Example of the broad absorption feature caused by the CRIRES optical ghost in the spectrum of 10 Leo. Spectra from different chips in the region of 1 μm are shown in black, demonstrating that most wavelengths are covered by 2−4 chips. This ghost is found on chip 2 of the 1005.6 nm observational setting. The feature caused by the ghost (marked with a solid green vertical arrow) often obscures important stellar features, but these are unobscured in neighbouring settings, a fact we exploit to remove the ghost feature. Examples of small spurious lines are marked with dashed blue arrows. See Sect. 3.4 for more detail.