Model specific intensities f_λ (in Equation (3)) observed at Earth from RSGs (in red) and BSGs (in blue) located at z = 0.03 hosted in a galaxy with extinction value E_B − V (median value from Figure 1). For each progenitor type, fiducial parameters are used (for RSGs R_* = 722 R_⊙, E = 10⁵¹ erg and f_ρ = 1.455; for BSGs R_* = 50 R_⊙, f_ρ = 0.0465, and E = 10⁵¹ erg), apart from the ejected mass M_ej that is let to vary between 1 and 10 M_⊙. $f_{\lambda }^{\lim }$ of ULTRASAT and ZTF in their observational wavelength bands, obtained following Equation (7), are indicated with the grey and black dashed horizontal lines, respectively.