Cryogenic optical performance of a lightweighted mirror assembly for future space astronomical telescopes: correlating optical test results and thermal optical model

ABSTRACT A 43cm diameter stacked core mirror demonstrator was interferometrically tested at room temperature down to 250 degrees Kelvin for thermal deformation. The 2.5m radius of curvature spherical mirror assembly was constructed by low temperature fusing three abrasive waterjet core sections between two CNC pocket milled face sheets. The 93% lightweighted Corning ULE ® mirror assembly represents the current state of the art for future UV, optical, near IR space telescopes. During the multiple thermal test cycles, test results of interferometric test, thermal IR images of the front face were recorded in order to validate thermal optical model. Keywords: Lightweighted ULE ® mirror, optical testing, optical model 1. INTRODUCTION In a 2012 National Research Council report, NASA Space Technology Roadmaps and Priorities, the committee identified 14 space technology areas (TA) an d roadmap for advancing space technology research and development over the next 3 decades. Space technology ar ea TA08, Science Instrument s, Observatories, and Sens or Systems, its roadmap addressed technologies for future space missions relevant to Earth science, heliophysics, planetary science, and astrophysics driven by science and mission priorities recommended in the 2010 decadal survey of astronomy and astrophysics in New Worlds, New Horizons in Astronomy and Astrophysics . The TA08 committee identified low cost, high performance telescopes as one of th e top technical challenges. The technology will be used to develop the next generation of large-aperture astronomical telescopes to detect elusive extrasolar planets, or exoplanets such as Earth-analogs. Earth-analogs are earth-sized worlds circling around a star similar to our sun. Large aperture telescopes are needed to help answer age old question, such as whether ther e are life on these exoplanets. In addition, large telescopes can be used for lightweighted, laser communication systems, and high-performance orbiting observatories for understanding how galaxies assemble their stellar populations, understanding the interactions of baryonic matter with intergalactic medium, and further understanding our earth formation and evolution in our own solar system. Large aperture, lightweighted, high stiffness mirrors, both monolithic 4 to 8 meters, and segmented mirrors greater than 8 meters, were identified as a high priority technology for futu re UV, visible, and IR orbiting observatories. Since large mirror development programs are inherently prohibitively costly to design, manufacture, and test; an integrated modeling software for the mirror technology would be necessary to va lidate the mirror system befo re cutting any hardware since cost of mirror materials, labor to manufacture the mirror system, and optical testing in relevant environment such as vacuum and cryogenic temperatures involve considerab le materials, labor hours and test facilities costs. *ron.eng@nasa.gov; phone 256-544-3603; optics.nasa.gov