  Synthesis performed under non Local Thermodynamic Equilibrium (non-LTE)
conditions usually overestimate stellar spectra. An approach widely adopted in
the literature to reduce the excess of UV radiation consists of artificially
increasing the continuum opacity by using multiplicative fudge factors, which
are empirically derived to impose the synthetized spectrum to match the
observed one. Although the method was initially developed to improve non-LTE
synthesis of spectral lines, it has been recently employed to model solar
spectral irradiance variability. Such irradiance reconstruction techniques
combine spectral synthesis of different types of structures, which are
performed making use of factors derived from a reference, quiet Sun model.
Because the opacity scales in a complex way with plasma physical properties,
the question arises whether, and to what extent, fudge factors derived using a
reference model can be used to adjust the opacity of models representing
different types of quiet and magnetic features. Here we investigate the effects
of opacity fudging on estimates of solar and stellar irradiance variability in
UV bands. We find that the use of fudge factors might underestimate the
variability by 19% and up to 20% in the ranges 230-300 nm and 300-400 nm,
respectively. These estimates are model dependent and should be considered as
upper limits. Finally, our analysis suggests the uncertainties generated by the
use of fudge factors to increase with the decrease of stellar metallicity and
to be significant for stars whose variability is facula-dominated and whose
effective temperature is larger than approximately 4000 K.
