V2 Mode - Astrometry (ASTRA-Astrometry)

The configuration for astrometry is completely identical to ASTRA-DFPR. The only difference is that instead of performing V2 sequences on the observed pair, the emphasis is on the measurement of the optical path difference between the two objects, to be turned into a measurement of the angular separation between the two objects projected on the interferometer baseline. This measurement is obtained through the combination of fringe tracker and internal metrology data for the optical path difference, and baseline motion data for the baseline itself.

To help meet the baseline stability requirement, the ASTRA team has designed a differential narrow angle astrometry observation scheme, which requires that the proposer find a reference pair close to the science pair of interest. This reference pair should have a similar separation distance and angle than the science pair. The geometry of the configuration is illustrated in the figure below.

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, Astrometry has not been demonstrated on-sky. All the required components for an astrometric measurement are in place and characterization of the astrometric performance is under way. Achieved performance will be confirmed in time for 2012A.

Proposals for Astrometry mode MUST include the following items (note that the PI is responsible for identifying appropriate science and reference pairs 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 reference pair for each target with magnitudes and separations. The sensitivity and separation parameters for the reference pair are identical to the parameters of the science pair.
3. A paragraph describing the science which would be done in regular K-mode (K<10.3) if Astrometry mode is not available.

• Sensitivity Limits:

• Reference stars:
• 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.
  
• 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: Proposers should assume that a single separation measurement requires at least 3 hours of observing time to provide a minimum UV coverage.
• Astrometric accuracy: Our goal is to achieve 100 uas for separations up to 25 arcsec.
• Requested number of observing epochs: Proposers must explain the number and temporal spacing of requested astrometry epochs to achieve the desired science. For orbital determinations, the number of epochs is at least 4.

• 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): including reference and target pair magnitudes and separations.

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