Fused silica / Quartz
Ion beam technology used for polishing fused silica surfaces produces high-quality and high-accuracy solar telescopes. Read on to find out more.
Astronomers make observations of the solar corona using extreme ultra-violet (EUV) imagery. The typical height of a solar corona that is observed from Earth 150 million kilometres away is about 100 metres. This requires high-resolution telescopes that have to be made from extremely stable components with highly sensitive detectors.
The most important component is a large-aperture mirror, or series of mirrors, whose substrate is a technical ceramic such as fused silica with a coating. The coatings could be dielectrics, metals or semiconductors and act to change the transmittance, reflectance, absorbency or polarisation of the fused silica quartz optic. The coatings reduce both ghost imagery and reflectance loss of the substrate.
The end shape of the surface and its roughness, or smoothness, will define the ultimate image it produces. In space optics this smoothness has to be at the nanometre level. Specifications for high-technology optical systems such as those used in extreme ultra-violet imagery and lithography are becoming ever more stringent.
Ion beam technology provides some of the most effective techniques for depositing a thin surface coating film on a large substrate of the dimensions necessary for solar astronomy. The process involves placing the substrate and a target containing the coating material close together in a plasma chamber.
The fused silica / quartz substrate is first cooled and then subjected to a heavy ion bombardment from the plasma. As the plasma ions hit the substrate, they knock out tiny particles from the target material. These particles are the ones that eventually settle on to the substrate and create a thin film coating.
The advantage of this method is that it can produce a highly homogeneous coating over a large area. It also produces an ultra-smooth surface. But if the conditions of the materials and the plasma chamber are not correct, the result could be the deposition of nanoscale microstructures on the substrate surface and that will affect its quality.
Manufacturers are developing a new technology that aims to correct any surface errors in coatings. This sometimes involves a further ion beam bombardment, also called spluttering, to smooth out the first coated surface. The final result is to reduce surface roughness values down to accuracies of plus or minus 0.1 nanometres, or one hundredth millionth of a centimetre.
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