Aluminum alloys for optical and eletronic require a low CTE (coefficient of thermal expansion) and often some high temperature capabilities. AMT has a range of alloys with a very low CTE to match these requirements. Most of the alloys have a high Silicon content to reduce the CTE. The very fine microstructure and grain size leads to extremly smooth surfaces required for mirror application. Some alloys match the CTE of Nickel. This is important as many mirrors are Nickle coated.
Aluminum optical, eletronic alloy overview.