Gaia Data Release 4: Modelling of drift-scan related effects in Gaia's point spread function

Abstract

An accurate model of the point spread function is required in order to estimate positions and brightnesses of stars in digitized images. The PSF of the Gaia space telescope is unusual due to the use of drift-scan mode and time-delayed integration, in which the satellite spins and precesses while images are captured. This induces several systematic and periodic distortions in the PSF that are unique to Gaia. These include systematic variations in the stellar image drift rate with respect to the charge transfer rate, and spatial variations in the CCD response that are, contrary to expectations, not marginalised by the use of TDI mode. These must be incorporated into the PSF model in order to reduce systematic errors in Gaia's data products. We have developed a semi-analytic model of the PSF, in which the blurring effects of along- and across-scan stellar image motion are modelled analytically, and dependences of the PSF shape on source colour and position within the CCD are calibrated empirically. Constraints on the PSF origin are introduced in order to break a degeneracy with the geometric instrument calibration. Our new PSF model leads to significant improvements in the modelling of observations, particularly around the 11-13 magnitude range in G. This will contribute to reductions in the astrometric and photometric uncertainties in the derived data products. Our PSF model was deployed in the Gaia cyclic data processing systems and used in the production of the forthcoming Data Release 4. The linear part of Gaia's PSF is now well understood. Future development work will focus on the handling of several nonlinear effects that depend on the signal level, including charge transfer inefficiency and the brighter-fatter effect. This work will provide a useful reference for users of Gaia data and for other missions that use the same observing principles, such as the proposed GaiaNIR mission.

Figures (30)

• Figure 1: Schematic diagram of one of Gaia's CCDs, as seen from the illuminated side and showing various features relevant to the PSF modelling. The ($\tau , \mu$) coordinates represent positions within the pixel grid. The images of stars move from left to right during integration, with the serial register lying on the right. The main AL and AC directions and their relation to the field angles $\eta$ and $\zeta$ are indicated at the top left. Fiducial lines and barriers for the five longest gates are indicated; NOGATE has no corresponding barrier and uses the entire AL range.
• Figure 2: Example window geometry and sampling strategy for a simulated $G\approx13$ source in window class 0 (WC0; upper left) and WC1 (upper right). These particular configurations apply to sources with onboard estimated $G<13$ (WC0) and $13<G<16$ (WC1), in CCD strips AF2-9. Faint grey lines mark the boundaries between individual samples. The red boxes mark the extent of the window region, with the internal black lines indicating the binning of individual samples. The lower panels depict the resulting downlinked 2D (left) and 1D (right) observations.
• Figure 3: Example of the observed AL (upper) and AC (lower) stellar image drift rates relative to the transferring charge over a two revolution (12 hour) period, for both FOVs and measured at the centre of a CCD in row 1.
• Figure 4: Effects of AL and AC source motion on the PSF. These plots depict the integrated effective PSF for three different regimes of $\dot{\tau}$ and $\dot{\mu}$. The $u$ and $v$ coordinates are relative to the PSF origin, as explained at the start of Sect. \ref{['sec:dr3psf']}. These plots have been generated using the calibrated PSF model presented later in this paper, and correspond to the FOV1 PSF for NOGATE observations in the ROW2 AF4 device. In the left panel $\dot{\tau}=\dot{\tau}_0$ and $\dot{\mu}=\dot{\mu}_0$ such that the stellar image motion is perfectly matched to the charge transfer and no smearing occurs in either dimension. In both the centre and right panels $\dot{\tau}-\dot{\tau}_0=0.226$ pix sec$^{-1}$, such that the stellar image lags 1 pixel behind the charge in the AL direction during the 4.42 second exposure. In the centre and right panels $\dot{\mu}-\dot{\mu}_0=0.974$ and $-0.974$ pix sec$^{-1}$ respectively, such that the stellar image moves $\pm4.3$ pixels in the AC direction, orthogonally to the charge transfer. Note that the $\dot{\tau}$ value is about 20 times larger than what is routinely observed in the real data, in order to make the impact on the PSF more obvious for the plots. Throughout this paper we make use of the cubehelix colour scheme introduced in 2011BASI...39..289G.
• Figure 5: Illustrative example of (artificial) flatfield images for a pair of CCDs manufactured from the same circular silicon wafer. This figure approximately reproduces the effect seen in industrial pre-launch flatfield data from Gaia's CCDs at 900nm, and which cannot be published. The CCD on the left is of TYPE-01 and the CCD on the right is of TYPE-02.