Nulling Data Calibration

The Level 2 processing of the nuller data uses the calibrator observations in each cluster to estimate the instrument leakage term so that it can be subtracted from the science observations, resulting in a leakage that depends, ideally, only on the target astrophysical terms. This process is analogous to estimation of the system visibility for V2 measurements, and thus we refer to L_S as the system leakage. The quality of the system leakage estimate depends strongly on the choice of the calibrators. In general, the calibrators should have a simple well determined astrophysical model so that their astrophysical leakage can be subtracted to high accuracy. An ideal calibration source would be unresolved, but given flux constraints, the calibrators used for the nulling are often resolved. Accurate diameters are essential for the calibrator stars, and one technique to generate these is to use N-band diameters computed from measured diameters from the contemporaneous K-band measurements taken by the cophasing fringe trackers.

Thus, the system leakage at the time of each calibrator is determined by subtracting the expected astrophysical leakage from the measured calibrator leakage. See the nullCalib manual for the relevant equations. The formal error for the system leakage associated with each calibrator includes the formal error associated with the L1 processing as well as a quadrature term incorporating the uncertainty in the adopted size. These discrete system leakage estimates must now be interpolated, and their errors propagated, to the epochs of the science observations, and subtracted from the measured science leakage. The result is a calibrated leakage, ideally containing only the science target's astrophysical terms, with a formal error computed from the quadrature combination of the formal error on the interpolated system leakage and the formal error on the science leakage.

nullCalib/nbNullCalib produce a single calibrated point for every nulling integration. The uncertainties given for these points are a combination of the uncertainty in the level 1 data and the uncertainties in the calibrator sizes. In some cases, the scatter between points is considerably larger than the per point error and in these cases, the scatter dominates the final uncertainty.

The nullCalib and nbNullCalib packages are available for the Level 2 calibration of wideband and narrowband (spectral) nulling data. See the V2Calib page for links to the manual and to download software. The recommended command line arguments are:

nullCalib -noangleProx -notimeWeight -chiReject 20 -nodelay

You may see warnings about calibrator system null estimates disgreeing by several sigma. This represents variations in the system null during the cluster.

The systematic uncertainties on the wideband data depend on the source flux and are (per scan):

• 0.6% for 1-1.5 Jy
• 0.5% for 1.5-2 Jy
• 0.4% for 1-5 Jy
• 0.3% for >5 Jy

See also the Nuller instrument and performance paper (Colavita et al. 2009).

Return to KI Support page

Last updated: 8-3-09 RLA