In this configuration, the K band light is split and sent to two
separate beam combiners. The fringes from the outputs of one combiner
are used to track the atmospheric piston at high rate (e.g. 200 Hz).
The fringes in the outputs of the other combiner are then
stable and longer integrations become possible (1 Hz frame rate
typically). Because both
combiners look at the same astrophysical object, the primary camera
(fast fringe tracker) sets the magnitude limit; the extra sensitivity
on the secondary combiner is used to enable spectral dispersion over
330 channels (R = 1700).
- primary fringe tracker:
- K5, optical path difference stabilization.
- 4.0 < K < 7.8 at 250 Hz frame rate.
- secondary fringe tracker:
- K330, long integrations (1Hz typically).
spectral coverage approximately 2.03 - 2.34 microns.
- magnitude limit set by primary fringe tracker.
Limits given are for unresolved objects: see the table of
sensitivity reduction as a function of object size in the
standard V2 page.
Accuracy and Relative Spectral
Based on preliminary characterization, the error floor on the
absolute visibility squared is 0.05, and the relative error among
spectral channels is 0.02. Over narrow spectral features, differential
phase measurements have been demonstrated to about 1 degree rms (or
equivalently, for unresolved objects, 16 microarsec of sky offset), per
single observation (120 sec of data), and for stars near the
sensitivity limit. However, differential phase slopes or curvatures
across the bandpass are dominated by instrument effects (mainly the
continously changing differential optical paths in air), and therefore
these broad features are not likely to contain astrophysical
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