In the Nuller mode the main data product is the coherent null leakage for a given source. The nuller operates in the N-band (8.5 microns effective wavelength). The nulling configuration enables accurate measurement of this quantity, which can also be converted to a correspondingly accurate visibility measurement. See the KI nulling description for more details on the nulling observations and how these observations can be used for measuring visibilities.

  • Fringe tracking: for nulling, the fringe tracker operates in K band with a 4 msec frame time.

    • K < 4.6 - X mag, where X is based upon the expected angular size of the target at K. This K band sensitivity level is for the mode in which an iris is used to filter the fringe tracker input and is the mode in which the nulling performance limits have been demonstrated. Additional fringe tracker sensitivity (to K < 6.0 - X mag) can be obtained under good seeing conditions by running without this iris, but the system has not been well characterized in this mode.
    • Note that the targets CANNOT be larger than 4.5 mas at K.

    • X diameter (mas)
      0.00 < 1
      0.25 1.6
      0.50 2.2
      0.75 2.7
      1.00 3.1
      1.25 3.4
      1.50 3.7
      1.75 3.9
      2.00 4.2

    • An 8 msec clock is available for the fringe tracker which can be used under good seeing conditions to increase the K-band limiting magnitude by 0.75 magnitudes over the values above at the expense of null leakage. The nuller has not been characterized with this clock; however, we would expect at least a factor of 2 degradation in the null leakage uncertainty given below. We would recommend this clock only for targets with a large expected excess, and it must be used for both target and calibrator.

  • Nuller: N band flux > 2.4*F Jy, where F is based upon the expected angular size of the target at 9.5 microns. Note that targets CAN NOT be larger than 20 mas at N band. Fainter objects (down to 1.7 Jy for sources less than 5 mas) can be observed at lower nulling performance (see below).
    F theta (mas)
    1.0 < 5
    1.1 7
    1.2 9
    1.5 13
    2.0 16
    2.5 18
    3.0 20

  • Spectral dispersion: in order to obtain best SNR, the default data reduction bins the spectral data as follows:
    • "wideband" channel: 8 - 9 microns.
    • 10 narrowband channels across the N-band (8-13 microns).

  • Null leakage uncertainty: The uncertainty in the null leakage from interleaved integrations of target/calibrator pairs is 0.25% (1 sigma) over 2.5 to 3 hours in moderately good weather when the source flux meets all stated sensitivity limits. For fainter N-band fluxes (1.7 to 2.4 Jy), the uncertainty increases to 0.5% in 3 hours. More detail on calibration is provided in nulling basics .

  • Observing efficiency: For sources where all magnitudes are > 1 magnitude above the sensitivity limits, KI in nulling mode can achieve 2 scans per hour under good weather and seeing conditions. A scan includes the fringe tracking data and detector calibrations. A separate scan on a nearby calibrator is required to determine the system visibility and nulling calibration. These scans are typically grouped into 2-3 hour blocks (depending on the FDL coverage and the observing goals) to provide 2-3 calibrated scans on the target. Pointing optimization on bright 10 micron sources is required between these observing blocks. These pointing observations will require 20-30 minutes each.

  • For sources at the sensitivity limit of one of the sub-systems, the efficiency is generally closer to 1.5 scans per hour. The first 30 minutes after the domes open may be necessary for other interferometer set-up and no science observations are guaranteed during this time.

Return to KI Sensitivities page
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