Kraaikamp, Emil ; Gissot, Samuel ; Shestov, Sergei ; Berghmans, David ; Verbeeck, Cis ; Stegen, Koen

Poster presented at EclipseSA - Joint Solar Orbiter, Parker Solar Probe, and DKIST Meeting , San Antonio, Texas, US on 2024-04-08

Abstract: The High-Resolution Extreme Ultraviolet imager (HRI-EUV) of the EUI instrument onboard Solar Orbiter observes the Sun at both high temporal and spatial resolutions. HRI-EUV uses a dual-gain CMOS active-pixel sensor (APS) that allows for a high dynamic range. For each pixel, the sensor outputs both a low-gain (LG) signal, with a high saturation level but a larger read-noise, as well as high-gain (HG) signal, with a low read noise and low saturation level. These signals are either calibrated and combined onboard to increase the dynamic range, or can be read out individually into LG- or HG-only images. However, accurately combining and calibrating the LG and HG channels either onboard or on-ground, is a complex task and can result in visible image artifacts, particularly around the transition regions of the LG and HG parts of combined gain images. Here we discuss several remaining challenges of calibrating HRI-EUV data. In particular we investigate the changing intensity ratio between the low- and high-gain channels depending on the location on the image detector, and we show how this explains a large part of the mismatch between the LG and HG parts in combined gain images. Additionally, we discuss a remaining fixed pattern noise (FPN) that is visible in LG (and parts of combined gain) images. As this bias signal is not captured in dark frames, we attempt to isolate the signal from sequences of LG LED images with varying exposures. Finally, we show how the resulting methods can be applied to improve the calibration of existing and new HRI-EUV data, either on-ground or onboard.

Keyword(s): Solar Orbiter ; EUI ; calibration
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