Extinction-corrected specific intensities f_λ observable at Earth from a RSG (in red) and BSG (in blue) located at z = 0.01, as expected from the model in Equation (3). As input of the computation, fiducial parameters are used for RSGs R_* = 722 R_⊙, E = 10⁵¹ erg, M_ej = 2.8 M_⊙, and f_ρ = 1.455, and for BSGs R_* = 50 R_⊙, M_ej = 10 M_⊙, f_ρ = 0.0465, and E = 10⁵¹ erg. The solid lines show the expected emission with the median Galactic extinction E_B − V = 0.04, and the corresponding shaded region scans within the minimum and maximum extinction values, i.e., E_B − V = 0.01 and E_B − V = 0.25, respectively (see Figure 1). The vertical shaded bands represent the wavelength range covered by each instrument: ULTRASAT (in grey), and ZTF with g filter (in cyan), r filter (in yellow), and i filter (in red). The dashed horizontal grey and black lines define $f_{\lambda }^{\lim }$ of the two instruments, obtained following Equation (7). Panels a–f refer to different evolution times, from 30 minutes to 20 days after the SBO event, highlighting for each stellar progenitor the time range when the emission becomes visible to ULTRASAT and ZTF. The temporal evolution from one minute to 30 days of f_λ coming from RSGs and BSGs shown here is available as an online animation (supplementary material in the digital version of the paper).