TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
degree of confidence, as the scaling of the control system with regard to the full telescope length
and optic weight will be straightforward.
Progress to Date
The field of precision control is extensive and well documented, including the use of the hexapod
for 6-DOF control. The challenge for TPF is to extend the hexapod control system to the subnanometer
level; this necessitates the use of laser interferometry to provide the control signal.
The most relevant area for the work described here is the ground-based gravitational wave field,
where laser interferometry is used to measure and control disturbances at the sub-picometer
level. For example, the Laser Interferometer Gravity Wave Observatory (LIGO) interferometer
has achieved control of the 30 cm diameter optics forming its resonant cavities (10 – 4000 m in
length) at the level of 10 -13 m over several tens of minutes 32 . In the LIGO case a frequency
stabilized laser is used to measure deviations of the cavity length from its nominal operating
position, and magnetic actuation is used to force the cavity deviations to zero. While there are
obvious differences between the LIGO and TPF configurations, the many similarities between
the two systems (stabilized laser for position error measurement, high gain control system,
precision mechanical actuation) give confidence that the TPF secondary mirror position control
requirements are achievable.
Stability information
Iodine stabilized laser
IFO output
“Evaluation (measurement)” beam
“Stabilization (locking)” beam
Iodine stabilized laser
IFO output
Hexapod PZT actuator
Figure 4-10. GSFC hexapod closed loop control testbed, showing vacuum system, hexapod
platforms, optics supported by hexapods, stabilized laser for error signal readout, and independent
stabilized laser to read out final residual noise.
32 P. Fritschel et al., “Readout and control of a power-recycled interferometric gravitational-wave
antenna”, Applied Optics 40, 4988-4998 (2001).
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