Mirrors thermal considerations

Paneth22 first demonstrated that the decomposition by heat of certain organic compounds furnished products which removed metallic mirrors of silver, tellurium and other metals from the walls of the tube. For example when vapor of lead tetraethyl was heated a silver coating on the inside of the exit tube was removed for a considerable distance. The results were interpreted to mean that ethyl radicals were liberated in the thermal decomposition and... 

Figure 24.6 The correlation between changes in volume (top), conductivity and thermal behavior with temperature for tetraethylammonium dicyanamide. There is an extremely large volume expansion on moving from phase II to phase I indicating considerable vacancy formation. This is mirrored by the simultaneous increase in conductivity.

There are a considerable number of oxidation and thermal stability tests, mirroring the complexity of the processes themselves. Oxidation tests used include the Indiana-stirred oxidation test [22], the rotary bomb oxidation by IP 229 and other standard procedures such as IP 48, IP 280 and IP 306. 

A simulation (Brammer et al 1988) showed that this mirror had a calculated temperature rise at the very centre of the mirror of 42 °C. Figure 5.19 shows the amount of increase of the mirror thickness as calculated. The maximum increase (between the centre and end of the mirror) is 0.2 m. This value should be compared with the vertical bend displacement of the centre of the mirror of 13.5/nn for a vertical radius of 5221 m. It is possible to polish such a mirror to a flatness of 2.5arcsec, i.e. over a length of 75cm this is a displacement of lO m. This is considerably larger than the thermal effect. 

Two types of specular references are used as standard materials. The first type, called first-surface mirrors, reflect incident radiation off the primary contact surface. These are typically vapor deposited or electroplated metals—gold, silver, rhodium, or aluminium on metallic or nonmetaUic (glass, quartz, or some other material that is thermally and mechanically stable) substrates. An important consideration is the flatness of the surface to be coated. The material must have minimal roughness and the surface must be as flat as possible to avoid measurement error. 

Reflector blanks, especially large ones, require special measures to maintain dimensional (surface) stability. The blank must be adequately supported to retain its shape under varying gravitational loads. It must also be stable during normal temperature cycles, and it should not heat the air in front of the mirror because that causes thermal turbulence and worsens the seeing. A mirror-to-air temperature difference less than about 0.5°C is usually acceptable. The support problem is a mechanical design consideration. Thermal stability may be approached in any of several ways ... 

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