TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Error Budgets
Figure 2-3. Models used to create the TPF-C dynamic and static error budget.
Performance Implications
The error budget enables the calculation of contrast at selected points in the image plane (e.g., 3,
3.5, and 4λ/D). This approach has allowed us to determine that the dynamic part of the budget is
dominated by beam walk. Both throughput and low-order aberration stability considerations
lead to a practical limit to the inner working angle (IWA) of the coronagraph. Contrast stability
requirements applied to the IWA then determine the dynamic engineering requirements (e.g.
mirror shape stability, pointing stability, secondary mirror position stability). Within the target
angular range in the image plane (i.e., between the IWA and the outer working angle), the
achieved stability will improve as we move outward from the IWA.
The TPF error tree depicts the quantitative contrast contributions and margins for a given image
plane point with the contrast requirements appearing at the top level, as shown in Figure 2-1. In
this case the image is evaluated at an angle of 4 λ/D, where λ is the wavelength of the light and
D is the length of the long axis of the telescope. The baseline 8-m version of TPF must achieve
the required contrast working as close in as 4 λ/D to provide an adequate IWA no larger than 62
milli-arcsecs (mas) at λ=600 nm.
Thermally induced deformations of optical surfaces must meet the requirements shown in
Table 2-1. The allowed structural motions of the secondary mirror relative to the primary are
shown in Table 2-2. Recognizing that the required relative stability of their positions along the
line of sight to the star is sub-micron, we plan to implement a laser metrology truss between
them to control a hexapod mounted on the back of the secondary mirror. The metrology system
has a resolution and stability of ~200 pm, but it responds slowly (