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

TL;DR

This paper presents a direct measurement of the intra-pixel response function of the Kepler CCD, revealing how pixel sensitivity varies within a pixel and depends on wavelength, which can enhance photometric precision.

Contribution

The study introduces a spot-scan technique to directly measure the intra-pixel response function of a Kepler CCD, providing detailed insights into pixel sensitivity variations and wavelength dependence.

Findings

Kepler CCD shows up to 50% sensitivity variation near pixel edges.

The intra-pixel response function varies with wavelength, showing more diffusion at shorter wavelengths.

The measurement method can be applied to TESS CCDs for improved photometry.

Abstract

Space missions designed for high precision photometric monitoring of stars often under-sample the point-spread function, with much of the light landing within a single pixel. Missions like MOST, Kepler, BRITE, and TESS, do this to avoid uncertainties due to pixel-to-pixel response nonuniformity. This approach has worked remarkably well. However, individual pixels also exhibit response nonuniformity. Typically, pixels are most sensitive near their centers and less sensitive near the edges, with a difference in response of as much as 50%. The exact shape of this fall-off, and its dependence on the wavelength of light, is the intra-pixel response function (IPRF). A direct measurement of the IPRF can be used to improve the photometric uncertainties, leading to improved photometry and astrometry of under-sampled systems. Using the spot-scan technique, 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 with a 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 other back-illuminated devices, with a decrease in responsivity near the edges of a pixel by $\sim$50%. The IPRF also depends on wavelength, exhibiting a large amount of diffusion at shorter wavelengths and becoming much more defined by the gate structure in the near-IR. This method can also be used to measure the IPRF of the CCDs used for TESS, which borrows much from the Kepler mission.