Error Budget Boundaries for Mounting Interface Design of Diamond Turned Mirrors

Abstract

Single point diamond turning can be a very powerful tool process in many medium-scale production environments for manufacturing large aperture metal freeform mirrors. However, as substrate sizes increase, achieving industry-demanded surface figure error becomes much more difficult due to errors incurred through mounting of the substrate onto a fixture plate. These mounting errors can be observed interferometrically and fit to Zernike polynomial terms like astigmatism, trefoil, and power, and can be mitigated through strategic design choices. Maintaining high surface flatness, applying appropriate bolt torques repeatably, and machining rings or pads into the mounting faces of substrates can significantly reduce the amount of mounting induced deformation. These decisions effectively trade an increase in upfront engineering labor and slightly higher production cycle times for improved performance. This process can lower the amount of error allocated to mounting effects in an error budget, freeing margin for other manufacturing steps or other components within a larger system.