TPF-I SWG Report - Exoplanet Exploration Program - NASA

TPF-I F L I G H T B ASELINE D ESIGN
This polarization control loop is needed because although the adaptive nuller can (in principle) correct for
the rotation error by adjusting intensities, it cannot do so on the timescales consistent with formation-drift
motions. Note that there is an unsensed rotational component caused by clocking of the telescopes around
the line-of-sight to the star; this would need to be known and sensed by means probably involving
transfer-mirror angle knowledge.
5.4.6 OPD Metrology
The OPD metrology system forms part of the fringe tracking system and effectively extends the
frequency response of that system up to several hundred hertz. The fringe tracker relies on light from the
star and typically will sense some 107 photons per second. Calculations show that the response time of
the fringe tracker will be ~ 0.01 second, and the OPD control-time constant will therefore be ~ 0.1
second. This is most likely inadequate given that the expected spacecraft vibration will extend to many
tens of hertz with appreciable amplitudes (see below). Therefore, a laser metrology system operating at
wavelengths near 1550 nm is used to sense and allow control of the higher frequency vibrations using
principally the high-speed stage of the delay line.
Key elements of the metrology system are that it fills the aperture of the science beam, it measures down
to the last beamsplitter, and it measures (as far as possible) only internal optical path. One area of the
beam train is unsensed at present, that is the section from the FOR mirror to the primary mirror. It is
worth noting that one possible motion cannot in any case be sensed, and that is high speed motion of the
primary mirror with respect to the star; more on this issue later. The metrology system is divided into two
components, internal and external, as shown in Figure 5-14. Internal metrology extends from the
launchers just ahead of the delay lines down to a single retroreflector following a dichroic mirror placed
after the cross-combiner beamsplitter. Beams from the four input paths are differentiated by having
different heterodyne frequencies. External metrology extends from the same launch dichroic to a
retroreflector placed behind the FOR mirror. These beams could all run at the same frequency shift,
assuming no interference via the beamcombiner retroreflector, but would have a different wavelength
from the internal metrology, nominally 1570 nm. The reference point for both internal and external
metrology is at the launch dichroic, a complex custom optic. Metrology pointing and shear correction is
active during calibration periods and consists of adjusting the mirrors placed behind the launch dichroic to
maximize the return signal from the collector and combiner retroreflectors. Once set, these should require
infrequent adjustment since the alignment is maintained by the separate alignment system.
5.5 Optomechanical Layout
5.5.1 Beamcombiner spacecraft
The four beams enter at the top of the spacecraft as far away as possible from the plane of the sunshades.
The maximum shading principle is followed so that the beams cross over the center of the spacecraft. One
side of the vertical bench supports the compressors and delay lines, the metrology launchers/receivers, the
K-mirrors and the tilt sensors. The other side supports the adaptive nullers, the high/low resolution
switch, and the shear and pointing sensors and actuators. The whole assembly will be contained within a
cylindrical cover. The adaptive nullers are angled away from the bench because they have prisms at the
entrance and exit, and these prisms refract the light away from the plane of the bench.
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