Example: Y derivatives for setting 860d2_1x1
X and Y derivative frames: The first raw frame of a stack has been shifted in X (dispersion direction) and Y (slit direction) by 1 px and subtracted. The result frames (two CCDs U and L) show any structure in X/Y direction enhanced. Interorder space is masked to not contribute to the resulting std dev.
The X structure consists of gain fluctuations and fringing. The Y structure consists of gain fluctuations, slit function and fringing. This example has been chosen to show the time evolution of the strongest kind of structure, fringing in the red setting 860d2.
For comparison, the corresponding master flat frames have been divided by each other.
The respective X and Y derivatives, though complex in structure and 1 month apart, are virtually indistinguishable. The ratio of the corresponding masters shows no differential structure except for the telluric absorption line (which is due to the different slit widths) and some components of the slit function (for the same reason).
With the time basis in this example, 1 month, no significant variations on any structure scale between 1 px and the slit height can be detected. We conclude that the coherence timescale for such variations is significantly larger than 1 month.
Orientation of all raw frames is rotated by 90 degrees.
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