Extraction of a small part of a Fabry-Pérot calibration frame. The plot shows the bias and scattered-light subtracted raw counts normalized by the trace profile. The region on the detector is identical to the one shown in Fig. 2. Clearly visible is the sequence of equally-spaced narrow emission lines. However, in contrast to the expectation, significant structure in cross-dispersion direction is present. The vertical purple lines correspond to the positions of the cuts shown in Fig. 4.