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tokens: quasar proximity zone (e.g., Eilers et al. 2017 ) allow an estimate of the intrinsic luminosity of the quasar. However, it is crucial to note that the maximum image separation for J0439+1634 is ∼02 (Fan et al. 2018 ), close to the highest-resolution limit obtainable with the Hubble Space Telescope ( HST ; see Figure 2 ). Therefore, it is a clear possibility that many of the detected z > 6 quasars are actually lensed, with image separations below the resolution threshold. Following Barkana Loeb ( 2000 ), in Figure 2 we show the distribution in angular separations of lensing images for z = 6.51, , where is the angular diameter distance between source and observer. The distribution indicates that a fraction of the lensed sources at z > 6 have image separations ≲01. Thus far, no multiple images have been detected with the HST in samples of ∼10 z > 6 quasars (e.g., Kim et al. 2009 ; McGreer et al. 2013 ). This is equivalent to stating that the fraction of μ > 2 magnified quasars at z > 6 is ≲10% in current samples, leading to an upper limit of β ≲ 3.1 (see Figure 1 ). It is worth noting that, while the diffraction limit for the HST is ∼01, its point-spread function (PSF) is very well characterized and stable. This would, in principle, allow to discern a point-source quasar from a multiply imaged one well below the diffraction limit, possibly at a level ∼0001. For example, Libralato et al. ( 2018 ) measured proper motions of stars by identifying the centroid of the PSF with a precision ∼00003. This remark could foster a re-examination of the HST images of z > 6 quasars thus far detected. Figure 2. Distribution of image separations at z = 6.51, obtained assuming a Press–Schechter distribution of galaxies, following Barkana Loeb ( 2000 ). The shaded area indicates the 1 σ uncertainty due to the distribution of velocity dispersion in galaxies σ v . The angular separation for J0439+1634 (∼02), the HST angular resolution (∼01), and the typical angular size of lens galaxies at z ∼ 1 (∼1′′) are also shown for reference. Additionally, as originally suggested by Fan et al. ( 2018 ), the current selection criteria might have missed an important fraction of the quasar population at z > 6 because a lens galaxy in the same line of sight might have contaminated the drop-out band of the spectrum, leading to a misclassification of the source. It is remarkable to note that, if we de-magnified J0439+1634 by a factor ∼5, reducing its total magnification to μ ∼ 10, its flux would have been comparable to the one from the foreground lens galaxy and, thus, the quasar would have been misclassified. This situation could be occurring systematically for z > 6 quasars with μ ≲ 10. As the typical image separation for strongly lensed z > 6 quasars is ≪1′′ and the typical angular dimension of a z ∼ 1 lens galaxy is ∼1′′ (see Figure 2 ), the probability of lensing is equivalent to the probability of having the lens galaxy and the quasar images within the same photometric aperture.