The wavelength day calibration consists in using the MIDI heated
blackscreen and taking 3 exposures through different narrow-band
filters [NeII], [SIV] and [ArIII] (having central wavelengths
accurately measured in the laboratory by the manufacturer). An extra
exposure is taken through a polycarbonic foil that has several
absorption lines in its N-band spectrum. Exposures without filters and
with closed shutter are also taken to process the exposures with
filters. From the spectra obtained through the filters a fit of the
lambda(X) function is performed. The procedure is performed for each
setup (PRISM or GRISM, and SCI_PHOT or HIGH_SENS). The MIDI HIGH_SENS mode
uses 2 windows on the detector (R1 and R2). The MIDI SCI_PHOT mode uses 4
windows (R1 to R4)

We monitor here the stability of the dispersion by plotting the
wavelength of a spectral channel given by the lambda(X) function
computed by the reduction of the calibration data. The selected
spectral channels corresponds to the expected central wavelength of the
three narrow-band filters.