Example: X and Y derivatives for selected blue and red settings
X/Y-derivative frames: The first raw frame of a stack has been shifted in X (Y) by 1 px and subtracted. The result frames (blue: one CCD, red: two CCDs U and L) show any structure in X (dispersion) and Y (slit) direction enhanced. Interorder space is masked to not contribute to the resulting std dev.
Generally the X structure consists of high-frequency gain fluctuations only. In the red settings, fringing starts showing up, but becomes pronounced only in the very red, at 860 nm. The REDL CCD shows also a lot of surface structure in that setting. Technically these add to the gain fluctuations.
The Y structure consists of gain fluctuations, slit function and fringing. Gain fluctuations and fringing are the same as for X. The slit function is very faint to undetectable in the blue, while it becomes increasingly stronger in the red.
Orientation of all raw frames is rotated by 90 degrees.
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