TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 4
4.2 Subsystem and System Testbeds
4.2.1 Closed-loop Secondary Mirror Position Control
Objective
The TPF telescope requires very high stability to enable transmission of a wavefront to the
coronagraph meeting the requirements for planet finding. A critical aspect of the telescope
stability is the position of the secondary mirror with respect to the primary. The TPF
requirements allow for no more than 26-nm variation in the piston and transverse directions, and
100-nrad variation in the tip and tilt directions, over a timescale of 24 hours. This level of control
cannot be maintained passively, but must be provided with an active feedback correction system.
The feedback system will include two main aspects: an error signal that will indicate the
deviation of the secondary mirror from its nominal operating point, and an actuator to correct the
mirror positioning so that the error signal is driven to zero. The error signal will be provided by
the laser metrology system described earlier. The actuation system will incorporate a hexapod, a
high precision positioning device with six actuator legs (described above). The hexapod allows
for 6-DOF positioning and possesses a high degree of stiffness that can provide relatively fast
(>1 Hz) control.
The objective of this activity will ultimately be the development and test of a sub-scale
secondary mirror position control system, which will include primary and secondary mirrors and
a support tower for the hexapod. The activity will proceed in the stages described below.
Approach
A testbed designed for the study of closed-loop interferometric control at low frequency is now
operational at GSFC, as in Figure 4-10. The testbed contains a vacuum system housing two
hexapods, which in turn will support a small, simple model of the TPF telescope optics. Two
frequency-stabilized lasers are used in the testbed. One utilizes the metrology scheme to measure
the variation of position of the secondary optic, and then feeds this signal back to the hexapod to
stabilize the optic position. The second laser system is used to independently confirm that the
residual noise in the motion of the model telescope is at the level required by TPF. The testbed
will operate in the presence of laboratory seismic and thermal disturbances (of order 10 -6 m over
1 hour), which is much larger than the disturbance expected in space. However, the stability of
the testbed is limited only by the sensitivity of the metrology system, thus control of the model
telescope at the 10 -10 m level will be possible. In fact, the development of this level of control in
the presence of environmental disturbances will allow the design of future larger-scale
prototypes that can be useful for end-to-end telescope testing in the laboratory.
This testbed will allow for the study and characterization of many elements that will be critical to
designing a realistic control system, including: testing of the metrology system, including its
sensitivity to motion in all 6 DOF; measurement of non-linear behavior and hysteresis in the
hexapod PZT actuators; the effect of the hexapod internal resonances on the control system; and
overall performance of the control system in real time. The insights gained from this testbed will
allow for the design of a realistic, full-size secondary mirror positioning system with a high
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