Data Reduction Frequently Asked Questions
I prefer to subtract the sky with external routines (i.e. in the case of extended objects that cover most of the slit(s), or dedicated sky observations). What are the relevant files I should use for this purpose?
The fors_science product MAPPED_ALL_SCI_<MODE> (MODE = LSS, MOS, MXU) contains the rectified and wavelength calibrated image (not flux calibrated). External routines for sky subtraction can be applied to this file.
After the sky subtraction the SPECPHOT_TABLE from the processing of the flux standard star can be used to perform the flux calibration. The relevant columns in the table are WAVE and RESPONSE (RESPONSE_FFSED for volume phase holographic grisms), which can be used to apply the response curve and obtain the desired sky-subtracted and flux-calibrated spectrum. For volume phase holographic grisms you will also need the spectral energy distribution of the flat field (FLAT_SED_<MODE> (MODE = LSS, MOS, MXU), provided by fors_calib). The correction for atmospheric dispersion and gain has to be performed independently as well.