TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 3
algorithms have limitations in achievable contrast, but they provide useful tools for diagnostic
and error modeling experiments. Combining speckle nulling with one of these approaches has
yielded improved control of the DM and better contrast. Other algorithms under development in
the TPF community will eventually be tested on the HCIT.
Illumination has been improved in the last year. Amplitude uniformity was improved
significantly by changing the input source from a fiber to a pinhole (by imaging the fiber onto a
pinhole inside the vacuum chamber). Increased efficiency in the illumination has made white
light operation more practical. Stability of the source has been studied, identifying the need to
carefully calibrate contrast measurements and to make further improvements to the illumination
design.
DM calibration was identified as an impediment to achieving convergence of the speckle nulling
algorithm. Due to the construction of the DM, the actuator gains differ slightly depending on
whether a single actuator or a group of adjacent actuators is moved. Experimentation using the
surface gauge, a Michelson interferometer in vacuum used to calibrate gains of the actuators, has
yielded a way to separate these effects and improve control of the DM in the testbed. Testbed
experiments have also led to improvements in the DM manufacturing process, notably in the
requirements on mirror polish.
Initial speckle nulling performance was limited by the 12-bit digitization used by the multiplexed
driver electronics that control the DM. An improved 16-bit multiplexer (MUX) was completed in
early 2004. The new MUX uses high voltage Application Specific Integrated Circuits (ASICs –
chips) situated in the vacuum chamber, simplifying cabling to the DM and replacing a large rack
of electronics with a compact system on the path toward a flight qualified MUX. With the MUX
no longer a limiting error source, contrast was improved to near its current levels.
Examples of laboratory results from the HCIT are shown in Figure 3-14. The HCIT has now
achieved a contrast of 0.9 × 10 -9 for laser light (λ = 785 nm), as shown in Figure 3-15. 30 This
contrast is an average measured in the half dark hole over a range of angles from 4 to 10 λ/D. At
the innermost speckle of interest (4 λ/D), the contrast is 3 × 10 -9 . Experiments in white light (40-
nm bandpass) have yielded an average contrast over the half dark hole of 5 × 10 -9 .
3.2.2 Planet Detection Simulator (PDS)
TPF-C is planning to add an industry-built planet detection simulator (PDS) to the HCIT. The
PDS will allow simulation of a variety of error sources expected in the flight telescope, including
phase, amplitude, polarization, and beam walk. The initial TPF-C plan was to let three study
contracts in FY04 with a downselect for hardware construction at the end of the fiscal year; due
to budget uncertainties, this was delayed. Current thinking has a single study contract starting at
the beginning of Phase A, followed by hardware design and fabrication. Delaying this activity to
Phase A ensures that the PDS will have an architecture similar to the baseline TPF-C design.
30 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,” in
Optical, Infrared, and Millimeter Space Telescopes, J. C. Mather, ed., Proc SPIE 5487, 1330-1336
(2004).
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