PTI V2 -- Description of L1 Data Plots

This page describes plots produced by KvisPlots which are used by the L1 database data display. For more details on some of these quantities and data analysis methods, please refer to the additional documentation in the Level 1 file description.


  1. Wideband Incoherent V2
  2. Composite SPEC Incoherent V2
  3. Wideband coherent V2
  4. Composite SPEC coherent V2
  5. Wideband flux
  6. Composite SPEC flux
  7. Total optical delay (m)
  8. Wideband ratio correction
  9. Composite SPEC ratio correction
  10. Phase jitter (rads)
  11. Lock breaks
All plots represent one of the above quantities as a function of UT. In all cases, the data is plotted only for times when the fringe tracker is locked or recording internal calibrations as part of a fringe acquisition sequence. In all cases, a data point is plotted for each average over xx secs, where xx equals the value of the blockTime parameter in the configuration file ini.params and is generally 25 seconds. All quantities representing a flux count (plots 5,6,7,8) are expressed in data number (DN).

  1. Wideband Incoherent V2
  2. Incoherent V2 for the FATCAT white-light (WL) pixel. This is the basic V2 measurement, and this plot is useful for a quick assesment of the V2 data quality and to determine whether or not a given object is resolved, and by how much. Unresolved calibrators should have a V2 equal to the system visibility (V2 ~ 0.6 to 0.7) and resolved objects will have lower values of V2. An estimate of the calibrated V2 can be obtained by dividing the V2 measured on a target by the V2 measured on its calibrator(s) (if the calibrator(s) are resolved, then an additional correction will be necessary).

  3. Composite SPEC Incoherent V2
  4. Same as (1) but based on an average of the V2 measured on the spectrometer pixels.

  5. Wideband Coherent V2
  6. Same as (1) but using a coherent V2 estimator. See the Kvis manual for the algorithm defining the coherent estimator.

  7. Composite SPEC Coherent V2
  8. Same as (3) but based on an average of the spectrometer pixels.

  9. Wideband flux
  10. Photon counts on the WL pixel. This plot is useful for assesing the overall flux throughput. For a given star and frame rate, the flux trace should remain approximately constant. Large changes may indicate an alignment or weather (clouds) problem. The flux trace should scale appropriately (as long as the fringe tracker is below non-linear or saturation regime) with stellar magnitude (K-band or H-band, depending on which filter is being used) and frame rate.

  11. Composite SPEC flux
  12. Same as (5) but for an average of the spectrometer pixels.

  13. Total optical delay (m)
  14. Total optical delay introduced by the KI delay lines, and defined as (FDL - LDL), where FDL and LDL are the optical delays introduced by the long  delay line and the fast delay line, respectively.

  15. Wideband ratio correction
  16. The value of the correction factor that needs to be applied to the measured V2 to compensate for the loss of coherence due to flux mismatches between the beams.

  17. Composite SPEC ratio correction
  18. Same as (11) but for an average of the spectrometer pixels.

  19. Phase jitter (rads)
  20. The frame-to-frame RMS of fringe phase. This quantity will be high when the differential (between the beams) optical path difference fluctuates significantly due to e.g. instrument vibrations and/or atmospheric piston. To the extent that it is dominated by atmospheric effects, it may used as a good indicator of seeing quality. A value of 0.8 radians or smaller is considered indicative of ``good'' conditions.

  21. Lock breaks
  22. The number of times the fringe tracker looses/re-acquires lock during a block integration. If greater than 1, it indicates that fringe lock was lost frequently, and therefore the observing conditions may be marginal (e.g. low V2, low flux, bad seeing).

    March 2004, rev. April 2005
    R. Millan-Gabet and R. Akeson (NExScI)