V2 Mode - Dual-Field Phase Referencing (ASTRA-DFPR)

In this configuration, the field is split, so that two different objects can be observed simultaneously and sent to separate beam combiners. One of the objects is a bright star used to track the atmospheric piston at a fast rate and stabilize the optical path difference for the other beam combiner, which can now integrate for much longer times and thus obtain science data for faint objects. The star used by the AO system can be the bright interferometer reference star, or it can be a third star, as described below. Angle tracking is done separately for the reference and science stars. Currently fringe tracking is done at K band and angle tracking at H band. As with the other observing modes, in this configuration observations of the target/reference pair will be interleaved with observations of the calibrator pair.

To maximize the science, performance characterization and engineering benefits of TAC-allocated science during the shared-risk availability, science teams are strongly encouraged to collaborate with the NSF-funded ASTRA development team throughout the science process from planning to publication (contact Julien Woillez, for more information about ASTRA). To recognize their contribution, publications based on DFPR data are strongly encouraged to include ASTRA team co-authors.

As of today, phase-referenced fringe tracking has been demonstrated on a K=12.5 star.

Proposals for DFPR mode MUST include the following items(note that the PI is responsible for identifying appropriate reference and calibrator objects as part of the proposal phase):

1. The reference star and target magnitudes and separations. The limits for these quantities are given below.
2. A calibrator pair for each target with magnitudes and separations. The sensitivity and separation parameters for the calibrator pair must follow reference star case 1, given below.
3. A paragraph describing the science which would be done in regular K-mode (K<10.3) if DFPR mode is not available.

• Sensitivity Limits:

• Reference stars: Two scenarios are supported:
• Same star used for AO and fringe tracking:
• AO: R mag < 14
• Angle tracking: H mag < 13
• Fringe tracking: K mag < 9
• Distance to target: 3 to 25 arcsec.
  
• Different star used for AO and fringe tracking:
  
• AO reference star:
• AO: R mag < 14
• Distance to target and fringe reference star: < 25 arcsec.
• Fringe tracking reference star:
• Angle tracking: H mag < 12
• Fringe tracking: K mag < 8
  
• Distance to target: 3 to 15 arcsec.
• Limits given are for unresolved objects. See the table of sensitivity reduction as a function of object size in the standard V2 page.
• Science target:
• Angle tracking: H mag < 13
• Fringe tracking: K < 14
  

• Efficiency: expected 3 pointings per hour.
• Accuracy: expected 0.05 in V2.
  

• Note: As with other modes, Principal Investigators are responsible for identifying and providing all the information required for observing in this mode (per limits given above): reference and target star magnitudes and separations, and calibrator pair for each target, also including magnitudes and separations.

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