Measurement of the thermal-vacuum defocus of an objective lens for an imaging payload on a CubeSat

The paper presents a laboratory methodology for determining the space environmental effect on the focus of an objective lens for an earth imaging payload on a small satellite. Components used in space applications should undergo rigorous screening and testing, including thermal-vacuum (TVac) testing. These tests reduce risk and can provide critical information leading to risk mitigation strategies contributing to mission success. The global value and market for CubeSats for both commercial and scientific applications is fast growing and this has led to the demand for environmental test equipment. Existing facilities around the globe for satellite TVac testing are typically expensive and difficult to access and also oversized for nanosat applications. As a result, CSIR Optronic Sensor Systems embarked on developing a laboratory test procedure and a small scale TVac chamber based on commercial off-the-shelf (COTS) components. The specific purpose was to quantify the defocus of a COTS imaging lens under vacuum and over a specific operating temperature range. The TVac defocus is caused by variation of the refractive index of air with pressure and variation of the refractive index of glass with temperature. Temperature-related dimensional changes also contribute. The tests provided repeatable measurements leading to a TVac model for the defocus behaviour of the lens. The model allows for setting of a compensatory or "preemptive" defocus before launch.