<p>Space missions designed for high precision photometric monitoring of stars often undersample the point-spread function, with much of the light landing within a single pixel. Missions such as 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 intrapixel response function (IPRF). A direct measurement of the IPRF can be used to improve the photometric uncertainties, leading to improved photometry and astrometry of undersampled 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 ≲3 μm across the range of 400 to 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 ∼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.</p>
Interpixel capacitance (IPC) is the mechanism for a type of deterministic electrostatic coupling between neighboring pixels in hybridized detector arrays. Indium bumps are used to connect the photo-diode detection layer to the read-out circuitry. These indium bumps act as wires, transmitting voltages and currents between the detector layer and the read-out layer. When placed in close proximity, the voltages of these pixels capacitively couple to each other. The result is that when the voltage on one pixel changes, the voltage on that pixel’s neighbors change as well. IPC coupling results in a blur where a fraction, called the coupling coefficient, of the signal that was generated and collected in a given pixel, appears instead as signal on the neighboring pixels. Semi-conductor and electrostatic physics simulations have been conducted which show that the IPC is expected to change depending on the relative voltages of nearby pixels. The coupling coefficient is a function of signal strength, decreasing as signal strength increases. Characterization of IPC coupling using isolated single pixel events such as hot pixels in dark frames and single pixel resets shows this same signal dependence. The signal dependence of IPC introduces a new wrinkle in using hybridized arrays for scientific imaging. Previous work has anticipated that IPC results in a blurring of signal that can result in decreased contrast, but a signal dependence can result in additional effects on astronomical data. For example, when using pointspread function (PSF) fitting techniques on crowded fields to do astrometry and photometry, the PSF distortions due to IPC can result in an underestimate for the flux of brighter sources and an overestimate for the weaker sources. For an IPC coupling coefficient on the order of 1% with a signal dependence on the order of 0.4% , the full-width-half-max (FWHM) for a source near the sensitivity limit can be as much as 1% wider than the FWHM of a source near saturation. This results in flux estimations being off on the order of 1%. PSF distortion systematically drives down measurements of separation between sources. This error in separation drops to zero for well isolated sources, but when the PSFs are confused, it can result in an underestimate of separation on the order of 1%. To correct these errors, a method to remove a signal dependent IPC using an iterative method of successive approximations has been developed.
Interpixel capacitance (IPC) is a deterministic electronic coupling resulting in a portion of signal incident on one pixel of a hybridized detector array being measured in adjacent pixels. Data collected by light sensitive HgCdTe arrays that exhibit this coupling typically goes uncorrected or is corrected by treating the coupling as a fixed point spread function. Evidence suggests that this coupling is not uniform across signal and background levels. Subarrays of pixels using design parameters based upon HgCdTe indium hybridized arrays akin to those contained in the James Webb Space Telescope’s NIRcam have been modeled from first principles using Lumerical DEVICE Software. This software simultaneously solves Poisson’s equation and the drift diffusion equations yielding charge distributions and electric fields. Modeling of this sort generates the local point spread function across a range of detector parameters. This results in predictive characterization of IPC across scene and device parameters that would permit proper photometric correction and signal restoration to the data. Additionally, the ability to visualize potential distributions and couplings as generated by the models yields insight that can be used to minimize IPC coupling in the design of future detectors.
Interpixel capacitance (IPC) is a deterministic electronic coupling by which signal generated in one pixel is measured in neighboring pixels. Examination of dark frames from test NIRcam arrays corroborates earlier results and simulations illustrating a signal dependent coupling. When the signal on an individual pixel is larger, the fractional coupling to nearest neighbors is lesser than when the signal is lower. Frames from test arrays indicate a drop in average coupling from approximately 1.0% at low signals down to approximately 0.65% at high signals depending on the particular array in question. The photometric ramifications for this non-uniformity are not fully understood. This non-uniformity intro-duces a non-linearity in the current mathematical model for IPC coupling. IPC coupling has been mathematically formalized as convolution by a blur kernel. Signal dependence requires that the blur kernel be locally defined as a function of signal intensity. Through application of a signal dependent coupling kernel, the IPC coupling can be modeled computationally. This method allows for simultaneous knowledge of the intrinsic parameters of the image scene, the result of applying a constant IPC, and the result of a signal dependent IPC. In the age of sub-pixel precision in astronomy these effects must be properly understood and accounted for in order for the data to accurately represent the object of observation. Implementation of this method is done through python scripted processing of images. The introduction of IPC into simulated frames is accomplished through convolution of the image with a blur kernel whose parameters are themselves locally defined functions of the image. These techniques can be used to enhance the data processing pipeline for NIRcam.