The normalised high-resolution disc-integrated quiet-Sun spectrum over a 10 Å interval centred at the core of the Ca II K line from the atlas of the Hamburg Observatory. Also shown (various coloured bands) are the nominal bandwidths of typical Ca II K archives (shown are 9, 7, 2.5, 1, 0.5, and 0.15 Å), including the two extreme cases from San Fernando (broadest, light grey) and Meudon (narrowest, lavender). The vertical light grey lines mark wavelength intervals of 1 Å from the core of the Ca II K line. The bottom part of the figure presents three exemplary observations taken with CCD cameras on the same day (11 July 2012) at the San Fernando, Rome/PSPT, and Meudon sites. The images have been compensated for ephemeris to show the North Pole at the top, but no other processing has been applied. The images are shown over their entire respective ranges of values.