The nonlinear photon transfer curve of CCDs and its effects on photometry

Abstract

The photon transfer curve (PTC, variance vs. signal level) is a commonly used and effective tool in characterizing CCD performance. It is theoretically linear in the range where photon shot noise dominates, and its slope is utilized to derive the gain of the CCD. However, recent researches on different CCDs have revealed that the variance progressively drops at high signal levels, while the linearity shown by signal versus exposure time is still excellent and unaffected. On the other hand, bright stars are found to exhibit fatter point spread function (PSF). Both nonlinear PTC and the brighter-fatter effect are regarded as the result of spreading of charges between pixels, an interaction progress increasing with signal level. In this work we investigate the nonlinear PTC based on the images with a STA1600FT CCD camera, whose PTC starts to become nonlinear at about 1/3 full well. To explain the phenomenon, we present a model to characterize the charge-sharing PSF. This signal-dependent PSF can be derived from flat-field frames, and allow us to quantify the effects on photometry and measured shape of stars. This effect is essentially critical for projects requiring accurate photometry and shape parameters.

Research on a back-illuminated CMOS sensor for LUV imaging

“…The Variance-ADU curve using 4 × 4 pixel and 56 × 34 pixel sub-region can be plotted in figure 4. By linear fitting, the slope can be obtained and the gain can be derived as the inverse of the slope [17]. The result of gain of is about 0.236 ± 0.025 e-/ADU and 0.220 ± 0.0034 e-/ADU using 4 × 4 pixel and 56 × 34 pixel sub-region respectively.…”

Repeatability and reproducibility of a hyperspectral imaging system for in vivo color evaluation

“…Field of view 19×14 cm 2 For calibration, we implement all the standard CCD characterizations (cf. [22][23][24][25] ), including subtraction of the CCD dark counts from the raw data, adjustment for the non-uniformity of the CCD response, and measurement of the absolute white response of the total system using a 99% reflectance standard to provide spectral calibration. We also fully characterized the linearity and noise performance of the CCD itself, and, combined with the calibration data, this allows us to convert the raw CCD count information into a spectral reflectance curve for each spatial pixel in the field of view.…”

Astronomy from Dome A in Antarctica

“…The CCD cameras went through detailed lab tests in order to thoroughly understand their performance as they cannot be easily accessed once they were deployed to Dome A with the telescopes (Ma et al 2012). A rare problem was found and studied that the photon transfer curve became non-linear at a level around just 25000 ADU, causing the brighter-fatter effect, and could affect photometry of bright sources (Ma et al 2014a). Other special treatments have also been studied for data reduction of AST3 images (Ma et al 2014b;Wei et al 2014) and will be discussed in Section 3.1.…”