Jacqueline A. Townsend

Wide Field Camera 3: a powerful new imager for the Hubble Space Telescope

Wide Field Camera 3 (WFC3) is a powerful UV/visible/near-infrared camera that has just completed development for installation into the Hubble Space Telescope during upcoming Servicing Mission 4. WFC3 provides two imaging channels. The UVIS channel incorporates a 4102 × 4096 pixel CCD focal plane with sensitivity from 200 to 1000 nm and a 162 × 162 arcsec field of view. The UVIS channel features unprecedented sensitivity and field of view in the near ultraviolet for HST, as well as a rich filter set that complements the visible capabilities of the HST/Advanced Camera for Surveys, whose repair will be attempted in the Servicing Mission. The IR channel features a 1024 × 1024 pixel HgCdTe focal plane covering 850 to 1700 nm with a 136 × 123 arcsec field of view, providing a major advance in IR survey efficiency for HST. We report here on the design of the instrument, on recent activities that have completed the integration of the instrument for flight, and on results of the ground test and calibration program.

Environmental Exposure Conditions for Teflon® Fluorinated Ethylene Propylene on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in the FEP materials retrieved from solar-facing surfaces on the HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a failure review board was convened to address the problem of degradation of MLI on the HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, the environmental exposure conditions for the FEP surfaces on the HST were estimated, including: the number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons and electrons and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the HST, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on the HST.

Hubble Space Telescope Metallized Teflon ® FEP thermal Control Materials: On-Orbit Degradation and Post-Retrieval Analysis

During the Hubble Space Telescope (HST) second servicing mission (SM2), degradation of unsupported Teflon® FEP (fluorinated ethylene propylene), used as the outer layer of the multilayer insulation (MLI) blankets, was evident as large cracks on the telescope light shield. A sample of the degraded outer layer was retrieved during the mission and returned to Earth for ground testing and evaluation. The results of the Teflon® FEP sample evaluation and additional testing of pristine Teflon® FEP led the investigative team to theorize that the HST damage was caused by thermal cycling with deep-layer damage from electron and proton radiation which allowed the propagation of cracks along stress concentrations, and that the damage increased with the combined total dose of electrons, protons, ultraviolet and x-ray radiation along with thermal cycling. This paper discusses the testing and evaluation of the retrieved Teflon® FEP.

Effects of Radiation and Thermal Cycling on Teflon ® FEP:

Surfaces of the aluminized Teflon® FEP (fluorinated ethylene propylene) multilayer thermal insulation on the Hubble Space Telescope (HST) were found to be cracked and curled in some areas at the time of the second servicing mission (SM2) in February 1997, 6.8 years after HST was deployed in low Earth orbit (LEO). In an effort to understand what elements of the space environment might cause such damage, pristine second-surface aluminized Teflon® FEP was tested for durability to various types of radiation, to thermal cycling and to radiation followed by thermal cycling. Types of radiation included synchrotron vacuum ultraviolet and soft x-ray radiation, simulated solar flare x-ray radiation, electrons and protons. Thermal cycling was conducted in various temperature ranges to simulate HST orbital conditions for Teflon® FEP. Resultsoftensiletestingoftheexposedspecimensshowedthatexposuretohighfluencesofradiation caused degradation in tensile properties of FEP. However, exposure to radiation alone in exposures comparable to those experienced by HST did not produce reduction in ultimate tensile strength and elongation of Teflon® similar to that observed for HST-retrieved aluminized Teflon®. Synergism of radiation exposure and thermal cycling was evident in the results of three experiments: thermal cycling following electron and proton irradiation, thermal cycling following x-ray exposure, and additional thermal cycling of a sample retrieved from HST. However, irradiation and thermal cycling with comparable HST SM2 exposure conditions did not produce the degradation observed in the FEP material retrieved during HST SM2.

Mechanical Properties Degradation of Teflon ® FEP Returned From the Hubble Space Telescope

After 6.8 years on orbit, degradation has been observed in the mechanical properties of second-surface metalized Teflon® FEP (fluorinated ethylene propylene) used on the Hubble Space Telescope (HST) on the outer surface of the multi-layer insulation (MLI) blankets and on radiator surfaces. Cracking of FEP surfaces on HST was first observed upon close examination of samples with high solar exposure retrieved during the first servicing mission (SM1) conducted 3.6 years after HST was put into orbit. Astronaut observations and photographs from the second servicing mission (SM2), conducted after 6.8 years on orbit, revealed severe cracks in the FEP surfaces of the MLI on many locations around the telescope. This paper describes results of mechanical properties testing of FEP surfaces exposed for 3.6 years and 6.8 years to the space environment on HST. These tests include bend testing, tensile testing, and surface micro-hardness testing.

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.

Ground-Based Testing of Replacement Thermal Control Materials for the Hubble Space Telescope

The mechanical and optical properties of the metallized Teflon® FEP thermal control materials on the Hubble Space Telescope (HST) have degraded over the nearly seven years the telescope has been in orbit. Given the damage to the outer layer of the multilayer insulation (MLI) blanket that was apparent during the second servicing mission (SM2), the decision was made to replace the outer layer during subsequent servicing missions. A Failure Review Board was established to investigate the damage to the MLI and identify a replacement material. The replacement material had to meet the stringent thermal requirements of the spacecraft and maintain mechanical integrity for at least ten years. Ten candidate materials were selected and exposed to ten-year HST-equivalent doses of simulated orbital environments. Samples of the candidates were exposed sequentially to lowand high-energy electrons and protons, atomic oxygen, x-ray radiation, ultraviolet radiation and thermal cycling. Following the exposures, the mechanical integrity and optical properties of the candidates were investigated using optical microscopy, scanning electron microscopy (SEM), a laboratory portable spectroreflectometer (LPSR) and a Lambda 9 spectroreflectometer. Based on the results of these simulations and analyses, the Failure Review Board selected a replacement material and two alternatives that showed the highest likelihood of providing the requisite thermal properties and surviving for ten years in orbit.

ATLAST-9.2m: a large-aperture deployable space telescope

We present results of a study of a deployable version of the Advanced Technology Large-Aperture Space Telescope (ATLAST), designed to operate in a Sun-Earth L2 orbit. The primary mirror of the segmented 9.2-meter aperture has 36 hexagonal 1.315 m (flat-to-flat) glass mirrors. The architecture and folding of the telescope is similar to JWST, allowing it to fit into the 6.5 m fairing of a modest upgrade to the Delta-IV Heavy version of the Evolved Expendable Launch Vehicle (EELV). We discuss the overall observatory design, optical design, instruments, stray light, wavefront sensing and control, pointing and thermal control, and in-space servicing options.

On-orbit performance of HST Wide Field Camera 3

The Wide Field Camera 3 (WFC3) was installed into the Hubble Space Telescope during Servicing Mission 4 in May 2009. This panchromatic camera considerably improves the ultraviolet, visible, and infrared imaging capabilities of HST. Commissioned over the summer of 2009, WFC3 is now fully functional and responsible for approximately half of the Cycle 17 HST Science Program. This paper will review the scientific performance of WFC3 including its sensitivity in absolute terms and relative to other HST instruments. The paper will also discuss the calibration programs for WFC3 and the achieved photometric and astrometric calibration accuracies. Lessons learned from the ground calibration and in-flight commissioning will also be considered.

Wide Field Camera 3: science capabilities and plans for flight operation

Wide Field Camera 3 is the next generation of Hubble Space Telescope imaging instruments. Designed to complement and extend the existing capabilities of the HST, WFC3 will provide large increases in scientific performance in the near ultraviolet and near infrared wavelength regions. This paper describes the scientific capabilities of WFC3, provides a projection of its anticipated scientific performance, and discusses the plans for on-orbit testing and calibration during the Servicing Mission Orbital Verification period.