Comparison of the NCPA calibration performed on the internal source (left, test ZT06) and on sky (center, test ZT05). Top row: two OPD maps with the amount of aberration integrated in the range 1–15 c/p. Main plot: σ(f) of the two OPD maps, and bottom plot: ratio of the on-sky and internal measurements. In this test, NCPA was first measured on sky, and subsequently the instrument was switched to internal source without changing anything other than the source-selection mirror before another NCPA measurement was acquired. The tip, tilt, and focus have been subtracted (see Sect. 3.3), which explains why the amount of integrated aberration is smaller than in Fig. 1).