Model specific intensities, f_λ (Eq. 3) observed at Earth from RSGs (in red) and BSGs (in blue) located at z = 0.03 and hosted in a galaxy with extinction value E_B − V (the median value from Fig. 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, which is allowed to vary between 1 and 10 M_⊙. The $f_{\lambda }^{\lim }$ of ULTRASAT and ZTF in their observational wavelength bands, obtained following Eq. (7), are indicated with the horizontal dashed grey and black lines, respectively.