This effect can be seen in figures 8.9 and 8.10 . Figure 8.9 shows the output of detector 13 against time along with the wavelength of light falling on the detector. It can be seen that the detector output rises from a low state at the start of the observation, and that it does not fall back to its low level at the end. In this example, the first up-scan should not be used to derive the spectrum. In the case of the observation lasting for several up-down scan pairs this does not cause a significant loss in total observing time.
Memory effects can also change the shape of line profiles.
As an example, as the grating scans across a strong gaussian line the sensitivity of the detectors will change. If this change is large enough it will result in one side of the gaussian having a different shape from the other. As lines are scanned in different directions by the up-down scans , the line will have a different shape in both scans. If the scans are averaged the line will be broader than it actually is. Users should be aware of this effect.