Abstract: Modelling of N21P (5,3) contamination at the O+ doublet emissions in the region of 732nm is presented. The method is derived from a known relationship between emission from the N21P (5,3) band and emissions from the N21P (5,2), (4,1) and (3,0) bands. A synthetic molecular spectrum is used to quantify a temperature-dependent emission ratio of these band systems as a function of filter characteristics and emission altitude. Five optical observations of high-energy auroral periods on 9 January 2008 are compared with results from the synthetic spectrum. Two cameras from a high sensitivity, high frame rate (20Hz) ground based imager in combination with a co-located high resolution spectrograph are used to identify events which are dominated by molecular nitrogen emissions. There is good agreement between the observed and modelled ratios. The temperatures associated with these ratios agree well with temperature profiles extracted from fitting the synthetic spectra to the spectrograph data. A synthetic spectrum is important for future work when the removal of N21P (5,3) contamination from O+ (2P) doublet emissions is required at high temporal resolution. [Copyright &y& Elsevier]