Selective Surfaces -What are the different selective coatings available for Solar thermal application?

It is well known that coating the absorbers with a selective surface helps in increasing the efficiency. I am building a air heater using parabolic trough and an evacuated tube. I've chosen Aluminium as the tube material...but I'm not able to figure out how to selectively coat the surface and which coat (paint or process) to be used for Aluminium. i mean all I could get through the web is some kind of electroplating under controlled environment. Isnt there any kind of pain available which can give good performance at 200 degree celsius? I read somewhere that heating the stainless steel to a temperature at which nickel oxidises, results in a surface which has absorptivity of 0.93 and emissivity as low as 0.08? Can anyone help with this?

You need to heat the tube to around 800C for good results, and you need a high-nickel stainless. En304 doesn't work well, EN 316 isn't bad. Anodising aluminum is the process, Google on the resulting absorption. Steve