FWHM of the seeing in arcsec. The seeing is assumed to be provided at the given airmass. No correction for airmass is performed.
We have performed Monte Carlo simulations of the PSF to see how much light falls onto the central spaxel, then onto the first spaxel ring (6 spaxels) around it, and finally onto the second spaxel ring (12 spaxels). To do so, we have used FiberSpecSim (available at http://t-rex.fis.ucm.es/fiberspecsim/wform.html)(Author: Belén Alcalde Pampliega). We used the following input parameters: The spectrograph: GTC [MEGARA] (in Firefox, choosing this autofills the form to some extent)
Since FiberSpecSim does not allow very large number of particles, we made 20 simulations each with a 1000 particles for each of the seeing value. Averaging the results, we get the mean, the standard deviation, and the standard deviation of the mean of the percentage of the enclosed light within each region (central, ring1, ring2) for each seeing. In any case, the standard deviation of the mean is less than 0.5%. Hence, we use the mean (of the 20 trials) as fixed values.