This plot monitors XSHOOTER flat field spectra. For this reason flat field spectra are generated and are compared to a reference flat field spectrum.

We fit the flat field spectrum via the following linear formular:

current(lambda) = reference(lambda) * ( a + b * ( lambda - lambda_0 )

where lambda_0 is the center wavelength of the considered wavelength range, parameter a is the relative overall flux of the spectrum, corresponding to the lamp power and parameter b is a first order chromatic deviation (the slope between the reference flat spectrum and current flat spectrum). The reference spectrum is from 2015-10-24.

This plot shows both parameters a and b, of which paramter b is the important one.

On 2015-10-24, there is a=1 and b=0 by definition.

E.g. a = 1.22 means the current flux calibrated flat spectrum is 22% brighter than the flux calibrated reference spectrum.

A negative b (e.g. b = -1e-4) means the spectral slope of the current spectrum is less steep, hence blueer than the reference spectrum. In the trending plot, increasing b means the spectrum becomes redder, decreasing b means the spectrum gets more blue.