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The next generation of space-based telescopes used for weak lensing surveyswill require exquisite point spread function (PSF) determination. Previouslynegligible effects may become important in the reconstruction of the PSF, inpart because of the improved spatial resolution. In this paper, we show thatunresolved multiple star systems can affect the ellipticity and size of the PSFand that this effect is not cancelled even when using many stars in thereconstruction process. We estimate the error in the reconstruction of the PSFdue to the binaries in the star sample both analytically and with imagesimulations for different PSFs and stellar populations. The simulations supportour analytical finding that the error on the size of the PSF is a function ofthe multiple stars distribution and of the intrinsic value of the size of thePSF, i.e. if all stars were single. Similarly, the modification of each of thecomplex ellipticity components (e1,e2) depends on the distribution of multiplestars and on the intrinsic complex ellipticity. Using image simulations, wealso show that the predicted error in the PSF shape is a theoretical limit thatcan be reached only if large number of stars (up to thousands) are usedtogether to build the PSF at any desired spatial position. For a lower numberof stars, the PSF reconstruction is worse. Finally, we compute the effect ofbinarity for different stellar magnitudes and show that bright stars alter thePSF size and ellipticity more than faint stars. This may affect the design ofPSF calibration strategies and the choice of the related calibration fields.

Evaluating the effect of stellar multiplicity on the PSF of space-based weak lensing surveys