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 :

1. reflectivity of >0.98 over the frequency range 30 - 950 GHz,
2. 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:
1. reflectivity over the frequency interval 150 - 1500 GHz,
2. scattering pattern of visible light over 180 degrees in two planes,
3. absorptivity and emissivity in visible and infrared.
3. 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.

Last modified: 2007-01-05