The James Webb Space Telescope (JWST) is a large space based astronomical telescope that will operate at
cryogenic temperatures. The telescope by virtue of its size must be stowed in an inoperable configuration for launch
and remotely reconfigured in space to meet the operational requirements using active Wave Front Sensing and
Control (WFSC). This paper will report on the optical budgeting process used to manage the performance of the
active system. The current status of the design and verification of the optical hardware, the WFSC processes, and
the total system verification modeling will be presented. More detailed discussions of the system verification by
analysis will be presented in separate accompanying papers.
The Kepler spacecraft and telescope were designed, built and tested at Ball Aerospace & Technologies Corporation in
Boulder, Colorado. The Kepler spacecraft was successfully launched from NASA's Kennedy Space Center on March 6,
2009. In order to adequately support the Kepler mission, Ball Aerospace upgraded its optical testing capabilities. This
upgrade facilitated the development of a meter-class optical testing capability in a thermal vacuum (TVAC)
environment. This testing facility, known as the Vertical Collimator Assembly (VCA), was used to test the Kepler
telescope in 2008. Ball Aerospace designed and built the VCA as a 1.5m, f/4.5 collimator that is an un-obscured system,
incorporating an off-axis parabola (OAP) and test flat coated for operations in the VIS-IR wavelength region. The VCA
is operated in a large thermal vacuum chamber and has an operational testing range of 80 to 300K (-315 to 80°F). For
Kepler testing, the VCA produced a 112nm rms wavefront at cryogenic temperatures. Its integral autocollimation and
alignment capabilities allowed knowledge of the collimated wavefront characteristics to <5nm rms during final thermal
vacuum testing. Upcoming modifications to the VCA optics will bring the VCA wavefront to <20nm rms. The VCA
optics are designed and mounted to allow for use in either a vertical or horizontal orientation without degradation of the
collimated optical wavefront.
Ball Aerospace has constructed a new collimator for interferometric and image quality testing of meter scale optical
systems under cryogenic, vacuum conditions. Termed the Vertical Collimator Assembly (VCA), it features 1.5 m
diameter off-axis parabolic and calibration flat mirrors. In order to preserve as large a volume as possible for the unit
under test, the main platform is suspended inside its vacuum chamber by a hexapod, with the parabolic mirror mounted
overhead. A simultaneous interferometer facilitates collimator alignment and monitoring, as well as wavefront quality
measurements for the test unit. Diffusely illuminated targets may be employed for through-focus image quality
measurements with pinholes and bar targets. Mechanical alignment errors induced by thermal and structural
perturbations are monitored with a three-beam distance measuring interferometer to enable mid-test compensation.
Sources for both interferometer systems are maintained at atmospheric pressure while still directly mounted to the main
platform, reducing vibration and stability problems associated with thermal vacuum testing. Because path lengths inside
the ambient pressure vessels are extremely short, problems related to air turbulence and layering are also mitigated. In-chamber
support equipment is insulated and temperature controlled, allowing testing while the chamber shrouds and test
unit are brought to cryogenic temperatures.