Reflectivity, scattering and emissivity... Abstract
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ALMA Memo # 566
Reflectivity, scattering and emissivity measurements on samples of prospective ALMA reflector panel.
Jacob W.M. Baars, Bernard Buzzoni, Hans-Peter Gemünd and A. Polsak
2007-01-05
The material choice and the surface finish of the primary reflector panels of the ALMA submillimeter antennas must satisfy (among others) the following conditions :
- reflectivity of >0.98 over the frequency range 30 - 950 GHz,
- capable of solar observation without thermal damage to
the antenna and receivers. The second requirement
necessitates a surface finish which scatters visible light
diffusely over a wide solid angle. Also, the panel should
not become too hot during solar observation. In the context
of the prototype antenna procurement a series of possible
surfaces were considered. Samples of these were measured
for the parameters:
- reflectivity over the frequency interval 150 - 1500 GHz,
- scattering pattern of visible light over 180 degrees in two planes,
- absorptivity and emissivity in visible and infrared.
- The results of these measurements are summarised here. For both antennas a good solution has been achieved. Vertex applies a chemical etching to machined aluminium panels. Alcatel/Media Lario employ electroformed nickel skins, replicated from a roughened mold. A coating of rhodium provides good thermo-optical properties.
View a pdf version of ALMA Memo #566.
