E. Todd Kvamme

Adhesive bond cryogenic lens cell margin of safety test

The Near Infrared Camera (NIRCam) instrument for NASAs James Webb Space Telescope (JWST) has an optical prescription which employs four triplet lens cells. The instrument will operate at 35K after experiencing launch loads at approximately 295K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus maintain an exceptional wavefront during operation. Lockheed Martin Space Systems Company (LMSSC) was tasked to design and qualify the bonded cryogenic lens assemblies for room temperature launch, cryogenic operation, and thermal survival (25K) environments. The triplet lens cell designs incorporated coefficient of thermal expansion (CTE) matched bond pad-to-optic interfaces, in concert with flexures to minimize bond line stress and induced optical distortion. A companion finite element study determined the bonded systems sensitivity to bond line thickness, adhesive modulus, and adhesive CTE. The design team used those results to tailor the bond line parameters, minimizing stress transmitted into the optic. The challenge for the Margin of Safety (MOS) team was to design and execute a test that verified all bond pad/adhesive/ optic substrate combinations had the required safety factor to generate confidence in a very low probability optic bond failure during the warm launch and cryogenic survival conditions. Because the survival temperature was specified to be 25K, merely dropping the test temperature to verify margin was not possible. A shear/moment loading device was conceived that simultaneously loaded the test coupons at 25K to verify margin. This paper covers the design/fab/SEM measurement/thermal conditioning of the MOS test articles, the thermal/structural analysis, the test apparatus, and the test execution/results.

A second generation low stress cryogenic mount for space-borne lithium fluoride optics

Single crystal Lithium Fluoride has been base-lined as one of the optical materials for the Near Infra-Red Camera (NIRCam) on the James Webb Space Telescope (JWST). Optically, this material is outstanding for use in the near IR. Unfortunately, this material has poor mechanical properties, which make it very difficult for use in any appreciable size on cryogenic space based instruments. In addition to a dL/L from 300K to 30K of ~-0.48%, and a room temperature CTE of ~37ppm/K, the material deforms plastically under relatively small tensile loading. This paper will update a paper presented in 2005 on the same optical mount [1]. The mount has been proven via vibration and thermal-vacuum testing to successfully mount large (70 mm-94 mm) Lithium Fluoride optics for application in space. An overview of Lithium Fluoride material properties and characteristics is given and updated yield strength test data is provided and discussed. A design limit load is determined for the material based on strength values from the literature as well as independent testing. The second generation mount design is then presented along with test data and results. Finally, the test results are discussed showing survival and performance of the optic and mount during cool-down to the operational thermal environment.

A low stress cryogenic mount for space-borne lithium fluoride optics

Single crystal Lithium Fluoride has been base-lined as one of the optical materials for the Near Infra-Red Camera (NIRCam) on the James Webb Space Telescope (JWST). Optically, this material is outstanding for use in the near IR. Unfortunately, this material has poor mechanical properties, which make it very difficult for use in any appreciable size on cryogenic space based instruments. In addition to a dL/L from 300K to 30K of ~-0.48%, and a room temperature CTE of ~37ppm/K, the material deforms plastically under relatively small tensile loading. This paper will present a mount that has been proven via vibration and thermal-vacuum testing to successfully mount a large (70mm-94mm) Lithium Fluoride optic for application in space. An overview of Lithium Fluoride material properties and characteristics is given. A design limit load is determined for the material based on strength values from the literature as well as independent testing. The original design option is shown and the pros and cons discussed. The final mount design is then presented along with analysis results showing compliance to the limit load requirement. Finally, testing results are discussed showing survival of the optic in a space launch vibration environment as well as survival during cool-down to the operational thermal environment of 30K.

Cryogenic bonding for lens mounts

Lockheed Martin Space Systems Company (LMSSC) has performed a feasibility study for bonded cryogenic optical mounts. That investigation represents a combined effort of design, experiments and analysis with the goal to develop and validate a working cryogenic mount system for refractive lens elements. The mount design incorporates thermal expansion matched bond pads and radial flexures to reduce bondline stress and induced optical distortion. Test coupons were constructed from lens and selected mount materials and bonded with candidate adhesives to simulate the designs bond pads. Thermal cycling of those coupons to 35K demonstrated both the systems survivability and the bonds structural integrity. Finally, a companion finite element study determined the bonded systems sensitivity to bondline thickness, adhesive modulus and adhesive CTE. The design team used those results to tailor the bondline parameters to minimize stress transmitted into the optic.

The opto-mechanical design process: from vision to reality

The design process for an opto-mechanical sub-system is discussed from requirements development through test. The process begins with a proper mission understanding and the development of requirements for the system. Preliminary design activities are then discussed with iterative analysis and design work being shared between the design, thermal, and structural engineering personnel. Readiness for preliminary review and the path to a final design review are considered. The value of prototyping and risk mitigation testing is examined with a focus on when it makes sense to execute a prototype test program. System level margin is discussed in general terms, and the practice of trading margin in one area of performance to meet another area is reviewed. Requirements verification and validation is briefly considered. Testing and its relationship to requirements verification concludes the design process.

Front Matter: Volume 8150

This PDF file contains the front matter associated with SPIE Proceedings Volume 8150, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Mounting small optics for cryogenic space missions

The Near Infrared Camera (NIRCam) instrument for NASAs James Webb Space Telescope (JWST) includes numerous optical assemblies. The instrument will operate at 35K after experiencing launch loads at ~293K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus maintain exceptional optical quality during operation. Lockheed Martin Space Systems Company (LMSSC) conceived, designed, analyzed, assembled, tested, and integrated the optical assemblies for the NIRCam instrument. With using examples from NIRCam, this paper covers techniques for mounting small mirrors and lenses for cryogenic space missions.

Flight build of the collimator and shortwave camera optics on NIRCam

The Near Infrared Camera (NIRCam) instrument for NASAs James Webb Space Telescope (JWST) has an optical prescription which employs three triplet lens cells. The instrument will operate at 35K after experiencing launch loads at ~293K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus maintain an exceptional wavefront during operation. The Lockheed Martin Advanced Technology Center (LMATC) has built and tested the collimator and camera optics for use on the NIRCam flight instrument. This paper presents an overview of the driving requirements, a brief overview of the changes in the opto-mechanical design and analysis since our last presentation, a discussion of the collimator and shortwave camera triplet assembly processes, and finally a summary of the mechanical and optical test results.

Opto-mechanical test results for the Near Infra-red Camera on the James Webb Space Telescope

The Near Infrared Camera (NIRCam) for the James Webb Space Telescope (JWST) has undergone Pathfinder component testing and evaluation. This paper presents the opto-mechanical test results. An overview of the optomechanical system requirements is provided, followed by a discussion of the opto-mechanical system design and assembly process. Tolerances in the opto-mechanical system as they relate to system level alignment are also presented. Mechanical analysis related to vibration and thermal behavior of the design is shown. Finally, the overall performance of the opto-mechanical system is discussed as it relates to instrument optical performance.