Since the synthesis of oligonucleotides requires up to a few hundred exposures over the course of up to 12 hours, the maximum image drift that can be tolerated over this period of time is on the order of the size of the gap between micromirrors, about 0.5 µm. Primarily due to this constraint, the mechanical support elements spanning the long axis of the relay were chosen to be Invar, a nickel-iron alloy with a thermal expansion coefficient of about 1.2 µm/m/°C at room temperature. Thermal expansion perpendicular to the optical axis of the relay were judged to be less critical and therefore cross-spanning structural components were fabricated with low-expansion stainless steel alloys, primarily 1.4104 (AISI/SAE 430F), with a coefficient of thermal expansion of ~10 µm/m/°C. The illumination system requires less strict positioning tolerances and therefore its support element along the optical axis was chosen to be 1.4104 stainless steel and the cross-spanning components, primarily commercial lens mounts, are made of aluminum.