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The Mg II h&k lines are the prime chromospheric diagnostics of NASA'sInterface Region Imaging Spectrograph (IRIS). In the previous papers of thisseries we used a realistic three-dimensional radiative magnetohydrodynamicsmodel to calculate the h&k lines in detail and investigated how their spectralfeatures relate to the underlying atmosphere. In this work, we employ the sameapproach to investigate how the h&k diagnostics fare when taking into accountthe finite resolution of IRIS and different noise levels. In addition, weinvestigate the diagnostic potential of several other photospheric lines andnear-continuum regions present in the near-ultraviolet (NUV) window of IRIS andstudy the formation of the NUV slit-jaw images. We find that the instrumentalresolution of IRIS has a small effect on the quality of the h&k diagnostics;the relations between the spectral features and atmospheric properties aremostly unchanged. The peak separation is the most affected diagnostic, butmainly due to limitations of the simulation. The effects of noise start to benoticeable at a signal-to-noise ratio (S/N) of 20, but we show that with noisefiltering one can obtain reliable diagnostics at least down to a S/N of 5. Themany photospheric lines present in the NUV window provide velocity informationfor at least eight distinct photospheric heights. Using line-free regions inthe h&k far wings we derive good estimates of photospheric temperature for atleast three heights. Both of these diagnostics, in particular the latter, canbe obtained even at S/Ns as low as 5.

The formation of IRIS diagnostics. III. Near-ultraviolet Spectra and Images