Standard errors (ϵ) of $\hat{q},\hat{u}$, and $\hat{v}$ measured for different standard stars plotted against numbers of detected photo-electrons (N_e). Blue triangles represent measurements using the Zyla data only (single-ratio method). Red circles are the same as the blue triangles except they are using the iXon data. Black squares denote measurements using the double-ratio method. A single symbol is from one take. In most cases, one take consists of 30 000 frames (Δt = 150 s). All the observations were conducted with 200 fps. The size of the red circles and the black squares represents the EM gain applied to iXon observations; the smallest and largest symbols correspond to gain factors of ×1 (no gain) and ×100, respectively. The solid line displays the photon noise limit, which is $ϵ=1/\sqrt{N_{\mathrm{e}}}$. The dashed line is 2.5 times the limit.