TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 2
2 Error Budgets
2.1 Contrast Error Budget
Error Budget Description
An efficient stellar coronagraph blocks virtually all of the light from a target star by presenting a
highly corrected wavefront to an efficient diffraction control system. At any point in the image
plane, the ratio of the residual light level to the light that would appear if the stellar image was
centered at that point is termed the “contrast.” For TPF-C, the residual diffracted and scattered
starlight in the image plane must be many orders of magnitude below the direct light from the
target star to detect terrestrial planets via reflected starlight. The contrast describes the
fundamental performance required of the observatory system, and so has been chosen as the toplevel
metric. The requirement is a contrast of 10 -10 over an angle representing the habitable zone
(where liquid water could exist), stable to a level of 2.5 × 10 -11 for up to 24 hours.
The contrast error budget (CEB) rolls up the allocations for individual error contributions into an
observatory system contrast. The CEB is an on-going process, with the document held by the
system architect and iterated regularly to reflect changing design baselines and system
understanding. It is used to manage the allocation of challenging requirements between system
components and to manage the reserve margins on each of those allocations. The CEB guides
directly the technology effort, serving to highlight requirements beyond the state-of-the-art,
thereby triggering development activities.
The error budget allocation process begins with a first-order sensitivity analysis. Engineering
judgment is used to partition allowable errors throughout the subsystems. In some cases, the
allocations point directly to the difficult requirements, such as primary mirror stability, while in
others requirements are derived indirectly through engineering analysis, as is the case for
temperature stability requirements on the primary mirror. Reserve factors are allocated for each
source and account for the performance reserve, the modeling uncertainty factor, and the error in
the modeling. The modeling uncertainty relates to aspects of the model that do not accurately
reflect physical behavior, while the modeling error refers to inaccuracies in the as-built model or
physical properties. These reserve allocations are initially chosen based on engineering
judgment and over time modified to reflect bounding of model calculations via testbed results.
The TPF-C high-level error budget is shown in Figure 2-1.
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