Temperature and spectral index fits of field G0.02+18.02 using Herschel data. Frames are: colour temperature and spectral index with SPIRE data and χ² minimisation (frames a)–b)), 250μm optical depth and colour temperature for SPIRE data and β = 1.8 (frames c)–d)), colour temperature and spectral index with SPIRE data and MCMC calculations (frame e)–f)) and the corresponding error maps (frames g)–h)), MCMC results for 160–500 μm fits (frames i)–j)) with corresponding error estimates (frames k)–l)). The last frames show the effect of zero point uncertainty in the three-band fits (frames m)–n)) and four-band fits (frame o)–p)) (χ² fits) based on Monte Carlo simulation using χ² fits. Temperature maps have contours drawn at intervals of 2.0 K, starting at 10.0 K. Spectral index maps have contours at intervals of 0.2, starting at 1.0.