Intrapixel effects of CCD and CMOS detectors

TL;DR

This paper investigates intrapixel nonuniformity in CCD and CMOS sensors, revealing that pixel-scale variations affect flux, shape, and position measurements, which are critical for high-precision astronomical observations.

Contribution

It provides detailed measurements of intrapixel effects in different detectors, highlighting their significance for precision astronomy and the need for further examination.

Findings

Intrapixel effects cause flux and ellipticity variations of 0.2-0.3% (rms).

Centroid displacement varies by about 0.01 pixel rms, uncorrelated with pixels.

Effects are more pronounced in CMOS sensors, especially frontside illuminated types.

Abstract

Intrapixel nonuniformity is known to exist in CCD and CMOS image sensors, though the effects in backside illuminated (BSI) CCDs are too small to be a concern for most astronomical observations. However, projects like the Large Synoptic Survey Telescope require precise knowledge of the detector characteristics, and intrapixel effects may need more attention. By scanning CCD and CMOS cameras with a small light spot (unresolved by the optics), we find in the images that the spot's flux, centroid displacement, and ellipticity vary periodically on the pixel scale in most cases. The amplitude of variation depends on not only the detector but also how well the spot is sampled by the pixels. With a spot radius of 2 pixels (encircling 80% energy) as measured, the flux and the ellipticity extracted from the BSI CCD camera vary by 0.2-0.3% (rms) and 0.005 (rms), respectively, while the deviation of the centroid position (rms ~ 0.01 pixel) is not correlated with the pixels. The effects are more pronounced for the BSI CMOS camera and even worse for the frontside illuminated CMOS camera. The results suggest that a closer examination of the intrapixel effects is needed for precision astronomy.