Prior to assembly of the new segments, a review of the original surface support system was carried out. Based on previous experience with the alignment and stability of the inner rings, it was decided to upgrade a large number of the early subpanel support and adjustment components. A key modification was the inclusion of lateral adjustment for subpanel support points, helping to minimize bending moments on the panels both during and after integration. Improvements in the ease of surface setting and greater surface stability were immediately observed following component overhaul. Form setting of individual segments was carried out at the LMT facilities in Puebla and again at the telescope site, using the iterative process developed previously that makes use of laser tracker surface measurements.
While the original implementation of the subpanel support system enabled the setting of individual segments to a mean surface error of around 30 micrometers RMS, this mean value was improved to around 20 μm for the entire set of 96 segments for the outer two rings, with the best segments coming in at around 18 μm RMS surface error. We believe this brings us close to the limit of achievable surface accuracy for the LMT design of laminated composite subpanels supported on simple mechanical differential adjusters.
We present an overview of primary surface improvements since 2011, and the main aspects of the LMT aperture expansion relating to the assembly and alignment of the surface segments for the outer two rings of the 50-meter primary.