Eve M. Wooldridge

Effects of Vacuum Ultraviolet Radiation on Thin Polyimide Films

This paper describes the vacuum ultraviolet (VUV) radiation durability screening testing of thin (12.7–25.4 μm) polyimide films proposed for use on the Next Generation Space Telescope (NGST) sunshield. Materials included in this screening test were Kapton®E, Kapton®HN, Upilex®S, CP1, CP1 with vapour deposited aluminium (VDA) on its back surface, and CP2 with a VDA coating on its back surface. Samples were exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation and examined for changes in solar absorptance, thermal emittance, ultimate tensile strength, and elongation at failure. Changes in the solar absorptance were observed for some materials, and, additionally, significant changes in spectral reflectance were observed in the ultraviolet to visible wavelength region for all of the polyimide materials tested. Changes in the ultimate tensile strength and elongation at failure were within the experimental uncertainty for all samples. Longer exposures are needed to verify the observed trends and to develop performance predictions for these materials on the NGST sunshield.

Passive thermal control of the NGST

Preliminary studies of passively cooling the NGST utilizing a lightweight deployable subshield are described. The NGST mission concept of a passively-cooled large-aperture optical telescope is unique from any other mission flown to date. We show that achieving operational temperatures of less than 50 K appears feasible by passive cooling alone through a combination of (i) operating the observatory far from the Earth so that the Sun becomes the only significant source of environmental heating, (ii) selecting an observatory configuration that isolates all significant heat dissipation from the cold telescope, and (iii) employing a high performance sunshield to attenuate the incident solar radiation. The observatory configuration consists of the sunshield with cold telescope and instrument elements on the anti-sun side, and warm spacecraft avionics and propulsion elements on the sun-side of the sunshield. A sunshield thermal configuration trade study, preliminary telescope thermal analyses, and a mechanical concept for a lightweight deployable sunshield are presented. Also discussed are the remaining issues to be addressed.

The Use of the Molecular Adsorber Coating Technology to Mitigate Vacuum Chamber Contamination During Pathfinder Testing for the James Webb Space Telescope

As a coating made of highly porous zeolite materials, the Molecular Adsorber Coating (MAC) was developed to capture outgassed molecular contaminants, such as hydrocarbons and silicones. For spaceflight applications, the adsorptive capabilities of the coating can alleviate on-orbit outgassing concerns on or near sensitive surfaces and instruments within the spacecraft. Similarly, this sprayable paint technology has proven to be significantly beneficial for ground based space applications, in particular, for vacuum chamber environments. This paper describes the recent use of the MAC technology during Pathfinder testing of the Optical Ground Support Equipment (OGSE) for the James Webb Space Telescope (JWST) at NASA Johnson Space Center (JSC). The coating was used as a mitigation tool to entrap persistent outgassed contaminants, specifically silicone based diffusion pump oil, from within JSC’s cryogenic optical vacuum chamber test facility called Chamber A. This paper summarizes the sample fabrication, installation, laboratory testing, post-test chemical analysis results, and future plans for the MAC technology, which was effectively used to protect the JWST test equipment from vacuum chamber contamination.

Contamination effects and requirements derivation for the James Webb Space Telescope

The James Webb Space Telescope (JWST) will carry on exploration of the early universe with a 6-m exposed primary mirror and cryogenically cooled instruments. The mirror and its instruments will perform extremely deep exposures at near infra-red wavelengths (0.6-30 microns), and will operate for 5-10 years. The contamination effects of foremost concern on JWST are those of scatter due to particulate contamination on the primary mirror, loss of transmission from particulate, molecular and ice contamination, and loss of detector operation due to ice forming during cool-down of the observatory. The effects on JWST science of these contamination sources will be described together with how requirements for cleanliness levels were subsequently established.

EFFECTS OF MANUFACTURING AND DEPLOYMENT ON THIN FILMS FOR THE NGST SUNSHADE

The Next Generation Space Telescope (NGST) is being developed as an advanced astronomical observatory. The NGST proposes to utilize several thin film membrane layers to create a shield for protection of the telescope from solar thermal energy and stray light, The shield will take the form of a polygon, approximately 15 x 30 m, with individual membrane layers positioned so that they do not come in contact with one another. The membrane shield will be deployed and supported by a series of booms, which will be packed into a small volume for launch. Finally, the shield will be deployed on orbit. Several film materials are being considered for the membrane shield, including CPI, Kapton E, Kapton RN, and Upilex. Each of these polyimide materials was tested to determine their durability over the 10-year mission. New facets of materials testing have been introduced in this study to develop performance data with greater realism to actual use, particularly that of degradation from packing, launch and deployment processing. Materials were exposed to handling that simulated the life of the materials from manufacture through deployment with standardized fixtures and then exposed to a simulated, L2, 10-year radiation environment. Mechanical and thermal radiative properties were measured before and after each phase of testing. This paper summarizes the program and test results.

Next Generation Space Telescope (NGST) thin film materials test program

A test program has been implemented to evaluate candidate thin film materials for the sun-facing layer of the Next Generation Space Telescope (NGST) sunshield. Various polymers are being tested to determine if any can survive the radiation environment of the proposed NGST orbit (the second Sun-Earth lagrangian point or L2). This testing will characterize the mechanical and thermal properties before and after exposure to a simulated NGST sunshield environment. In addition, because the sunshield will be folded and stowed before launch, the candidate materials will be folded, stowed and unfolded (deployed) to determine if they can survive this type of handling and storage. Based on the results of this testing, candidates will be down selected for further development and testing. Future development will include the addition of optical coatings, rip-stop for tear resistance, and seaming techniques.

Contamination control requirements implementation for the James Webb Space Telescope (JWST), part 2: spacecraft, sunshield, observatory, and launch

This paper will continue from Part 1 of JWST contamination control implementation. In addition to optics, instruments, and thermal vacuum testing, JWST also requires contamination control for a spacecraft that must be vented carefully in order to maintain solar array and thermal radiator thermal properties; a tennis court-sized sunshield made with 1-2 mil Kapton™ layers that must be manufactured and maintained clean; an observatory that must be integrated, stowed and transported to South America; and a rocket that typically launches commercial payloads without contamination sensitivity. An overview of plans developed to implement contamination control for the JWST spacecraft, sunshield, observatory and launch vehicle will be presented.

Contamination control requirements implementation for the James Webb Space Telescope (JWST), part 1: optics, instruments and thermal vacuum testing

The derivation of contamination control (CC) requirements for the JWST Optical Telescope Element (OTE) was presented at the SPIE conference in 20081. Since then, much work has been done to allocate contamination at each phase of Integration and Test (IandT) and to plan for achieving the allocations. Because JWST is such a large and complicated observatory, plans for meeting the requirements are many and varied. There are primary mirror segments that must be cleaned early and maintained clean; there are four science instruments that each have tight contamination requirements but cannot be cleaned after they are integrated onto the Integrated Science Instrument Module (ISIM) structure; there is the composite ISIM structure that is fragile and must be minimally handled; there are numerous cryo-vacuum tests that must be controlled and monitored in order to minimize molecular contamination during return to ambient; … and more. An overview of plans developed to implement contamination control for JWST optics, instruments, and thermal vacuum testing for JWST will be presented.

Evaluation of thin films for the Next-Generation Space Telescope (NGST) sunshield

The optics and detectors for the NGST, will operate at IR wavelengths between 0.5 and 30 micrometers . To accomplish the requirements set for NGST, the telescope and science module will have to operate at temperatures below 60 K. To achieve cryogenic temperatures, several of the current designs for NGST use a large deployable sunshield to passively cool the telescope. The current concepts for the sunshield consist of 4 to 6 layers of thin film thermal control material supported by deployable struts. The sunshield will need to be about 30 by 15 meters, and will have to survive for 10 years in a deeps space environment. A program has been initiated to identify thin film materials that will meet the NGST sunshield requirements. The first step in this program is a literature research that has identified potential thin film materials and coatings for the sunshield. The second step will involve an initial screening of these materials, followed by more rigorous testing of selected candidate materials. This testing will characterizes the mechanical, thermal and optical properties before and after exposure to a simulated NGST sunshield environment. In addition, because the sunshield will be folded and stowed before launch, the candidate materials will be folded, stowed and unfolded before exposure to the simulated environment.