TPF-I SWG Report - Exoplanet Exploration Program - NASA

D ESIGN AND A R C H I T E C T U R E T RADE S TUDIES
4.8.3 Post-Nulling Calibration
As previously explained, instability noise can wash out any planet signal unless the various electric fields
are matched in all particulars at the level of ~0.1%. The difficulty of attaining such levels of control (e.g.,
~1 nm path control) has motivated some research into ways to measure and remove nulling leakage after
beam combination (Guyon 2005; Lane, Muterspaugh and Shao 2006).
One such approach, dubbed “coherent calibration”, is outlined in Figure 4-20. In this approach, one takes
advantage of the fact that light leaking through the null is coherent with light from the star, but not with
light from the planet. Hence it is possible to use the bright output from the nuller (which would otherwise
simply be discarded) as a “reference beam” to be mixed with part of the nulled output from the
interferometer. This mixing process will yield an interferometric fringe, the amplitude of which is a
direct measure of the amount of starlight leaking through the nuller.
Simulations of coherent calibration shown in Fig. 4-21 indicate that it is possible relax the required levels
of field matching, possibly by factors of 10 or more. However, further work is required to demonstrate the
approach in the laboratory, and to understand the ultimate limits to the level of calibration precision that
can be attained before other limitations become manifest.
Figure 4-20. Schematic outline of coherent calibration. Part of the science beam (I d ) is mixed with a
reference beam (I b ) taken from discarded starlight; the resulting fringes are measured at I C1 and I C2 and
used to infer the amount of starlight leaking through the null and contaminating the science
measurement. Although only a single-Bracewell nuller is shown, the concept can be applied to more
complicated nulling architectures (e.g., the X array) in a straightforward manner.
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