This is the instrument performance plot for the FORS2 response level from spectroscopic data. The spectrophotometric standard star observations are taken within three days of the corresponding science observations and are meant to provide a response curve for data reduction. They can also be used to determine the efficiency of the spectroscopic modes of FORS2, provided they have been taken under photometric conditions which however is not guaranteed.

Plotted here are:
a) values for the relative efficiency of the system 'telescope + spectroscopic arm of FORS2' (top), at a representative wavelength (indicated in the comment); these values can be used for trending
b) all values for the grism and the order separation filter of the upper plot, displayed over wavelength to obtain a chromatic overview (bottom); the date range is the same as for the upper plot.

Only data points for the upper CCD are plotted. The lower CCD usually has no standard stars and no response curve.

Major mirror interventions (M1, recoating) are marked by a blue vertical bar.

The best current estimate of the efficiency of the spectroscopic modes of FORS2 is the upper envelope of the data points in the upper panel. Under 'data downloads' you find further information regarding the photometric stability of the night ('S' if stable, 'N' if unstable, 'U' if unknown).

More information can be found on the plot tutorial page