The primary environmental specification was maintenance of optical alignment with a ± 2°C change in room temperature during the course of a synthesis. This is a temperature range that should be readily achievable in a normal modern lab, or in a room equipped with a standard split system inverter air conditioner. A dimensional tolerance analysis of the 0.12 NA catadioptric relay revealed that, relative to correct (aligned) positioning of the relay components, the system would remain in focus (Strehl ratio at all positions of the 1080p DMD image > 0.8) with changes in the front lens to back lens distance of <20 µm, and with a change in the separation of the back lens and mirror of <2 µm. Here, the limiting constraint is the former, as the distance between the front and back lens is large, 642.3 mm. This constrains the thermal expansion coefficient α of the support structure to satisfy 0.642 10 𝜇𝑚 • 2°𝐶 • 𝛼 ≲ 20 𝜇𝑚. This gives a thermal expansion coefficient of less than ~16 µm/m/°C, which is a property of some stainless steel alloys, but not, e.g., aluminum. An additional concern was differential heating of the relay support structure that would cause a bend in the relay and a shift in the image position, that is, the image would remain in focus, but move laterally during the synthesis. This effect was observed in the MAS 1.0 when room temperature was changed quickly resulting in a large drift in the image position within the flow cell . This potential for drift-in temperature-uncontrolled environments-led to the implementation of an active realignment system ("image lock") based on slight movements to the primary mirror of the Offner relay using piezoelectric actuators. This system was functional, but complex to maintain and significantly increased the synthesis time. Therefore, the MAS 2.0 was designed to avoid the need for active realignment.