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Space missions designed for high precision photometric monitoring of starsoften under-sample the point-spread function, with much of the light landingwithin a single pixel. Missions like MOST, Kepler, BRITE, and TESS, do this toavoid uncertainties due to pixel-to-pixel response nonuniformity. This approachhas worked remarkably well. However, individual pixels also exhibit responsenonuniformity. Typically, pixels are most sensitive near their centers and lesssensitive near the edges, with a difference in response of as much as 50%. Theexact shape of this fall-off, and its dependence on the wavelength of light, isthe intra-pixel response function (IPRF). A direct measurement of the IPRF canbe used to improve the photometric uncertainties, leading to improvedphotometry and astrometry of under-sampled systems. Using the spot-scantechnique, we measured the IPRF of a flight spare e2v CCD90 imaging sensor,which is used in the Kepler focal plane. Our spot scanner generates spots witha full-width at half-maximum of $\lesssim$3 microns across the range of 400 nm- 850 nm. We find that Kepler's CCD shows similar IPRF behavior to otherback-illuminated devices, with a decrease in responsivity near the edges of apixel by $\sim$50%. The IPRF also depends on wavelength, exhibiting a largeamount of diffusion at shorter wavelengths and becoming much more defined bythe gate structure in the near-IR. This method can also be used to measure theIPRF of the CCDs used for TESS, which borrows much from the Kepler mission.

Direct measurement of the Kepler Space Telescope CCD's intra-pixel response function