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gaia data release 3 documentation

3.4 Processing steps

3.4.3 Raw data reconstruction

Author(s): Javier Castañeda

The raw data reconstruction establishes the detailed circumstances for each observation, including SM, AF, BP, and RP windows of normal observations, RVS windows for some brighter sources, as well as the BAM windows. The result is stored in persistent raw data records, separately for SM and AF (into AstroObservations); for BP and RP (into PhotoObservations); for RVS (into SpectroObservations); and for BAM (into BamObservations). These records need no later updates and are therefore only created in IDT.

The telemetry star packets with the individual observations include the samples of each window, but do not include several vital pieces of information, e.g., the AC position of each window line, if some lines of the window are gated, or if there is a charge injection within or close to the window. These details, which are common to many observations, are instead sent as auxiliary science data (ASD).

As previously described (Section 1.1.3), there are several kinds of ASD files. ASD1 files detail the AC offsets for each CCD for each telescope. These offsets give the AC positions of window lines in the CCD at a given instant, relative to the position of the window in AF1. Due to the precession of the spin axis, the stellar images will have a drift in the AC direction, which can reach 4–5 pixels while transiting a single CCD. This shift changes during a revolution and must therefore be updated regularly. When an update occurs, it affects all window lines immediately, but differently for the two telescopes. Windows may therefore end up with a non-rectangular shape, or windows may suddenly enter into conflict with a window from the other telescope.

The regular charge injections for AF and BP/RP CCDs are recorded in the ASD5 records. IDT must then determine the situation of each window with respect to the more recent charge injection. This task has an added twist, because charge injections that encounter a closed gate will be held back for a while, and are actually diluted.

Also the TDI gating is recorded in an ASD file, and here IDT must determine the gate corresponding to each window line. The detection causing the gate will have the same gate activated for the full window, but other sources observed around the same time will generally have only gating in a part of their windows. Any awkward combination may occur. An added twist is that the samples immediately after a release of a gate are contaminated by the charge held back by the gate, rendering them useless.