This paper describes the opto-mechanical design of a large instrument for sub-mm, SCUBA-2, to be commissioned at JCMT. The scientific requirements, specially the large fov and the constraints of the telescope mechanical structure, lead to a complex optical design using freeform aluminium mirrors . The mechanical design is also challenging with large modules to be mounted and aligned in the telescope as well as the cryogenic instrument containing the mirrors, the filters, the dichroics and the detector modules. The cryogenic isostatic mounting, the structural and thermal designs are presented. This includes details of the fabrication of the structure and design of a shutter mechanism for operation at 4K. The results of the first AIV cool-down are also presented.
A new universal non-contact measurement machine design for measuring free-form optics with 30 nm expanded uncertainty is presented. In the cylindrical machine concept, an optical probe with 5 mm range is positioned over the surface by a motion system. Due to a 2nd order error effect when measuring smoothly curved surfaces, only 6 position measurement errors are critical (nanometer level). A separate metrology system directly measures these critical errors of the probe and the product relative to a metrology frame, circumventing most stage errors.
An uncertainty estimation has been performed for the presented design, including a calibration uncertainty estimation and a dynamic analysis. Machine dynamics certainly cause relative motion between probe and product, but due to the non-contact nature of the measurement and the short metrology loop, these motions do not cause significant measurement errors. The resulting shape measurement error for aspheres up to medium free-forms is between 24 and 37 nm, and 30 - 85 nm for medium to heavily free-form surfaces. The suitability of the proposed design is herewith confirmed. A detailed design and a prototype of the machine are currently being developed.
TNO Science and Industry was contracted by the UK Astronomy Technology Center to deliver nine aluminum freeform mirrors for a new sub-millimeter instrument called SCUBA-2. With a much larger field-of-view and sky-background limited sensitivity, SCUBA-2 will map large areas of sky up to 1000 times faster than the original SCUBA camera. The requirement for freeform optics on SCUBA-2 is due to the fact that conventional spherical optic systems do not fit into the allotted space. TNO, who has developed a number of enabling technologies for freeform fabrication and metrology, applied its skills to the mechanical design and analysis of the mirrors. Due to the diverging requirements of the nine mirrors in size, form, and accuracy, TNO needed to spread the fabrication and metrology over additional organizations. The National Aerospace Laboratory of the Netherlands machined and hand polished the five largest mirrors (dimensions exceeding 1 meter and mass exceeding 150kg). They obtained form accuracies between 10 and 15μm peak to valley and roughness down to 40nm RMS. TNO fabricated the smaller and more accurate mirrors (dimensions between 300 and 700mm) on a Precitech Nanoform 350 with a slow tool servo. Measurements by the Dutch National Metrology Institute indicated form accuracies between 4 and 6μm peak to valley and roughness below 20nm RMS for the smaller mirrors.
A polishing robot, using the fluid jet polishing technique (FJP), in combination with in-situ monitoring systems will be presented. One of the monitoring systems is used to check the surface roughness, while the other checks the local removal. Owing to this removal monitoring system the presented system is ideally suited for corrective polishing applications where normally a time consuming iterative approach would be required.