TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 3
observations. The time it takes to sense and null the speckles must take no longer than the time it
takes to detect a planet.
Typically a sequence of images is taken with a camera to observe residual light from the on-axis
source in the presence of a known induced diversity. For example, on HCIT the DM actuators
are moved in a predetermined way while a series of images is collected at either the postcoronagraph
pupil 10 or post-coronagraph focal plane. 11 Other approaches have considered using
phase-retrieval using imagery collected about the occulter focal plane, 12,13 by inducing diversity
through the optical element alignments or by the introduction of a coherent reference beam to
conduct direct interferometry on the speckles. 14
Ultimately the wavefront sensing schemes must be well matched to the wavefront control and
coronagraph architectures in a way the enables efficient and reliable high contrast imaging.
Progress to Date
Experiments on the HCIT are in part the demonstration of a single-DM-based wavefront sensing
and control architecture. Aside from having a single DM at a pupil, the HCIT employs a Lyot
coronagraph that has a HEBS glass occulting spot. The camera at the final focal plane represents
the science camera. Using a phase-retrieval approach at the HCIT occulter focus, it has been
demonstrated that the testbed is stable to the l/10000 level. 13 Using a focal-plane speckle nulling
approach 11 the residual light from the on-axis source has been suppressed to levels approaching
10 -9 over a portion of the controllable field of view. See Section 3.2.1 for recent HCIT
experimental results.
3.1.4 Deformable Mirrors
Objective
Unlike adaptive optics systems designed for correction of atmospheric seeing in ground based
observatories, the active optical system for a space telescope needs only to correct for wavefront
errors created in the telescope itself. The magnitude of wavefront errors is reduced to the
magnitude of errors expected in a diffraction-limited optical system, and the bandwidth required
to follow significant wavefront drift is reduced from kHz rates to the time scales associated with
mechanical and thermal stability of spacecraft systems. The accuracy with which the wavefront
can be corrected is fundamentally limited to the accuracy of wavefront error information that can
10 S. B. Shaklan, D. Moody, and J. J. Green, “Residual wave front phase estimation in the reimaged Lyot
plane for the Eclipse coronagraphic telescope,” Proc. SPIE Int. Soc. Opt. Eng. 4860, 229 (2003).
11 J. T. Trauger, C. Burrows, B. Gordon, J. J. Green, A. E. Lowman, D. Moody, A. F. Niessner, F. Shi,
and D. Wilson, “Coronagraph contrast demonstrations with the high-contrast imaging testbed,” Proc.
SPIE Int. Soc. Opt. Eng. 5487, 1330 (2004).
12 J. J. Green, D. C. Redding, S. B. Shaklan, and S. A. Basinger, “Extreme wave front sensing accuracy
for the Eclipse coronagraphic space telescope,” Proc. SPIE Int. Soc. Opt. Eng. 4860, 266 (2003).
13 J. J. Green, S. A. Basinger, D. Cohen, A. F. Niessner, D. C. Redding, S. B. Shaklan, and J. T. Trauger,
“Demonstration of extreme wavefront sensing performance on the TPF high-contrast imaging testbed,”
Proc. SPIE Int. Soc. Opt. Eng. 5170, 38 (2003)
14 R. Angel, “Imaging Extrasolar Planet From the Ground,” ASP Conference Series, Vol. 294, (2003).
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