Reflection anisotropy in evaporated aluminum: Consequences for telescope mirror coatings

Aluminum coatings deposited in the presence of small quantities of oxygen onto substrates at oblique incidence are optically anisotropic, even at normal incidence. Values of the normal incidence reflection anisotropy are given for films deposited under controlled pressures and rates and for the range of angles 0°–45° which is appropriate to a circular array of sources for coating a large telescope mirror. Experimental arrangements for measuring anisotropy are described. One of these leads to a map showing the angular displacement of an incident linearly polarized beam which is reflected by a coated concave mirror.