tokens: and systematics), the resulting co-added maps will now also contain a simulated signal that is correlated with that present in the FIRST simulations. We have passed these co-added ‘real + simulated’ maps through our power spectrum estimation pipeline, and the results are shown in Fig. 11 . These recovered power spectra have undergone noise bias subtraction and correction of the mask in exactly the same way as before. Fig. 11 shows that the recovery of the SDSS auto-power spectrum is very heavily biased by the presence of unaccounted-for large-scale systematics in the data. 17 However, the right-hand panel of Fig. 11 shows that the recovery of the SDSS–FIRST cross-power spectrum remains unbiased, despite the presence of the SDSS systematics. Note that, although it is unbiased, the cross-power spectrum is not completely unaffected by the presence of the SDSS systematics: their presence results in a significant increase in the error bars of the cross-power spectrum recovery (compare the right-hand panel of Fig. 11 and the central panel of Fig. 10 ), due to chance correlations between the noise in the FIRST simulations and the SDSS systematic effects. We will account for these additional contributions to the errors in our final analysis. Figure 11. The recovered mean SDSS auto-power spectra ( left-hand panel) and SDSS–FIRST cross-power spectra ( right-hand panel) from 100 simulations in the presence of large-scale systematic effects in the real SDSS shear maps. For a definition of the symbols, see Fig. 10 . Systematic effects in the SDSS data severely bias the recovery of the auto-power spectrum, shown as the solid curve in the left-hand panel. The cross-power spectrum recovery is unbiased in the presence of the same systematics, though the errors are amplified. To assess the impact of the large-scale systematics in the FIRST shear maps, we have performed an equivalent analysis to what is described above but now we superimpose the FIRST signal-only simulations on to the real FIRST maps and cross-correlate these with the SDSS simulations. The results of this test are displayed in Fig. 12 where we observe similar effects to what is described above – the FIRST auto-power spectrum is heavily biased by large-scale systematics while the cross-power spectrum remains unbiased. Figure 12. The recovered mean FIRST auto-power spectra ( left-hand panel) and SDSS–FIRST cross-power spectra ( right-hand panel) from 100 simulations in the presence of large-scale systematic effects in the real FIRST shear maps. For a definition of the symbols, see Fig. 10 . Systematic effects in the FIRST data severely bias the recovery of the auto-power spectrum, shown as the solid curve in the left-hand panel. As in Fig. 11 , the systematics contribute additional errors into the cross-power spectrum measurement though it remains unbiased. 6 REAL DATA MEASUREMENTS In the previous section, we have demonstrated, using simulations, that we can extract an unbiased cosmic shear
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