Diamond turning of large off-axis aspheric mirrors using a fast tool servo with on-machine measurement

We describe a systematic approach to diamond turning which has been devised to fabricate large aspheric off-axis mirrors demanded for the construction of three-mirror-anastigmatic telescopes. To obtain sub-micrometer form accuracy, the machine axes errors are compensated in real time using a fast tool servo driven by a piezoelectric actuator along with an on-machine measurement scheme. Besides, a special chucking device is used to mount the mirror substrate on the spindle with a required large offset to the axis of rotation, which has been specially designed to minimize elastic deformation caused by centrifugal inertia and clamping force. Actual machining data are discussed in detail to prove that the proposed approach is capable of fabricating aluminum mirrors of 620 mm diameter to a form accuracy of 0.7 μm in peak-to-valley error value.