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The 4-Metre Multi-Object Spectrograph Telescope (4MOST) is a second-generation instrument build for ESO’s VISTA telescope in Chile, enabling large-scale spectroscopic surveys of the night sky. 4MOST will complement several European space-based observatories and future ground-based survey facilities, furthering our understanding of the universe. The instrument uses over 2 400 science optical fibres to collect and transmit light simultaneously from various astronomical targets to three spectrographs. Two thirds of the fibres will go to low-resolutions spectrographs and the remaining to a high-resolution spectrograph. Each spectrograph has three channels. Each channel uses a charge-coupled device (CCD) 231-C6 from Teledyne e2v, which gives a total of 9 science detectors. The detectors have a resolution of 6k × 6k with a pixel size of 15 μm which accounts for a total image area of 92.2mm × 92.4 mm. The image area has four separately connected sections that allow the read-out to be conducted through four output circuits. The data acquisition and signal processing unit of each detector is a new general detector controller (NGC), which is a versatile platform for infrared and optical detectors developed by ESO that is already employed in several state-of-the-art instruments. During the testing phase of the different spectrographs, flat frames were acquired that showed an unidentified image structure manifesting mostly as diagonal lines across all quadrants with central-to-edge preferential pathways. The observed fingerprint showed a slightly elevated charge amount over a few pixels wide when compared to the rest of the array. Due to the dynamic variation of the affected pixels across successive frames, the feasibility of mitigating the described phenomenon through calibration was impractical. While large format CCDs of this nature find extensive application, and the NGC is a prevalent choice for ESO instrumentation, the observation of this particular artifact appears to be previously undocumented, although it shows some similarities with the tearing patterns observed in other deep-depletion devices which are associated with field distortions in thick silicon. In this work, we describe, evaluate, and present a removal technique for the undefined image structure observed in the science detectors of the 4MOST instrument.
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