Observation in vacuum is very important and often the only means to monitor and acquire measurements by external instruments.
Our visible spectrum viewports are constructed from a high quality silicate glass, joined to the stainless steel flange through a matched expansion kovar seal.

Although bake-able, a safe maximum temperature rise across the seal of 2-3° per minute is recommended.
Covering the viewport with aluminium foil during bakeout helps to achieve, particularly during cool down.