For the reduction of astrometric delays, corrections from atmospheric dispersion and array metrology have to be computed. They are applied during averaging of the point data into scans. Metrology correction can also be applied to the scan data later, in which case one would initialize these data before averaging.
The procedure is as follows: for the metrology, open the appropriate INCHWORM output file, and load a motion group using ACCESSLOADMETROLOGY. (A motion group is a solution for the siderostat pivot motion for all or a subset of the laser readings.) As part of the motion group, two types of data are provided, one being the coordinates of the pivot, the other one being the delay correction. In CHAMELEON, metrocorr uses the former to compute delay corrections, whereas astrocorr only does the interpolation of the corrections computed by INCHWORM to the observed epochs of the data points. The result is stored in only one variable, metrodelay, and can be plotted using the PointData plot widget.
The dispersion correction is computed from the observed variation of the visibility phase with wavelength. Due to the low SNR of the blue channels, REDUCEDELAYSPHASEEDIT can be used to flag these. Also, before computing the correction, the closure relations have to be enforced using REDUCEDELAYSPHASECLOSE, and the phases have to be unwrapped using REDUCEDELAYSPHASEWRAP. After completion of dispcorr, the corrections for the white light scans (if present), can be set to zero if these are too noisy and one would rather use the raw fdl delays.