2.3.3 Astrophysical background

Author(s): Nigel Hambly

The astrophysical background incident on the focal plane of Gaia is dominated by stray light (Gaia Collaboration et al. 2016), in part because the compact and folded design (Safa et al. 2004) of the various optical instruments leaves little opportunity for stray light path baffling. The lower rows (1–4) of the focal plane have a diffuse background signal that is dominated by sunlight scattered around the DSA while in the higher rows (5, 6, and 7) it is the diffuse optical background from the Zodiacal Light and Milky Way that dominates. Hence, the background consists of a high amplitude, rapidly changing photo-electric component that repeats on the satellite spin period. Furthermore, this component evolves slowly in both amplitude, with the L2 elliptical orbital solar distance, and phase, as the scanning attitude changes with respect to the ecliptic and Galactic planes. Superposed on this are transient spikes in background due to very bright stars and bright Solar system objects transiting across or near the focal plane. Moreover, there are two non-photo-electric components to the background signal. These are a charge release signal associated with electronic charge injections which are used for on-board radiation damage mitigation (Prod’homme et al. 2011) and the dark current signal. The photo-electric background signal varies routinely over three orders of magnitude depending on instrument and spin phase with values as low as 0.1 and up to 100 electrons per pixel per second. Regarding the non-photo-electric signals, the charge release signal currently varies between 1 and 10 electrons per pixel per second in the first TDI line immediately after the last injection line but rapidly diminishes to 1% of this level after the following 20 TDI lines, while the dark signal is all but negligible (see Section 2.3.4).

The parametric model for the astrophysical background consists of the outer product of two one-dimensional spline functions (van Leeuwen 2007) which defines a flexible, two-dimensional surface model known colloquially as a ‘bispline’. In the along-scan direction, the spline function is quadratic with knots evenly spaced at intervals that adapt to the available data density. In regions of high density that generally exhibit higher background fluctuations on smaller spatial scales, and in which there is a higher density of data to constrain the model, knots are more closely spaced. Typically, the along-scan knot interval is in the range 1 to 10 arcminutes. In the across-scan direction, the spline function is linear, allowing for sudden discontinuities in the gradient of the stray light pattern. Knot positions are placed at fixed, unevenly spaced positions to best follow rapid changes in the stray light pattern in the across-scan direction. The charge release model consists of a simple, empirical look-up table (LUT) as a function of TDI line index following the last charge injection line with a power-law scaling as a function of that injection level present at the same column position on the CCD. This enables a single charge release LUT calibration per device when applied in conjunction with the across-scan profile of the charge injection for the same device which is also characterised via a LUT. Compared to Gaia DR1, which employed background following from the daily processing chain (IDT), in Gaia DR2 the astrophysical background spline models are improved by being refit in the cyclic processing (IDU). In particular, additional data beyond the along–scan limits of each independent processing interval are employed to better constrain the bispline surface edge along that dimension. This leads to more accurate background following near the boundaries between each interval’s independently fitted model and consistent behaviour across those boundaries.