D3.6.1 Report/documentation on impact of observational effects and their spatial variability on photo-z, based on simulations

Qianjun Hang, Benjamin Joachimi (University College London)

The WP3.6 team investigated how different observational effects such as sky brightness, seeing, and number of exposures can affect the photometric redshift (photo-z) distribution for LSST. The observation strategy for Rubin is to cover a large survey region before building up the depth. During the first few years of observation, therefore, it is expected that the inhomogeneity in depth due to e.g. varying weather condition is large. Effectively, one can regard each pointing as a ‘mini’ survey with different observational systematics and limiting magnitudes. This could potentially be a problem for weak lensing analysis because the signal is sensitive to the mean redshift of the tomographic bin, especially for the precision required by LSST. In this investigation, the WP3.6 team focus on the ‘gold’ sample from the first (Y1) and fifth year (Y5) data release, for which the simulated observation conditions for the Rubin Observatory (OpSim) as well as the DC2 DM catalogue are used. We split the sample into tomographic binning for lens sample between 0.1 in 0.2 ≤ z ≤ 1.2 according to Y1 and Y5 requirements respectively. One of the main aims is to check whether the spatially varying observing conditions introduce fluctuations to the mean and scatter of each tomographic bins that are larger than the Rubin requirement. The photo-z is estimated using a template-fitting algorithm, BPZ lite. We find that for the sample with reduced photo-z outliers, the shifts in the mean redshift and the scatter in each tomographic bin is consistent with the random noise of the sample, and comparable to the Rubin requirement. Additionally, we also looked at the impact of spatially varying observational conditions in each band on the cModel magnitude, the magnitude error, colour, and galaxy over-density