\label{Appdx:1}

This section contains details of the FLOYDS specific FDR1 FITS keywords and their definitions for users.

\label{Appdx:cosmic}

FLOYDS uses the Laplacian cosmic ray rejection algorithm^{1} of van Dokkum (2001). If the boolean value is set to T, then the rejection algorithm has been applied, otherwise it has not.

`LACOSMIC= T / TRUE if Laplacian cosmic ray rejection has been applied `

\label{Appdx:wavecal} The arc frame used for wavelength calibration is always recorded and the frame is available in the data product. The frame is labelled with the object name that it is attached to, the date of observations, grism and slit combinations used, and the MJD\(^{4}\) when it was reduced. The number of arc lines used in the fit is given by LAMNLIN, and the root mean square of the residuals to the fit is listed as LAMRMS, formally calculated as

\[{\rm LAMRMS}_B = \frac{\sqrt{\sum_{i=1}^N {R_i^2}}}{N}\]

\[{\rm LAMRMS}_R = \frac{\sqrt{\sum_{i=1}^N {R_i^2}}}{N}\]

where \(R_i\) is the residual of the wavelength fit for the \(i\)th arc line and \(N\) is the number of arc lines (LAMNLIN). This assumes that errors are randomly distributed and without any systematic errors, which is true as far as we can tell for FLOYDS. Hence the statistical uncertainty in the wavelength solution at any point is approximately given by the value (SPE_ERR_R), where

\[{\rm SPE\_ERR}\_B = \frac{\rm{LAMRMS}_B}{\sqrt{\rm{LAMNLINB}}}\]

\[{\rm SPE\_ERR}\_R = \frac{\rm{LAMRMS}_R}{\sqrt{\rm{LAMNLINR}}}\]

As described above, the wavelength positions of the skylines in the science frame (or telluric lines for bright standard stars) are checked and a linear shift is applied. This is listed in the keyword SHIFT_B in Angstroms. The keyword SPEC_SYE aims at capturing any further systematic error in the spectral coordinate system that is found during the observation and reduction process. After the systematic SHIFT is applied to correct the skylines to rest, we find no further systematic effects in FLOYDS wavelength calibration and hence set this to zero in all cases. After the wavelength solution is determined and the SHIFT applied, the values for minimum and maximum wavelength are determined and the following are calculated and inserted as keywords:

```
ARC = 'arc_SN2013XYZ_20130401_Gr11_Free_slit1.0_56448_1.fits'
LAMRMS_R= 0.0136 / residual RMS [nm]
LAMRMS_B= 0.0136 / residual RMS [nm]
LAMNLINR= 9.0 / Nb of arc lines used in the fit of the wavel.
LAMNLINB= 9.0 / Nb of arc lines used in the fit of the wavel.
SPE_ER_R= 0.004533333333333334 / statistical uncertainty
SPE_ER_B= 0.004533333333333334 / statistical uncertainty
SHIFTR = 2.0
SHIFTB = 2.0
SPE_RESB= 418.0955936041426 / Spectral resolving power
SPE_RESR= 430.0955936041426 / Spectral resolving power
```