Astrometry with NPOI is based on the measurement of fringe delays, which
are related to the baseline orientation and star position. There are
three different corrections to the uncalibrated FDL delays, which are
due to atmospheric refractive index fluctuations, path length variations
in the feed system, and motions of the siderostat pivots. We call the
application of these corrections to the raw FDL delays calibration of
the delays.
, similar to the visibility calibration, has storage
for three calibrated delays, i.e. group, dry, and wet delays. These are
the result of the dispersion correction procedure (dispcorr), which
computes cumulative corrections based on the white light fringe position
(group delay), the dry and wet air dispersion constants, respectively. All
subsequent metrology corrections are only applied to these three
calibrated delays. Note that
does not have separate variables
for delays with and without metrology corrections, for example. The user
has to keep track of the actual calibration status of the delays.