V2 Mode - Standard and Split-Pupil

The L-band and simultaneous K/L or H/L visibility modes are available for shared risk proposals. Backup observing plans using the standard V2 mode are required when proposing for these configurations. During the shared-risk period, up to 1 hour of testing or calibration time may be used by the KI team during a science night. The timing of these tests will be coordinated with the PI to minimize disruption of the observing program.

  • Spectral bands: one of the following
    • K band (2.18 microns)
    • H band (1.65 microns)
    • L band (3.8 microns)
    • split pupil simultaneous K/L or H/L
  • Spectral resolution options:
    • 5, 10 or 42 pixels across K band
    • see the self-phase referencing (ASTRA-SPR) configuration for higher spectral resolution in K band
    • 4 pixels across H band
    • 10 pixels across L band
  • Sensitivity:
    • Limits given here are for unresolved objects. For resolved objects, the sensitivity is reduced: see the table of sensitivity reduction as a function of object size, below.
    • In the low spectral resolution configurations, an aperture can be used to shift the limits by about 2.0 mag. If used, such an aperture must be used for all calibrator and target observations in a cluster. (For the K42 configuration, an iris is already used on the white-light output).
    • In K-band, the default rate is 200Hz. For good seeing, the 100Hz rate may be used.

    Standard low resolution configurations:
      • 5 spectral pixels across K band (K5):
        • 6.0 < K < 9.8 at 200 Hz frame rate.
        • 6.5 < K < 10.3 at 100 Hz frame rate.
      • 10 spectral pixels across K band (K10):
        • 6.0 < K < 9.3 at 200 Hz frame rate.
        • 6.5 < K < 9.8 at 100 Hz frame rate.
      • 4 spectral pixels across H band (H4):
        • 5.3 < H < 9.0 at 200 Hz frame rate.
      • 10 spectral pixels across L band (L10):
        • 2.5 < L < 6.0 at 100 Hz frame rate.

    Standard medium resolution configurations:
      • 42 pixels across K band (K42):
        • 4.0 < K < 7.1 at 200 Hz frame rate.
        • 4.5 < K < 7.6 at 100 Hz frame rate.
        • see the note above about the bright limit.

    K/L and H/L Split Pupil configuration:
      • Available only during Nuller runs.
      • Making use of the Nuller Split-Pupil configuration, L band can be used simultaneously with either K or H band in low or medium resolution. In this configuration, the sensitivity limits for all bands are 1.2 mag brighter than the standard limits, above.

      Sensitivity reduction X (mag) as a function of object size:
      X (K mag)
      Diam (mas)
      X (H mag)
      Diam (mas)
      < 1
      < 0.8

      • Note: Because the KI is a fringe tracking interferometer, all objects must exceed a threshold in visibility in order tobe observable. Objects must not have a characteristic diameter larger than 4.4 mas in K-band or 3.3 mas in H-band.

    • Accuracy and Relative Spectral Precision (see also additional notes below):
      • H and K bands: absolute accuracy is 0.03 in squared visibility
      • L band: see notes below
      • relative precision among spectral channels: 0.02 in squared visibility for H4 and K5 configurations.
      • relative precision among spectral channels: 0.01 in squared visibility for K10 and K42 channel configurations.

    • Efficiency: for objects where the magnitudes relevant to all sub-systems (AO, angle tracker, fringe tracker) are about 1 magnitude over the sensitivity limits, KI can achieve 6 observations (i.e. 6 pointings to different stars followed by the data acquisition sequence) per hour under good weather and seeing conditions. Observations of a target of interest must be bracketed by calibrator observations, therefore in one hour there will be 3 target and 3 calibrator observations. For objects closer to the sensitivity limits, the efficiency is closer to 4 observations per hour.

    • Additional Notes:
      • The accuracy of the KI V2 measurements was studied via observations of binaries with well known orbits (comparing calibrated KI measurements to predictions). Although within a night a given cluster of observations has V2 scatter of about 0.01, the binary studies reveal that the absolute calibration cannot be assumed to be better than the level given above (0.03 for H and K), and we recommend that this be used as an error floor. Our programs for calibrating KI data ( V2 Calib package ) provide options for including this error in the data calibration. For details on how the error floors and relative spectral precisions were derived, please see the "recommended settings" memo and the "flux bias" memo at our memos page. .

      • The relative spectral channel V2 calibration has only been characterized using the 200 Hz frame rate. Although this is considered low risk, data obtained at the slower 100 Hz frame rate may have higher relative errors.

      • The L-band configuration has not been throughly characterized. However, the instrument has been used several times for shared risk science. The absolute accuracy of the L-band V2 measurements on fainter objects will be limited by the accuracy of the background measurements that form a part of the automated sequence. We estimate that the background uncertainty will contribute an additional uncertainty in V2 of approximately 0.05 at L=5; proportionately more for fainter objects. This uncertainty will not be apparent in the scatter of the measurements (blocks) comprising an observation (scan), as a single value for the background is used to process the scan.

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