The next generation ground-based giant telescope, the European Extremely Large Telescope (E-ELT), under
development by the European Southern Observation (ESO) 1, will have nearly 1000 hexagonal segments of 1.45m across
the flats. Fast processing of these segments with high form and edge specifications has proven to be a challenge. The
Zeeko Precessions sub-aperture bonnet polishing plays an important role providing capability for polishing the surface
and correcting the form to meet this target 2,3.
BoXTM grinding has been adopted. This technology has the advantage of fast generating of aspheric surface with very
low subsurface damage (SSD) 4. This will avoid the need of removing thick layer of stock at polishing stage to remove
SSD. However the result grinding signatures has proven to be problematic for direct polishing with Zeeko’s standard
bonnet technology. A novel ‘grolishing’ process which stands between ‘grinding’ and ‘polishing’ has been developed to
deal with mid-spatial features left by BoXTM grinding. This tool is designed base on Zeeko’s R80 bonnet which will fits
directly into the company’s IRP series machines. The process parameters have been optimised to have signatures less
than 10 nm PV. The edge profile is 1μm upstand within 40 mm edge zone.
The ‘grolished’ surface can be directly pre-polished together with all the form corrections. To meet the fabrication time
target, R160 bonnet is used with 50 mm polishing spot, this will provide removal rate of 9.8 mm3/minute, which can be
employed at pre-polishing stage and some form correction. Process parameters have been developed to leave slow
upstand at edge zone without any form of sharp edge downturn. The following form correction stage, which employs
smaller polishing spot of about 20 mm diameter, will continue to remove form errors of spatial frequency between 0.02 –
0.05 1/mm. Furthermore, the upstand edge will be, to a large part, removed at this stage. It is demonstrated that the form
specs can be achieved after this process. The following smoothing process will improve surface textures and remove
edge errors. Local edge rectification is normally necessary to bring the edge at same level. A final smoothing process
will bring the bulk area and edge zone to meet all the specifications.