TPF-I SWG Report - Exoplanet Exploration Program - NASA

TPF-I F L I G H T B ASELINE D ESIGN
the way to L2, the cruise stage performs a slow “barbecue” roll to maintain a benign thermal environment
for the spacecraft within its shell. After arrival at L2, the cruise stage is used to deploy the individual
spacecraft of the observatory one at a time. Ground operators verify successful deployment of each
spacecraft before deploying subsequent spacecraft. After all the spacecraft are deployed, the formation is
formed, and calibration begins. Following initial calibrations, the observatory is commissioned, and the
prime mission begins.
Table 1-2 has a summary of the properties of the current TPF-I observatory concept. The prime mission
lasts 5 years with approximately 3 years budgeted for star-system surveys and 2 years budgeted for
detailed, follow-up studies of targets found by the surveys. Enough expendable resources are carried to
permit extending the mission another 5 years if consumption of these resources is as predicted and the
observatory remains healthy.
Observations consist of aligning the observatory’s viewing axis to a target star, adjusting the formation
baseline length to an optimum value (tuning), and then rotating about that axis for multiple hours until a
sufficient signal-to-noise ratio (SNR) is attained. Depending on the length of the observation, data are
either downlinked before slewing to the next target system or recorded and played back after completing
the observation of a multiple-target set. It is envisioned that the observatory will be capable of
completing slews and observations of multiple targets autonomously. However, it is not certain that this
capability will be used since the frequency of calibrations requiring ground interaction has not been
analyzed yet.
Geometrical thermal constraints will limit the target set to stars within ~± 45° of the ecliptic. This band
of stars will be observed multiple times as the Earth/observatory system orbits the Sun. The target set
includes many of the stars to be observed by the TPF Coronagraph.
5.8 Performance of Flight Baseline Design
5.8.1 Inner and Outer Working Angles
Figure 5-18 shows how the rms signal from a planet of unit flux varies with position relative to the star.
The rms is taken over the full range of array-rotation angles. These maps of the ‘rms modulation
efficiency’ show how the peak of the PSF varies with position. The example on the left is for the smallest
array size of 120 × 20 m, at a wavelength of 10 μm. The response for the largest array size of 612 × 102
m is shown on the right. The overall response is a product of the primary beam taper and the effect of the
interferometric fringes.
The outer working angle (OWA) is taken to be the half-power point of the primary beam taper. With a
collector diameter of 3.8 m, this is 280 mas, although the value is increased if we consider the nonuniform
response of the single-mode fiber over the input aperture. The OWA is independent of the array
size. At longer wavelengths the taper is reduced.
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