Our project aims to create a time-saving device for testing infrared sensors, called Focal Plane Arrays (FPAs), which are used in mission-critical military and space applications. Current test chambers used to validate the output of FPAs include a fixed-sized aperture, placed in a cryogenically-cooled vacuum chamber, which controls the amount of light that hits a FPA during testing. The necessity of having to continually exchange this fixed-sized aperture within the vacuum chamber with apertures of many different sizes extends the testing process over multiple days, as it can take more than six hours to pump the vacuum chamber down to the necessary testing pressure after exchanging the aperture. We have designed a variable-aperture mechanism housed within a vacuum chamber to greatly shorten the time it takes to test FPAs. Our design features a cryogenically-cooled commercial iris centered within a vacuum chamber. The iris is actuated with a linear feedthrough to an aperture size set by an operator.