ESPaDOnS: an Echelle SpectroPolarimetric Device
for the Observation of Stars at CFHT

Calibrations

In all observing modes, the following calibration frames are needed:

• 1-3 biases; only one bias is actually needed because the data reduction software does not do a pixel-by-pixel bias subtraction, but rather averages pixel values in 8px by 8px squares, fits the averages, and subtracts the fit
• one comparison lamp exposure with mostly Thorium lines; exposures times are between 30sec and 60 sec, depending on the lamps currently installed; some lines in the red are saturated, but the data reduction software takes that into account
• 20-40 flats, with exposure times between 2 sec and 4 sec, depending on the lamps installed and the readout speed

For the spectroscopic mode 'star only' and for the polarimetric mode, a Fabry-Perot exposure is also needed to better track the slit. Using a FP exposure for the 'star + sky' mode will not work.

The flat field exposures are used to find the orders, track them on the chip, fit their position, and produce a geometric calibration. The Thorium exposures are used for wavelength calibration (and to fit the shape and orientation of the slit in the 'star + sky' spectroscopic mode). The Fabry-Perot exposure is used to fit the shape and orientation of the slit.

All these calibrations can easily be taken through the GUI in Classical Observing, or are taken care of by the Service Observer in QSO mode.

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