After you have obtained your level 1(L1) data, the next step in data analysis is to use your calibrator and target observations to produce calibrated visibilities, or level 2 (L2) data.
The visibility that KI measures on an astronomical object (Vm) is the product of the true object (Vt) visibility and a factor, called the system visibility (Vs), which represents the loss of coherence due to instrumental and atmospheric factors: Vm = Vt * Vs. In order to obtain the true object visibility, the system visibility must be estimated. The system visibility can be estimated using observations of stars of known size, and thus of known true visibility; or more typically using observations of unresolved stars, which have a true visibility of 1.0 and directly provide an estimate of the system visibility (Vm = Vs). Once the system visibility has been estimated, the calibrated or true object visibility is simply obtained as: Vt = Vm/Vs. (Note: the Keck Interferometer data reduction method uses visibility amplitude squared, rather than visibility amplitude, as the basic observable; a distinction with no consequences to the present discussion).
In general the system visibility is an instantaneous quantity, and may depend on other factors such as the object location in the sky (e.g. as the zenith angle increases, the atmospheric effects may become more severe) and the instrument configuration (e.g. integration time). Insuring that target and calibrators can be observed under the same instrument configuration (e.g. have similar magnitudes for the different sub-systems) and are close in the sky, is part of the planning process . Insuring that a target observation is bracketed by calibrator observations with a few minutes cycle time, so that a valid estimate of the system visibility can be formed, is the responsibility of the WMKO/NExScI staff executing the schedule.
Although data calibration is not
performed at the NExScI, we make
available two software packages,
wbCalib
(for calibration of
wide-band data) and
nbCalib
(for calibration of
narrow-band data), which which greatly automate and simplify this
process. The Calib packages take as input a L1 data file and a
calibration
script
detailing which objects are targets and which
are calibrators, and their estimated sizes, and produces one file of
The NExScI distributes a tool for
modelling interferometric observables, that can be used to compare the
calibrated data with predictions from geometrical or physical models.
Please see the
Visibility
Modelling Tool page.
Clearly the above data shows that the source myTarget is clearly resolved, as its raw visibilities are much lower than those of the two calibrators used (HD157546 and HD174596).
# Example calibration script for myTarget
myTarget
17 56 21.288 -21 57 21.880 -0.009 -0.040 0.01
---
2
HDC157546 17 24 37.040 -18 26 44.743 0.016 -0.008 0.02 0.17 0.1
HDC174596 18 52 08.375 -21 55 17.498 0.048 -0.048 0.01 0.21 0.1
Note that the getCal package contains a function that facilitates the generation of properly formatted calibration scripts.
Please pay particular attention to your choice of calibrator angular sizes and errors, as those will directly impact the resulting calibrated visibilities. While for observation planning it is acceptable to provide only approximate calibrator angular sizes, it is crucial to provide accurate estimates with realistic errors in the calibration step. Recall that in addition to providing stellar size estimates based on standard lookup tables for normal stars, the getCal package also contains routines to provide better estimates by fitting spectro-photometric data , see the getCal documentation for details.
wbCalib -WL -ratioCorrection example.calScript
example.sum >
example.wbcalib
A variery of command line options are available for both wbCalib and nbCalib . The NExScI science team has evaluated these options for KI data, and documented the recommended Calib settings . Note that the recommended settings for KI and PTI are not the same.
In the example above, we have told wbCalib that we want to use data from the white-light pixel, and to apply a correction for un-balanced fluxes between the interferometer arms. The calibrated data is now in the ASCII file called "example.wbcalib". Please see the wbCalib documentation for a detailed description of the output file fields. Using this file, a plot of the calibrated visibilities and errors versus UT time (columns 8, 9 and 5 respectively) is shown below:
last updated Nov 12, 2008 %hr
last updated November 16, 2015
The NASA Exoplanet Science Institute is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.
