Results of the beryllium AMSD mirror cryogenic optical testing

Abstract

The 1.4-meter semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD) mirror completed initial cryogenic testing at Marshall's X-ray Calibration Facility (XRCF) in August of 2003. Results of this testing show the mirror to have very low cryogenic surface deformation and possess exceptional figure stability. Additionally, the mirror substrate exhibits virtually no change in surface figure over the James Webb Space Telescope (JWST) operational temperature range of 30 to 62 Kelvin. The lightweighted, semi-rigid mirror architecture approach demonstrated here is a precursor to the mirror technology being applied to the JWST observatory. Testing at ambient and cryogenic temperatures included the radius of curvature actuation system and the rigid body displacement system. These two systems incorporated the use of 4 actuators to allow the mirror to change piston, tilt, and radius of curvature. Presented here are the results of the figure change, alignment change, and radius change as a function of temperature. Also shown will be the actuator influence functions at both ambient and cryogenic temperatures.

Minimizing actuator-induced errors in active space telescope mirrors

“…Cryogenic performance of this mirror also met requirements and results were used to guide JWST segment design. 13 Finally, the Primary Mirror Segment Assembly (PMSA) demonstrated TRL-6 for the JWST primary mirror by verifying segment requirements in a space-equivalent thermal and vacuum environment. 14,15 Of particular relevance to the present study is the work of Budinoff and Michels 5 and Park et al, 16 both of which consider optimization over geometric design parameters to reduce residual figure error.…”

Herschel Space Telescope: Optical test and model correlation

“…Cryogenic mirrors have been demonstrated to change figure as they are cooled. [14,15,16,17] This has often been attributed to material non-uniformity and in some cases adhesives (epoxy/CFRP or frit/glass); specifically thermal expansion properties. This type of aberration can lead to erroneous wavefront measurement in under sampled optical measurements.…”

Webb Space Telescope primary mirror development: summary and lessons learned

“…Ball Aerospace, Goodrich, and Kodak were the winning vendors 9 , 20 – 25 All of these mirrors were 1.3 to 1.4-m point to point—just the size needed to produce a segmented primary mirror 6 to 8 m in diameter—and had an areal density of ∼15 kg/m2.…”

“…2). 20 – 23 A key element of the BATC approach is that the mirrors required cryo-null polishing to remove cool down cryodistortions.…”