The stressed lap on the Large Polishing Machine (LPM) at the University of Arizona Richard F. Caris Mirror Lab has recently been used to polish the M1 and M3 surfaces of the 8.4-m mirror for the Large Synoptic Survey Telescope (LSST). Loadcells in the three 4-bar links that connect this lap to the spindle of the machine allow the translational forces and torque on the lap to be measured once a second. These force readings and all other available machine parameters are recorded in history files that can be used to create a 2D removal map from one or more polishing runs. While the Preston equation has been used for many years to predict removal in a conventional polishing process, we have adopted a new equation that assumes that removal is proportional to the energy that is transferred from the lap to the substrate via friction. Specifically, the instantaneous removal rate at any point is defined to be the product of four parameters – an energy conversion factor which we call the Allen coefficient, the coefficient of friction, the lap pressure, and the speed of the lap. The Allen coefficient is the ratio of volumetric removal to frictional energy for a particular combination of pad material, abrasive, and substrate. Because our calculations take into account changes in the coefficient of friction between the lap and mirror, our 2D removal maps usually correlate well with optical data. Removal maps for future polishing strokes are created in simulations that track the position and speed of individual lap pads.
The Steward Observatory Mirror Lab is nearing completion of the combined primary and tertiary mirrors of the Large
Synoptic Survey Telescope. Fabrication of the combined mirror requires simulation of an active-optics correction that
affects both mirror surfaces in a coordinated way. As is common for large mirrors, the specification allows correction of
large-scale figure errors by a simulated bending of the substrate with the 156 mirror support actuators. Any bending
affects both mirrors, so this active-optics correction is constrained by the requirement of bending the substrate so both
mirrors meet their figure specifications simultaneously. The starting point of the simulated correction must be
measurements of both mirrors with the substrate in the same shape, i. e. the same state of mechanical and thermal stress.
Polishing was carried out using a 1.2 m stressed lap for smoothing and large-scale figuring, and a set of smaller passive
rigid-conformal laps on an orbital polisher for deterministic small-scale figuring. The primary mirror is accurate to about
25 nm rms surface after the active-optics correction, while work continues toward completion of the tertiary.
The Giant Magellan Telescope (GMT) primary mirror is a 25 meter f/0.7 surface composed of seven 8.4 meter circular
segments, six of which are identical off-axis segments. The fabrication and testing challenges with these severely
aspheric segments (about 14 mm of aspheric departure, mostly astigmatism) are well documented. Converting the raw
phase data to useful surface maps involves many steps and compensations. They include large corrections for: image
distortion from the off-axis null test; misalignment of the null test; departure from the ideal support forces; and
temperature gradients in the mirror. The final correction simulates the active-optics correction that will be made at the
telescope. Data are collected and phase maps are computed in 4D Technology's 4SightTM software. The data are saved
to a .h5 (HDF5) file and imported into MATLAB® for further analysis. A semi-automated data pipeline has been
developed to reduce the analysis time as well as reducing the potential for error. As each operation is performed, results
and analysis parameters are appended to a data file, so in the end, the history of data processing is embedded in the file.
A report and a spreadsheet are automatically generated to display the final statistics as well as how each compensation
term varied during the data acquisition. This gives us valuable statistics and provides a quick starting point for
investigating atypical results.