Out gassed carbon from stainless steel can be a serious source of contamination and can damage soft x-ray optics as well as deep Ultra Violet (DUV) and Extreme Ultra Violet (EUV) optics for semiconductor lithography. The carbon from stainless also reacts with the elemental hydrogen and departs the surface in a variety of hydrocarbons. Additionally, CO and CO² are produced by the abundance of carbon from the stainless steel when reacting with oxygen.
Many labs are eliminating any potential carbon from stainless by using aluminum vacuum systems.
For some optical applications, the vacuum level is less critical than the purity of the environment. This is typical in the semiconductor lithography market. Companies in this market are pulling UHV then backfilling with a pure gas and operating at relatively low vacuum but incredibly high purity levels: Ultra High Purity (UHP)