TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 5
5.4.4 Shear Metrology
Shear metrology uses the same laser beam as the pointing system; the two systems are closely coupled.
By placing a pupil stop at the FOR mirror, the alignment laser beam and the starlight have the same initial
shear. Shear is first sensed after transfer to the next collector by a dichroic mirror with a set of lenses
placed behind it (Figure 5-13). This dichroic lets through a portion of the 850-nm radiation, but the
dichroic reflects most of it together with all the other useful light. The sensor output is run back to the
transfer mirror on the first spacecraft to point the beam at the center of the dichroic shear sensor mirror.
An identical method is used for transfer to the beamcombiner spacecraft. Once inside the beamcombiner,
the shear is again sensed after the delay lines and corrected before entry to the adaptive nullers.
5.4.5 Polarization Rotation
Polarization-rotation metrology uses the same laser beam as the pointing system. The beam is initially
polarized, and this forms a reference for beamtrain-induced rotations of the polarization angle. On arrival
at the beamcombiner spacecraft, the beam is compressed and then passed through a K-mirror assembly,
which allows the polarization angle to be rotated. A polarization-angle sensor is placed immediately after
the K-mirror; thus, a loop can be formed to allow continuous correction of the polarization angle.
Figure 5-14. Metrology scheme for simplified nulling beamcombiner.
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