Modeling a point-spread function originating from multiple reflection of light in the substrate of array sensor: the case of Akatsuki/IR2

Abstract

The 2-μm near-infrared (NIR) camera, IR2, onboard Japan's Venus orbiter Akatsuki acquired Venus images after the successful orbit insertion in December 2015. IR2 utilizes a platinum silicide (PtSi) Schottky-barrier array sensor (1040x1040 pixels) in which photon is detected by the photo-electric effect. This is by nature not a very high efficiency mechanism therefore unused light is subjected to multiple reflection within the silicon substrate (400-μm thick in IR2). Because very intense day crescent (some ~3 orders of magnitudes brighter) of Venus exists in the same field of view when the night-side disk is imaged, light spread from the former significantly affects the photometry of the latter. To restore the night-side features to a level that can be measured photometrically, we have developed a simulation to model the pointspread function (PSF) of IR2 in which effect of multiple light reflection is accounted for. Different elements in the array sensor (the NIR-sensitive PtSi pixels, the vertical scanning lines, and the charge-sweep device area) are considered and the light reflection is traced until the beam becomes weaker than a threshold. While the multiple rings (the innermost one corresponds to the critical angle of total internal reflection) are successfully reproduced, the cross pattern did not show up from this simulation and we had to artificially add it. The concept of simulation may be useful for other sensors of which substrate is relatively transparent for the wavelengths of interest while the target objects contain large dynamic range.