and any other surface near the optical path. These surfaces may be critical, and therefore steps should be taken to ensure they are not illuminated as well. An example of a black surface treatment (anodized aluminum) used in an optical system is included in this chapter. This image shows the inside of the barrel of the baseline Maksutov-Cassegrain telescope.
Although the use of black surface treatments can significantly reduce the magnitude of first-, second-, and higher-order stray light paths, the improvement gained in stray light performance by their use is often not enough to compensate for a system whose optical and/or baffle design is inadequate for its stray light requirement. Therefore, selection of black surface treatments should occur at the appropriate time in the stray light design process.
This chapter begins with a discussion of the physics of light scattering from black surface treatments and a discussion of the methods used to model its BRDF (only BRDF models will be discussed, as the BTDF of a black surface treatment is usually zero). BRDF models of a couple of typical black surface treatments (Aeroglaze Z306 paint and anodized aluminum) are provided, as are references to publications that contain measured BRDF data. Caution should be used when using any set of published BRDF data because it is often difficult to know how similar the measured sample is to the surface being modeled. The BRDF model data is followed by a discussion of the criteria that should be considered when selecting a black surface treatment, as there are usually many more criteria to consider than just its BRDF in the sensor waveband. The chapter ends with a survey of popular black surface treatments.
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