Brian Cuerden

Active supports and force optimization for the MMT primary mirror

We describe the active support system and optimization of support forces for the 6.5 m primary mirror for the Multiple Mirror Telescope Conversion. The mirror was figured to an accuracy of 26 nm rms surface error, excluding certain flexible bending modes that will be controlled by support forces in the telescope. On installation of the mirror into its telescope support cell, an initial optimization of support forces is needed because of minor differences between the support used during fabrication and that in the telescope cell. The optimization is based on figure measurements made interferometrically in the vibration- isolated test tower of the Steward Observatory Mirror Lab. Actuator influence functions were determined by finite- element analysis and verified by measurement. The optimization is performed by singular value decomposition of the influence functions into normal modes. Preliminary results give a wavefront accuracy better than that of the atmosphere in 0.11 arcsecond seeing.

Deformable secondary mirrors for the LBT adaptive optics system

We describe the manufacture of thin shells for the deformable secondary mirrors of the LBT adaptive optics system. The secondary mirrors are thin shells, 910 mm in diameter and 1.6 mm thick. Each mirror will have its shape controlled by 672 voice-coil actuators. The main requirement for manufacture of the shell is smoothness on scales too small to be adjusted by the actuators. An additional requirement is that the rear surface match the reference body within 30 μm peak-to-valley. A technique was developed for producing smooth surfaces on the very aspheric surfaces of the shells. We figure the optical surfaces on a thick disk of Zerodur, then turn the disk over and thin it to 1.6 mm from the rear surface. Figuring is done primarily with a 30 cm diameter stressed lap, which bends actively to match the local curvature of the aspheric surface. For the thinning operation, the mirror is blocked with pitch, optical surface down, onto a granite disk with a matching convex surface. Because the shell may bend during the blocking operation and as its thickness is reduced to 1.6 mm, figuring of the rear surface is guided by precise thickness measurements over the surface of the shell. This method guarantees that both surfaces of the finished shell will satisfy their requirements when corrected with small actuator forces. Following the thinning operation, we edge the shell to its final dimensions, remove it from the blocking body, and coat the rear surface with aluminum to provide a set of conductive plates for capacitive sensors.

Scanning pentaprism measurements of off-axis aspherics

The pentaprism test is based on the property of a paraboloidal surface where all rays parallel to the optical axis will go through its focal point. We have developed a scanning pentaprism system that exploits this geometry to measure off-axis paraboloidal mirrors such as those for the Giant Magellan Telescope primary mirror. Extension of the pentaprism test to off-axis mirrors requires special attention to field effects that can be ignored in the measurement of an axisymmetric mirror. The test was demonstrated on a 1. -m diameter off-axis mirror and proved to have about 50nm rms surface accuracy. This paper gives detailed performance results for the measurement of the 1.7 m mirror, and designs and analysis for the test of the GMT segments.

Lightweight mirror technology using a thin facesheet with active rigid support

The next generation of space telescopes will require primary mirrors that push beyond the current state of technology of mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/m2 and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes--diffraction limited optical performance of a 0.5-m diameter mirror and launch survival of a 1-m diameter mirror. We have also built and tested a prototype actuator that achieves 25 nm resolution at cryogenic temperatures. We are now building a 2-m mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators and control electronics. It will be actively controlled and interferometrically measured at 35 K.

Manufacture of 8.4-m off-axis segments: a 1/5-scale demonstration

We describe the requirements for manufacturing and maintaining alignment of the 8.4 m off-axis segments of the Giant Magellan Telescope’s primary mirror, and a demonstration of the manufacturing techniques on the 1.7 m off-axis primary mirror of the New Solar Telescope. This mirror is approximately a 1/5 scale model of a GMT segment. We show that the stressed lap polishing system developed for highly aspheric primary and secondary mirrors is capable of figuring the GMT segments and the NST mirror. We describe an optical test with a null corrector consisting of a tilted spherical mirror and a computer-generated hologram, and derive accuracy requirements for the test. The criterion for accuracy of low-order aberrations is that the active support system can correct any figure errors due to the laboratory measurement, with acceptably small forces and residual errors.

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.

Scanning pentaprism test for the GMT 8.4-m off-axis segments

The scanning pentaprism system for testing the 8.4 m off-axis segments for the Giant Magellan Telescope has recently been completed. The system uses a fiber source and a carriage mounted pentaprism to scan a 40 mm collimated beam across the surface of the segment under test. Since the scanning beam is parallel to the optical axis of the parent mirror, it comes to focus on a detector at the telescopes prime focus, where displacement of the spot is proportional to the slope error. A second collimated beam from a stationary reference pentaprism is used to compensate for any changes in the relative positions of the optical components during testing. The optical components are suspended over the mirror on a rail system that can be rotated so that scans can be made across any diameter of the segment. The test is capable of measuring wavefront slope errors to 1 μrad rms, adequate to verify that power, astigmatism, coma, and other low-order aberrations are small enough to be corrected easily at the telescope with the segments active support system.

The Giant Magellan Telescope (GMT) structure

A concept design has been developed for the Giant Magellan Telescope (GMT). The project is a collaboration by a group of U.S. universities and research institutions to build a 21.5-meter equivalent aperture optical-infrared telescope in Chile. The segmented primary mirror consists of seven 8.4-meter diameter borosilicate honeycomb mirrors that will be cast by the Steward Observatory Mirror Laboratory. The fast primary optics allow the use of unusually compact telescope and enclosure structures. A wide range of secondary trusses has been considered for the alt-az mount. The chosen truss employs carbon fiber and steel and, due to its unique geometry, achieves high stiffness with minimal wind area and primary obscuration. The mount incorporates hydrostatic supports and a C-ring elevation structure similar in concept to those implemented on the Magellan 6.5-m and LBT dual 8.4-m telescopes. Extensive finite element analysis has been used to optimize the telescope structure, achieving a lowest telescope resonant frequency of ~5 Hz. The design allows for removal and replacement of any of the 7 subcells for off-telescope mirror coating with no risk to the other mirrors. A wide range of instruments can be used which mount to the top or underside of a large instrument platform below the primary mirror cells. Large instruments are interchanged during the day while small and medium-sized instruments can be enabled quickly during the night. The large Gregorian instruments will incorporate astatic supports to minimize flexure and hysteresis.

Ultralightweight active mirror technology at the University of Arizona

Lightweight mirrors for space can be made using a thin flexible substrate for the optical surface and a rigid lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. The University of Arizona is now in the final stages of fabricating two such mirrors. A 2-m NGST Mirror System Demonstrator, with an areal density of 13 kg/m2, is being built for NASA and will be tested at cryogenic temperatures. A 50 cm development mirror, with an areal density of only 5 kg/m2, is also being fabricated. This paper discusses the fabrication processes involved with both of these mirrors.