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effective illumination
effective illumination
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From the beginning of operations, the ISAAC calibration plan includes the regular acquisition of illumination correction calibrations. A photometric standard star is observed 16 times on a regular grid over the field of view. Calibrations are taken once or twice a year in SW: H, J, Js, Ks and LW: H, J+Block and K. In 2007 illumination correction frames have been investigated and all OBs have been reprocessed. The pipeline generates one FITs table file containing the extracted fluxes of the photometric standard star and a FITs imaging product with a polynomial. Illumination correction calibrations can be used to retrieve more accurate photometry from imaging science frames. It is recommended to use the same flatfield for the science recipe and the illumination recipe in order to eliminate the gradients intrinsic to the twilight flat.
Within the ESO VLT dataflow and QC process illumination correction frames are used only as technical calibrations to monitor the throughput of the instrument across the FOV. Hence no twilight flat correction is applied.

As an example all SW-arm K-band illumination correction products are shown:

1999-04-03 1999-06-02 2001-02-30 2001-08-14 2001-09-12 2001-11-09
2002-03-29 2002-07-22 2003-04-07 2003-04-08 2004-02-09 2004-10-27
2005-04-11 2005-12-12 2006-04-24 2006-12-12 2007-04-24 2007-09-12
2007-11-05 2008-05-09 2008-09-16      

For each OB/AB a QC report is generated showing on the left panel measured fluxes and the grid points of the polynomial fit. On the right panel a comparison fit is shown.

For the LW-arm, an example QC report is given here:

In the LW-arm there appears a local depression matching star position #8 on the chip. The second order polynomial does not resolve the local feature, but lowers the quality of the fit

top illumination parameters

QC1 parameters

parameter QC1 database: table, name procedure
source acquisition parameters isaac_illu, qc_x1, qc_y1, qc_f1 - The position [pixel] and the counts [ADU] of the source on the first frame
- scales with
square root of signal
polynomial coefficients isaac_illu, qc_illum1, qc_illumx, qc_illumy, qc_illumxx, qc_illumxy, qc_illumyy - polynomial coefficients that fit the 17 flux values over the field of view.
effective illumination isaac_illu, qc_eff_ul, qc_eff_ur, qc_eff_ll, qc_eff_lr, qc_eff_cc - polynomial coefficients applied to five selected points in the FOV: the four centers a quadrants and the center of the detector, the effective illumination
mean effective illumination isaac_illu, qc_corr_mean - the mean of the five qc_eff values
rms of mean effective illumination isaac_illu, qc_corr_stdev - the standard deviation of the five qc_eff values
mean flux isaac_illu, qc_flux_mean - the mean of 17 flux values, used to normalize the polynomial
rms of mean flux isaac_illu, qc_flux_stdev - the standard deviation of the sample of 17 flux values


In addition to the flux values, the star identification including RA, DEC are stored in the QC1 database.