Ideal threshold of the EUV images derived from the comparison of near-side EUV and activity masks as a function of time. Blue dots represent the ideal threshold determined for individual cases. Darker blue regions illustrate a higher concentration of cases. The orange line shows the linear fit of the thresholds, which gives the actual threshold applied to far-side EUV data to compute the EUV masks. Fit coefficients can be found at the top of the figure. Dates range from June 2010 to May 2016, with a gap in STEREO/SDO composite images from August 2014 to November 2015.