Accuracy of the wavelength calibration. Top: SEDM and a CalSpec spectrum of the HZ4 standard stars. The gray band shows the wavelength region used to fit for the wavelength shift between the two spectra. Bottom: histogram of the wavelength shift fitted on the 94 HZ4 star spectra obtained by SEDM in September 2018. The lower axis presents the shift in redshift shift (in z or in km s⁻¹), the upper axis shows the corresponding offset in Å or pixel for a typical wavelength of 6000 Å. The vertical orange line represents the mean and error on the mean. The light-gray band shows the standard deviation. SEDM spectra automatically extracted by pysedm have a precision of ∼3 Å or 4  ×  10⁻⁴ in redshift. A small 1 s.d. offset is visible suggesting that the SEDM spectra a lightly blue-shifted of approximately a tens of a pixel. The origin of this shift is under investigation.