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
NASA's James Webb Space Telescope (JWST) will be a premier space science program for astrophysics following
launch scheduled for 2014. JWST will observe the early universe, with emphasis on the time period during which the
first stars and galaxies began to form. JWST has a 6.5 m diameter (25 square meters of collecting area), deployable,
active primary mirror operating at cryogenic temperatures.
The James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space
astronomy (~40K). The JWST Observatory architecture includes the Optical Telescope Element and the Integrated Science
Instrument Module (ISIM) element that contains four science instruments (SI) including a Guider. The ISIM structure must meet
its requirements at the ~40K cryogenic operating temperature.
The SIs are aligned to the structure's coordinate system under ambient, clean room conditions using laser tracker and theodolite
metrology. The ISIM structure is thermally cycled for stress relief and in order to measure temperature-induced mechanical,
structural changes. These ambient-to-cryogenic changes in the alignment of SI and OTE-related interfaces are an important
component in the JWST Observatory alignment plan and must be verified.
We report on the planning for and preliminary testing of a cryogenic metrology system for ISIM based on photogrammetry.
Photogrammetry is the measurement of the location of custom targets via triangulation using images obtained at a suite of digital
camera locations and orientations. We describe metrology system requirements, plans, and ambient photogrammetric
measurements of a mock-up of the ISIM structure to design targeting and obtain resolution estimates. We compare these
measurements with those taken from a well known ambient metrology system, namely, the Leica laser tracker system.
This paper will discuss the details of the metrology associated with the integration and testing of spacecraft systems and scientific instruments at the NASA Goddard Space Flight Center (NASA GSFC). Specifically, this paper will outline the process for correlating theodolite autocollimation measurements with theodolite coordinate triangulation measurements, laser tracker coordinate measurements, photogrammetry camera system, and other coordinate measurement techniques. For theodolite autocollimation data, NASA GSFC developed a Microsoft Excel-based spreadsheet program to calculate the transformation matrices from reference cube pointing directions into spacecraft coordinates defined by physical features. The autocollimated image return from the mirrored faces of the reference cubes are measured relative to each other and define unit vectors that point in the direction perpendicular to the cube face surface. The roll, zenith, pitch, and yaw are calculated from the direction cosines of the unit vectors that define the directional pointing rotations around coordinate axes. The theodolite-based pointing vectors are then transformed to the spacecraft coordinate system. The Brunson Spatial Analyzer<sup>TM</sup> coordinate measuring software program is used to analyze data from theodolites using triangulation on target positions, a laser tracker coordinate measuring system, a photogrammetry system or any other coordinate measuring system. All the coordinate data is tied into theodolite coordinate data by measuring common targets. To correlate theodolite autocollimation on cube faces to the point coordinate location data, one must first measure the test object with the Spatial Analyzer<sup>TM</sup> theodolite triangulation coordinate system. From coordinate features, a spacecraft coordinate system is defined by the blueprint design. One of the Spatial Analyzer<sup>TM</sup> theodolites is used as the primary reference for the auto-collimation measurements. This ties together the coordinate target point locations to the pointing directions of mirrored surfaces of cubes.