Determination of the F444W limiting magnitude. The dependence of the 90% limiting magnitude on circularized effective radii from $0\stackrel{″}{.}06$ to $1\stackrel{″}{.}2$ is shown for an exponential profile (n = 1, blue solid line) and a de Vaucouleurs profile (n = 4, red solid line), both with null ellipticity (e = 0). In the top right-hand corner, we show the completeness curves for a Sérsic profile n = 1 with size $R_{\mathrm{e}}=0\stackrel{″}{.}42$ and variable ellipticity and the projection of the 90% completeness on the e = 0 curve that is the most restrictive one. The grey shaded region depicts the FWHM size of the F444W-band. The green solid lines defines the F444W = 25 limiting magnitude as the projection on the most restrictive, while realistic, galaxy size ($R_{\mathrm{e}}=0\stackrel{″}{.}42$) and ellipticity (e = 0) at z = 3.5.