M. T. Tuell

Production of 8.4m segments for the Giant Magellan Telescope

Production of segments for the Giant Magellan Telescope is well underway at the Steward Observatory Mirror Lab. We report on the completion of the first 8.4 m off-axis segment, the casting of the second segment, and preparations for manufacture of the remaining segments. The complete set of infrastructure for serial production is in place, including the casting furnace, two 8.4 m capacity grinding and polishing machines, and a 28 m test tower that incorporates four independent measurement systems. The first segment, with 14 mm p-v aspheric departure, is by some measures the most challenging astronomical mirror ever made. Its manufacture took longer than expected, but the result is an excellent figure and demonstration of valuable new systems that will support both fabrication and measurement of the remaining segments. Polishing was done with a 1.2 m stressed lap for smoothing and large-scale figuring, and a series of smaller passive rigid-conformal laps for deterministic figuring on smaller scales. The interferometric measurement produces a null wavefront with a 3-element asymmetric null corrector including a 3.8 m spherical mirror and a computer-generated hologram. In addition to this test, we relied heavily on the new SCOTS slope test with its high accuracy and dynamic range. Evaluation of the measured figure includes simulated active correction using both the 160-actuator mirror support and the alignment degrees of freedom for the off-axis segment.

Aspheric optics: smoothing the ripples with semi-flexible tools

A well-known fabrication problem with aspheric optical surfaces lies in high-frequency surface irregularities inherent in the figuring process. Optical grinding and polish- ing tools can smooth these ripples, yet retain the flexibility required to fit the aspheric surface. Anf/0.52, paraboloidal, 17-in. convex surface is produced with conventional rigid tools. A transmission ronchigram is obtained showing high- spatial-frequency errors of large magnitude. After four hours of grinding with a semi-flexible multiple-segment ring tool, almost all high-frequency error is removed. This shows good potential for smoothing finished aspheric optics. Flexible tools can also be involved in the figuring process itself.

Manufacture of a combined primary and tertiary mirror for the Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope uses a unique optomechanical design that places the primary and tertiary mirrors on a single glass substrate. The honeycomb sandwich mirror blank was formed in March 2008 by spin-casting. The surface is currently a paraboloid with a 9.9 m focal length matching the primary. The deeper curve of the tertiary mirror will be produced when the surfaces are generated. Both mirrors will be lapped and polished using stressed laps and other tools on an 8.4 m polishing machine. The highly aspheric primary mirror will be measured through a refractive null lens, and a computer-generated hologram will be used to validate the null lens. The tertiary mirror will be measured through a diffractive null corrector, also validated with a separate hologram. The holograms for the two tests provide alignment references that will be used to make the axes of the two surfaces coincide.

Manufacture of the second 8.4 m primary mirror for the Large Binocular Telescope

The second 8.4 m primary mirror and its active support system were delivered to the Large Binocular Telescope in September 2005. The mirror was figured to an accuracy of 15 nm rms surface after subtraction of low-order aberrations that will be controlled by the active support. The mirror was installed into its operational support cell in the lab, and support forces were optimized to produce a figure accurate to 20 nm rms surface with no synthetic correction. The mirror was polished on a new 8.4 m polishing machine that gives the Mirror Lab the capacity to process up to four 8.4 m mirrors simultaneously, with each mirror going through a sequence of stations: casting furnace, generating machine, polishing machine, and integration with its support cell. The new polishing machine has two carriages for polishing tools, allowing use of two 1.2 m stressed laps during loose-abrasive grinding and early polishing, followed by final figuring with a stressed lap and a small tool for local figuring.

Optical data storage readout with quadrant pupil detection

A novel detection scheme that uses combinations of quadrant signals derived in a pupil of the optical system is described for optical storage devices. The signals arise because of an asymmetry in the reflected light distribution when the focused spot scans data with a nonzero tracking offset. Theoretical and scalar diffraction characterization indicates that the signals may be useful for improved data density by reducing intertrack interference (cross talk). The signals may also be useful for providing a tracking error signal.

Wavefront control of the large optics test and integration site (LOTIS) 6.5m collimator.

The LOTIS Collimator provides scene projection within a 6.5m diameter collimated beam used for optical testing research in air and vacuum. Diffraction-limited performance (0.4 to 5microm wavelength) requires active wavefront control of the alignment and primary mirror shape. A hexapod corrects secondary mirror alignment using measurements from collimated sources directed into the system with nine scanning pentaprisms. The primary mirror shape is controlled with 104 adjustable force actuators based on figure measurements from a center-of-curvature test. A variation of the Hartmann test measures slopes by monitoring the reflections from 36 small mirrors bonded to the optical surface of the primary mirror. The Hartmann source and detector are located at the f/15 Cassegrain focus. Initial operation has demonstrated a closed-loop 110 nm rms wavefront error in ambient air over the 6.5m collimated beam.

Production of primary mirror segments for the Giant Magellan Telescope

Segment production for the Giant Magellan Telescope is well underway, with the off-axis Segment 1 completed, off-axis Segments 2 and 3 already cast, and mold construction in progress for the casting of Segment 4, the center segment. All equipment and techniques required for segment fabrication and testing have been demonstrated in the manufacture of Segment 1. The equipment includes a 28 m test tower that incorporates four independent measurements of the segments figure and geometry. The interferometric test uses a large asymmetric null corrector with three elements including a 3.75 m spherical mirror and a computer-generated hologram. For independent verification of the large-scale segment shape, we use a scanning pentaprism test that exploits the natural geometry of the telescope to focus collimated light to a point. The Software Configurable Optical Test System, loosely based on the Hartmann test, measures slope errors to submicroradian accuracy at high resolution over the full aperture. An enhanced laser tracker system guides the figuring through grinding and initial polishing. All measurements agree within the expected uncertainties, including three independent measurements of radius of curvature that agree within 0.3 mm. Segment 1 was polished 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. For the remaining segments, the Mirror Lab is building a smaller, orbital stressed lap to combine the smoothing capability with deterministic figuring.

Alignment and use of the optical test for the 8.4-m off-axis primary mirrors of the Giant Magellan Telescope

The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100 microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT measurement, allowing active correction and compensation for residual errors. We describe the details of the design, alignment, and use of this unique off-axis optical test.

Manufacture and final tests of the LSST monolithic primary/tertiary mirror

The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in nearsimultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope’s active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small highresolution maps, and used these to show the impact of the small features on images is acceptably small.

Status of mirror segment production for the Giant Magellan Telescope

The Richard F. Caris Mirror Lab at the University of Arizona is responsible for production of the eight 8.4 m segments for the primary mirror of the Giant Magellan Telescope, including one spare off-axis segment. We report on the successful casting of Segment 4, the center segment. Prior to generating the optical surface of Segment 2, we carried out a major upgrade of our 8.4 m Large Optical Generator. The upgrade includes new hardware and software to improve accuracy, safety, reliability and ease of use. We are currently carrying out an upgrade of our 8.4 m polishing machine that includes improved orbital polishing capabilities. We added and modified several components of the optical tests during the manufacture of Segment 1, and we have continued to improve the systems in preparation for Segments 2-8. We completed two projects that were prior commitments before GMT Segment 2: casting and polishing the combined primary and tertiary mirrors for the LSST, and casting and generating a 6.5 m mirror for the Tokyo Atacama Observatory.