Stray light suppression components are built into the optomechanical system, particularly the catadioptric relay. Because photodeprotection kinetics are first order reactions, low levels of stray light reaching the DMD image in the flow cell will have a disproportionate impact on error rates (insertion errors). Particularly important in the MAS 2.0 is the suppression of light reflecting from OFF-state micromirrors. In typical usage scenarios, most photoexposures are performed with a large majority of the mirrors in the OFF position. In the case of nucleic acid synthesis with four monomers, ~75% of the mirrors would be in this position in most cycles, with this number increasing with the number of monomers. Furthermore, in many syntheses it is beneficial or even necessary to reduce the density of synthesis pixels in order to reduce insertion error rates, particularly those resulting from diffraction. Due to the optical resolution having a larger value (1.6 µm at 365 nm) than the gap between micromirrors, there is crosstalk between immediately adjacent synthesis pixels. Light scattering also contributes to unintended photodeprotection and is proportional to the number of mirrors in the ON position . Relative to the error rate when all synthesis pixels are used, a checkerboard pattern such that diffraction pattern crosstalk takes place only at the corners, or a 1:4 pattern such that all synthesis pixels are separated by twice the pitch, greatly reduces the insertion error rate . Mirrors permanently in an OFF-state to reduce synthesis density combined with the ~75% OFFstate mirror rate for the remaining mirrors requires very efficient absorption of this light to prevent it from reaching the flow cell. In the MAS 2.0, OFF-state light is directed to the front lens of the relay (see Figure ) where it can Figure . a. Illumination system of the MAS 2.0. All the optical components, including the homogenizer and holographic diffuser are off-the-shelf components. To reduce the numerical aperture of the system, an appropriate aperture should be inserted into the lens mount at the Fourier plane. b. LED and primary heat sink. The LED is soldered directly to the primary heat sink and thermally linked to the actively cooled secondary heat sink with thermal paste. produce a large amount of scattered light directly in front of the flow cell. For this reason, it is important to block this path with a very efficient absorber. For this, we use a base layer of Acktar Metal Velvet with a hemispherical reflectance of about 1% in the near UV, and at least one layer of Acktar Hexa Black to suppress most of the remaining light. The position of this light stop is shown in Figure . In addition, the front lens is equipped with light baffles to absorb light not following the optical path from the DMD to the flow cell (Figure ). Additional light baffles are located about 5 cm in front of the flow cell (Figure ), and surrounding the focusing lens of the illumination system (Figure ).