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Blind source separation techniques are used to reanalyse two exoplanetarytransit lightcurves of the exoplanet HD189733b recorded with the IR camera IRACon board the Spitzer Space Telescope at 3.6$\mu$m during the "cold" era. Theseobservations, together with observations at other IR wavelengths, are crucialto characterise the atmosphere of the planet HD189733b. Previous analyses ofthe same datasets reported discrepant results, hence the necessity of thereanalyses. The method we used here is based on the Independent ComponentAnalysis (ICA) statistical technique, which ensures a high degree ofobjectivity. The use of ICA to detrend single photometric observations in aself-consistent way is novel in the literature. The advantage of our reanalysesover previous work is that we do not have to make any assumptions on thestructure of the unknown instrumental systematics. Such "admission ofignorance" may result in larger error bars than reported in the literature, upto a factor $1.6$. This is a worthwhile trade-off for much higher objectivity,necessary for trustworthy claims. Our main results are (1) improved and robustvalues of orbital and stellar parameters, (2) new measurements of the transitdepths at 3.6$\mu$m, (3) consistency between the parameters estimated from thetwo observations, (4) repeatability of the measurement within the photometriclevel of $\sim 2 \times 10^{-4}$ in the IR, (5) no evidence of stellarvariability at the same photometric level within 1 year.

A new look at Spitzer primary transit observations of the exoplanet HD189733b