Ray tracing showing how light is reflected through Proba-V's Vegetation instrument. Its mirrors are arranged within a compact ‘three-mirror anastigmat’ (TMA) design, but need to be curved in just the right ‘aspherical’ shape. Achieving this required a manufacturing technique of nanometre-scale precision known as ‘single-point diamond turning’ by Belgium-based specialist AMOS. Whether it could be achieved at all was initially uncertain. In 2009 a prototype TMA telescope was produced through ESA’s General Support Technology Programme (GSTP), which helps to develop space hardware to flight readiness.