KEYWORDS: Systems modeling, James Webb Space Telescope, Observatories, Thermal modeling, Mathematical modeling, Space telescopes, Integrated modeling, Performance modeling, Systems engineering, Neodymium
The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope. System-level verification of
critical performance requirements will rely on integrated observatory models that predict optical response accurately
enough to verify that the allocated top-level wavefront error of 150 nm root-mean-squared (rms) through to wavefront
sensor focal plane is met. The assembled models themselves are complex and require the insight of technical experts
working as a team across multiple disciplines and organizations. Furthermore, responses in several key disciplines are
strongly cross-coupled. This paper describes the breadth of the systems engineering and modeling approach used on the
JWST including technical performance metrics management. The scope of the systems-level modeling includes stowed
dynamics, deployed dynamics, thermal, thermal distortion, straylight, optics and attitude control systems.
KEYWORDS: Systems modeling, James Webb Space Telescope, Observatories, Thermal modeling, Data modeling, Mathematical modeling, Solid modeling, Space telescopes, Performance modeling, Distortion
The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014.
The imaging performance of the telescope will be diffraction limited at 2μm, defined as having a Strehl ratio >0.8.
System-level verification of critical performance requirements will rely on integrated observatory models that predict the
wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms)
through to the wave-front sensor focal plane is met. Furthermore, responses in several key disciplines are strongly crosscoupled.
The size of the lightweight observatory structure, coupled with the need to test at cryogenic temperatures,
effectively precludes validation of the models and verification of optical performance with a single test in 1-g. Rather, a
complex series of incremental tests and measurements are used to anchor components of the end-to-end models at
various levels of subassembly, with the ultimate verification of optical performance is by analysis using the assembled
models. The assembled models themselves are complex and require the insight of technical experts to assess their ability
to meet their objectives. This paper describes the modeling approach used on the JWST through the detailed design
phase.
Michael Menzel, Marie Bussman, Michael Davis, Gary Golnik, Sandra Irish, Jon Lawrence, Richard Lynch, Peiman Maghami, Landis Markley, Kimberly Mehalick, Gary Mosier, Danniella Muheim, Keith Parrish, Shaun Thomson, Paul Geithner, Joseph Pitman, James Wehner, Jonathan Arenberg, Brian Costanza, Satya Anandakrishnan, William Burt, Reem Hejal
KEYWORDS: Observatories, James Webb Space Telescope, Thermal modeling, Space telescopes, Cryogenics, Solid modeling, Systems engineering, Systems modeling, Integrated modeling, Space operations
The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014.
System-level verification of critical performance requirements will rely on integrated observatory models that predict the
wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms)
through to the wave-front sensor focal plane is met. This paper describes the systems engineering approach used on the
JWST through the detailed design phase.
KEYWORDS: Wavefronts, James Webb Space Telescope, Point spread functions, Mirrors, Space telescopes, Wavefront sensors, Motion models, Observatories, Telescopes, Spatial frequencies
This paper is part five of a series on the ongoing optical modeling activities for the James Webb Space Telescope
(JWST). The first two papers discussed modeling JWST on-orbit performance using wavefront sensitivities to
predict line of sight motion induced blur, and stability during thermal transients. The third paper investigates the
aberrations resulting from alignment and figure compensation of the controllable degrees of freedom (primary and
secondary mirrors), which may be encountered during ground alignment and on-orbit commissioning of the
observatory, and the fourth introduced the software toolkits used to perform much of the optical analysis for JWST.
The work here models observatory operations by simulating line-of-sight image motion and alignment drifts over a
two-week period. Alignment updates are then simulated using wavefront sensing and control processes to calculate
and perform the corrections. A single model environment in Matlab is used for evaluating the predicted
performance of the observatory during these operations.
KEYWORDS: Space telescopes, Telescopes, Helium, Thermal modeling, James Webb Space Telescope, Cryocoolers, Liquids, Space operations, Infrared telescopes, Copper
To take advantage of the unique environment of space and optimize infrared observations for faint sources, space
telescopes must be cooled to low temperatures. The new paradigm in cooling large space telescopes is to use a
combination of passive radiative cooling and mechanical cryocoolers. The passive system must shield the
telescope from the Sun, Earth, and the warm spacecraft components while providing radiative cooling to deep
space. This shield system is larger than the telescope itself, and must attenuate the incoming energy by over one
million to limit heat input to the telescope. Testing of such a system on the ground is a daunting task due to the
size of the thermal/vacuum chamber required and the degree of thermal isolation necessary between the room
temperature and cryogenic parts of the shield. These problems have been attacked in two ways: by designing a
subscale version of a larger sunshield and by carefully closing out radiation sneak paths. The 18% scale (the
largest diameter shield was 1.5 m) version of the SPIRIT Origins Probe telescope shield was tested in a low cost
helium shroud within a 3.1 m diameter x 4.6 m long LN2 shrouded vacuum chamber. Thermal straps connected
from three shield stages to the liquid helium cooled shroud were instrumented with heaters and thermometers to
simulate mechanical cryocooler stages at 6 K, 18-20 K, and 45-51 K. Performance data showed that less than 10
microwatts of radiative heat leaked from the warm to cold sides of the shields during the test. The excellent
agreement between the data and the thermal models is discussed along with shroud construction techniques.
Conference Committee Involvement (2)
Modeling, Systems Engineering, and Project Management for Astronomy V
1 July 2012 | Amsterdam, Netherlands
Modeling, Systems Engineering, and Project Management for Astronomy IV
27 June 2010 | San Diego, California, United States
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