<title>Design progression of an all-composite primary mirror for the FIRST Telescope</title>

Abstract

Composite materials are an ideal choice for the FIRST Telescope, since they provide dimensional stability, excellent stiffness to weight ratios, near zero thermal expansion, and manufacturing flexibility. The most challenging aspect of producing an all-composite FIRST telescope, is the development of the lightweight primary mirror. The design of the primary mirror must satisfy requirements for surface accuracy at operating temperatures of 80 ¬= K as well as stillness and strength considerations during launch. The design of the all-composite FIRST primary mirror is a sandwich construction with the front and back facesheets separated by an egg-crate core structure. The front and back face-sheets are each comprised of six petals, or laminates, a feature that greatly simplifies the manufacturing process. In addition, it allows for optimizing the location of each petal, based upon tested material properties, to improve the overall performance of the mirror. Although the facesheets are discontinuous due to the gaps between the petals, the core structure is bonded to the facesheets in an arrangement that makes the mirror act as a monolithic structure. Invar fittings are embedded in the core structure, and are used to connect the primary mirror to its support structure via flexures. The design developed for the primary mirror balances considerations of surface accuracy, mechanical performance, producibility, and cost. The results of trade studies used to arrive at the current configuration are discussed. The analysis predictions presented in this study demonstrate the feasibility of the all-composite primary mirror design for meeting the requirements of the ESA FIRST mission. With a segmented facesheet approach, the requirements for CTE uniformity are relaxed, as the location of each faceskin segment is optimized for surface accuracy based upon its as-manufactured thermal strain behavior. Furthermore, to ensure the 10 μm wavefront error (WFE) requirement for the telescope is met, correction of low-order errors (for example, errors characterized by the first 36 Zernike polynomials) in the primary mirror will be corrected through the prescription of the secondary mirror, M2. To validate the all-composite primary mirror design concept, a 2-meter aperture demonstrator mirror was built and tested optically at ambient and cryogenic temperatures. The RMS surface accuracy of this mirror is 2.32 + 48 μm at ambient, with a figure stability of 1.84 ±.34 μm at 200K. The error at both ambient and 200K is dominated by a simple astigmatism. The residual surface error (after removal of first 36 Zernike terms) at ambient and 200K is 1.00 ±.48 μm and 1.21 ±.34 μm, respectively. Based on these results, confidence is high that a COI all-composite primary mirror will be able to meet the accuracy requirements at 80K needed for FIRST. Further testing and prototyping will be on-going over the next year to further improve surface accuracy prior to the final design and fabrication of the FIRST flight mirror.