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The technique of photometric redshifts has become essential for theexploitation of multi-band extragalactic surveys. While the requirements onphoto-zs for the study of galaxy evolution mostly pertain to the precision andto the fraction of outliers, the most stringent requirement in their use incosmology is on the accuracy, with a level of bias at the sub-percent level forthe Euclid cosmology mission. A separate, and challenging, calibration processis needed to control the bias at this level of accuracy. The bias in photo-zshas several distinct origins that may not always be easily overcome. Weidentify here one source of bias linked to the spatial or time variability ofthe passbands used to determine the photometric colours of galaxies. We firstquantified the effect as observed on several well-known photometric cameras,and found in particular that, due to the properties of optical filters, theredshifts of off-axis sources are usually overestimated. We show using simplesimulations that the detailed and complex changes in the shape can be mostlyignored and that it is sufficient to know the mean wavelength of the passbandsof each photometric observation to correct almost exactly for this bias; thekey point is that this mean wavelength is independent of the spectral energydistribution of the source}. We use this property to propose a correction thatcan be computationally efficiently implemented in some photo-z algorithms, inparticular template-fitting. We verified that our algorithm, implemented in thenew photo-z code Phosphoros, can effectively reduce the bias in photo-zs onreal data using the CFHTLS T007 survey, with an average measured bias Delta zover the redshift range 0.4<z<0.7 decreasing by about 0.02, specifically fromDelta z~0.04 to Delta z~0.02 around z=0.5. Our algorithm is also able toproduce corrected photometry for other applications.

Euclid preparation. XXXI. The effect of the variations in photometric passbands on photometric-redshift accuracy