Model specific intensities f_λ (in Equation (3)) observed at Earth from RSGs (a) and BSGs (b) located at z = 0.03 hosted in a galaxy with extinction value E_B − V (median value from Figure 1). Blue, green, and orange shaded regions show results for explosion energy E = 10⁵¹ erg, E = 10⁵² erg, and E = 10⁵³ erg, respectively. (a) RSG progenitor: by fixing the ejected mass at M_ej = 2.8 M_⊙ (fiducial parameter), the shaded regions reflect the variation of the progenitor radius between 200 and 1500 R_⊙. (b) BSG progenitor: by fixing the ejected mass at M_ej = 10 M_⊙ (fiducial parameter), the shaded regions reflect the variation of the progenitor radius between 10 and 50 R_⊙. $f_{\lambda }^{\lim }$ of ULTRASAT and ZTF in their observational wavelength bands, obtained following Equation (7), are indicated with the grey and black dashed horizontal lines, respectively.