The Fabry-Perot Etalon is the primary wavelength calibrator for MAROON-X. Based on an initial solution anchored to ThAr, the etalon provides a refined wavelength and drift solution for the instrument. Wavelength and drift solution follow a multistep process.
STEP 1: As a first step the etalon (and when needed ThAr) lines need to be fitted to determine their centroid in the 1D extracted spectr. This is done in the 1D extracted spectrum. See Etalon Line Fitting for details.
STEP 2: A static wavelength solution is built from ThAr and etalon spectra. The ThAr data are used an absolute anchor to determine the absolute order number for the etalon lines (aka m0) as well as the dispersion of the etalon. The primary products of this step are the spacer thickness and dispersion curve. These data are stored in a parameter file that is valid for as long as no major hardware changes are made to the etalon and as long as no new orders are included into the spectral format of MAROON-X. The last valid set of parameters was calculated in May 2020.
STEP 3: Using the etalon parameters determined in step 1, the 1D pixel positions and wavelengths of the etalon lines can then be fitted with a smoothing spline (default is a cubic spline with 30 equidistant knots per order) to determine a wavelength solution. This 'static' wavelength solution is saved in the config files for both camera arms and subsequently written into the hdf files during the Flux Extraction procedure. The validity of this wavelength solution depends on the long-term drifts of MAROON-X and of the etalon. It is expected that this solution is good to 500 m/s (0.5 pix) at any given time.
STEP 4: A new 'dynamical' wavelength solution can be computed for each new etalon spectrum in combination with the etalon parameters determined in step 1. Typically this step involves injected etalon light into the science fibers at the frontend and into the simultaneous calibration fiber, resulting in a DEEEE frame. The lines in these spectra are then refitted and a new spline-based wavelength solution is computed. See Dynamic Wavelength Solution for details.
STEP 5: A series of dynamical wavelength solutions over time form a drift measurement, i.e. they inform how much the RV zeropoint of the spectrograph has shifted over time since the construction of the first 'static' solution. To determine the instrumental drift at the for a science exposure, two methods can be used. (1) The 'bracketing technique' interpolates the wavelength solutions of two etalon calibration spectra taken right before and after a science spectrum to determine the wavelength solution of the science spectra. The precision of this technique is limited by a number of factors, incl. the time difference between science and etalon spectra and the linearity of the instrument drift. (2) A more precise way to determine the instrumental drift is to use the simultaneous calibration fiber. By comparing the etalon line positions in this fiber between an etalon calibration spectrum and a science spectrum, a drift can be computed and applied to the etalon calibration spectrum to 'predict' the correct wavelength solution for the science fibers in the science spectrum. Depending on the implementation of this technique, drifts at the 10-20 cm/s level should be recoverable, see Drift Solution.