Solar irradiance above the atmosphere in W m⁻² nm⁻¹ as a function of wavelength in nm. The blue curve is the FTS high resolution solar spectrum near Hα, Kurucz et al. (1984), which we use to calibrate the images taken with l₁ (λ = 656.28 nm; Δλ = 50 pm). The red curve is shown for comparison. It is a calibrated solar spectrum taken at the ISS by Meftah et al. (2018). The gray zone around the red curve represents the area covered by its error bars. The wavelength axes of these two curves have been adjusted to each other. The superposition of the red and blue curves indicates a good agreement between data from the International Space Station and our calibration of the high resolution spectrum.