NASA’s James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element which contains four science instruments (SIs). Prior to integration with the spacecraft, the JWST optical assembly is put through rigorous launch condition environmental testing. This work reports on the metrology operations conducted to measure changes in subassembly alignment, including the primary mirror segments, the secondary mirror to its support structure, the tertiary mirror assembly to the backplane of the telescope and ISIM.
NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Primary Mirror Backplane Support Structure (PMBSS) and Integrated Science Instrument Module (ISIM) Electronics Compartment (IEC) which is designed to integrate to the spacecraft bus via six cup/cone interfaces. Prior to integration to the spacecraft bus, the JWST observatory must undergo environmental testing, handling, and transportation. Multiple fixtures were developed to support these tasks including the vibration fixture and handling and integration fixture (HIF). This work reports on the development of the nominal alignment of the six interfaces and metrology operations performed for the JWST observatory to safely integrate them for successful environmental testing.
The NASA Goddard Space Flight Center (GSFC) and its partners have broad experience in the alignment of flight optical instruments and spacecraft structures. Over decades, GSFC developed alignment capabilities and techniques for a variety of optical and aerospace applications. In this paper, we provide an overview of a subset of the capabilities and techniques used on several recent projects in a “toolbox” format. We discuss a range of applications, from small-scale optical alignment of sensors to mirror and bench examples that make use of various large-volume metrology techniques. We also discuss instruments and analytical tools.
NASA’s James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element which contains four science instruments (SI), including a guider. The SIs and guider are mounted to a composite metering structure with outer envelope approximate measurements of 2.2x2.2x1.7m. These SI units are integrated to the ISIM structure and optically tested at NASA Goddard Space Flight Center as an instrument suite using an Optical telescope element SIMulator (OSIM). OSIM is a high-fidelity, cryogenic JWST simulator that features a ~1.5m diameter powered mirror. The SIs are aligned to the flight structure’s coordinate system under ambient, clean room conditions using opto-mechanical metrology and customized interfaces. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors and metrology in six degrees of freedom. SI performance, including focus, pupil shear, pupil roll, boresight, wavefront error, and image quality, is evaluated at the operating temperature using OSIM. This work reports on the as-run ambient assembly and ambient alignment steps for the flight ISIM, including SI interface fixtures and customization and kinematic mount adjustment. The ISIM alignment plan consists of multiple steps to meet the “absolute” alignment requirements of the SIs and OSIM to the flight coordinate system. In this paper, we focus on key aspects of absolute, optical-mechanical alignment. We discuss various metrology and alignment techniques. In addition, we summarize our approach for dealing with and the results of ground-test factors, such as gravity.
While efforts within the optics community focus on the development of high-quality systems and data products, comparatively little attention is paid to their use. Our standards for verification and validation are high; but in some user domains, standards are either lax or do not exist at all. In forensic imagery analysis, for example, standards exist to judge image quality, but do not exist to judge the quality of an analysis. In litigation, a high quality analysis is by default the one performed by the victorious attorney’s expert. This paper argues for the need to extend quality standards into the domain of imagery analysis, which is expected to increase in national visibility and significance with the increasing deployment of unmanned aerial vehicle—UAV, or “drone”—sensors in the continental U. S.. It argues that like a good radiometric calibration, made as independent of the calibrated instrument as possible, a good analysis should be subject to standards the most basic of which is the separation of issues of scientific fact from analysis results.
The optical alignment of the star trackers on the Global Precipitation Measurement (GPM) core spacecraft at NASA
Goddard Space Flight Center (GSFC) was challenging due to the layout and structural design of the GPM Lower Bus
Structure (LBS) in which the star trackers are mounted as well as the presence of the star tracker shades that blocked
line-of-sight to the primary star tracker optical references. The initial solution was to negotiate minor changes in the
original LBS design to allow for the installation of a removable item of ground support equipment (GSE) that could be
installed whenever measurements of the star tracker optical references were needed. However, this GSE could only be
used to measure secondary optical reference cube faces not used by the star tracker vendor to obtain the relationship
information and matrix transformations necessary to determine star tracker alignment. Unfortunately, due to
unexpectedly large orthogonality errors between the measured secondary adjacent cube faces and the lack of cube
calibration data, we required a method that could be used to measure the same reference cube faces as originally
measured by the vendor. We describe an alternative technique to theodolite autocollimation for measurement of an
optical reference mirror pointing direction when normal incidence measurements are not possible. This technique was
used to successfully align the GPM star trackers and has been used on a number of other NASA flight projects. We also
discuss alignment theory as well as a GSFC-developed theodolite data analysis package used to analyze angular