V2 Mode - Self-Phase Referencing (ASTRA-SPR)

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).

• Sensitivity:
• 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 Precision:
• 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 information.

• Efficiency: 4-5 pointings per hour.

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