Precision grinding for rapid fabrication of segments for extremely large telescopes using the Cranfield BoX

Abstract

An ultra precision large optics grinding machine, BoX ® , was developed and produced at Cranfield University. BoX ® offers a rapid and economic solution for grinding large off-axis aspherical and free-form optical components. Grinding high accuracy surfaces with low subsurface damage reduces subsequent polishing time. This efficient grinding process provides the capacity to grind 1.5 m parts. This paper presents an analysis of Astrositall ® optical ground parts: a hexagonal 84 m radius of curvature mirror of 1 m across corners and an off-axis 350 mm diameter mirror. The 1 m hexagonal part is representative of segments under study for making extremely large telescope (ELT) segmented mirrors. The second part was machined off-axis to demonstrate free-form fabrication capability. These operations demonstrate the scalability of the rapid grinding process developed for large free-form optics. The use of an error compensation procedure improved an initial ground form accuracy to +/-1 µm p-v over 1 metre surface. The results highlighted the effect of grinding parameters and machine dynamics on form accuracy and fabrication time.

Fully automating fine-optics manufacture - why so tough, and what are we doing?

“…2 University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK. 3 Nottingham University Business School, Jubilee Campus, Wollaton Road, Nottingham NG8 1BB, UK. 4 Research Center for Space Optical Engineering, Harbin Institute of Technology, Harbin 150001, China.…”

“…From our experience, a state-of-the-art CNC grinding machine, working hard, brittle materials, may deliver~10 μm of sub-surface damage (SSD) with a few microns RMS form error, depending on material, size and geometry of the part. One such machine is the Cranfield BoX™ machine [3]. Subsequent processing using CNC free-abrasive polishing [4] and similar techniques such as 'grolishing' [5], aims to remove the SSD layer, correct form errors, and refine texture to 1-2 nm Sa for most optics, but down to 01-0.2 nm Sa in critical applications.…”

Profile error compensation in cross-grinding mode for large-diameter aspheric mirrors

“…To avoid these issues, an on-machine measurement and compensation grinding method was proposed for manufacturing large-diameter aspheric mirrors by using a noncontact displacement sensor [16]. In this method, profile errors can be reduced by calibrating the sensor tilt and offset errors [17,18]. A grinding compensation method of onmachine contact measurement for the fabrication of smalldiameter mold was presented through analyzing the probe radius error [19].…”

Edge-control and surface-smoothness in sub-aperture polishing of mirror segments

“…The grinding leaves clean edges with no measurable misfigure. Grinding an off-axis aspheric full-size segment, the form-error is <1 μm RMS and 6 μm p-v form-error (measured by coordinate measuring machine, with the segment still mounted on its precision diamond-turned, hard grinding fixture) [11,12]. Sub-surface damage is ~ 6 μm on Zerdor; a little more on ULE [13].…”