3.4.9 Low-resolution background

Author(s): Michael Davidson

An IDU task creates low-resolution background records for each OBMT interval following the model described in Section 3.3.4. In contrast to previous IDU cycles (and IDT processing) in Gaia EDR3 these records do not contain a spline surface fit over OBMT and AC for each device. Instead they simply contain a set of data points with the background estimate at the sample positions, i.e. OBMT and AC. The storage of the data points allows the user to choose how they wish to estimate the background for a given science window. The method used here is a k nearest neighbours (KNN) approach where the 30 data points closest to the requested OBMT and AC position are obtained and a robust median computed. The key conclusions on the low-resolution background for Gaia EDR3 are:

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  The background is complex but is well-covered by the model over the full OBMT range.

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  Gaps are present throughout the interval; these are considered acceptable due to availability of local backgrounds. Note also that there may be no science data in many of these gaps.

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  The gross background levels are becoming more regular due to the stabilisation of the throughput.

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  Most features in the background distribution profiles have known explanations, e.g. scan law changes, decontaminations, galactic plane scans, VPU resets, onboard calibration activities or similar.

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  Other features can arise due to activity of various known straylight paths.