The switch from all reflective optics in the MAS 1.0 relay to primarily refractive optics in the MAS 2.0 relay has potential implications for the amount of scattered light reaching the synthesis surface and hence the optical contrast and error rate (insertion errors) in the synthesis. Photodeprotection kinetics are first order , and therefore small amounts of unintended light exposure results in disproportionally large unintended photodeprotection . The increase in numerical aperture from 0.08 to 0.12 results in an increase in the solid angle from which light is collected and directed to the image. For small angles, the solid angle is proportional to the square of the angular aperture, and therefore an up to 2-fold increase in stray light due to the NA increase is possible, although the separation between scattering surface and image, the sizes of the imaging elements relative to the image size, and the angular distribution of scattered light are also important. Reflective and refractive optics differ in their influence on this loss of contrast. These effects are very difficult to model accurately due to their sensitivity to incompletely quantified optics manufacturing parameters including microroughness of the optical polish or optical coating, as well as isolated point defects, such as dust particles or inclusions within the glass of refractive components. Nevertheless, an estimate is that reflective surfaces contribute four to ten times more scattered light than refractive optics, and that the aluminum reflective coatings in particular can exceed this estimate significantly . Therefore, to first order, in the catadioptric relay, with each photon traveling twice through each of two lenses and reflecting once is equivalent to the Offner relay with three reflections. This leaves the factor of possibly as large as two due to the increase in the numerical aperture. To approximately compensate for this, the quality of the fused silica substrate was chosen to be highly homogenous (ISO 10110-5 2/4;4 -refractive index inhomogeneity class 4, striae class 4), and the specifications for the figure and finish of the surfaces were held to strict tolerances: sagitta deviations of 3 fringe spacings, total irregularity not exceeding 0.2 fringe spacing (ISO 10110-5 3/3(0.2); 546 nm), five maximum surface imperfections of less than 0.16 mm, three long scratches less than 0.005 mm (ISO 10110-5 5/5×0.16; L3×0.005). Nevertheless, an advantage of the catadioptric relay in regards to scattering is that most of its surfaces can be easily sealed off from dust, with the front lens acting as a window into the relay compartment. Achieving a similar isolation of an Offner system would require entrance and exit windows that would themselves contribute to light scattering. The complete specifications are available in the supporting information, and are achievable, e.g., using magnetorheological finishing . Detailed specifications for all the optical components can be found in the supplementary materials.