Point spread function modelling for astronomical telescopes: a review focused on weak gravitational lensing studies

The accurate modelling of the Point Spread Function (PSF) is of paramount importance in astronomical observations, as it allows for the correction of distortions and blurring caused by the telescope and atmosphere. PSF modelling is crucial for accurately measuring celestial objects' properties. The last decades brought us a steady increase in the power and complexity of astronomical telescopes and instruments. Upcoming galaxy surveys like Euclid and LSST will observe an unprecedented amount and quality of data. Modelling the PSF for these new facilities and surveys requires novel modelling techniques that can cope with the ever-tightening error requirements. The purpose of this review is three-fold. First, we introduce the optical background required for a more physically-motivated PSF modelling and propose an observational model that can be reused for future developments. Second, we provide an overview of the different physical contributors of the PSF, including the optic- and detector-level contributors and the atmosphere. We expect that the overview will help better understand the modelled effects. Third, we discuss the different methods for PSF modelling from the parametric and non-parametric families for ground- and space-based telescopes, with their advantages and limitations. Validation methods for PSF models are then addressed, with several metrics related to weak lensing studies discussed in detail. Finally, we explore current challenges and future directions in PSF modelling for astronomical telescopes.

Rethinking data-driven point spread function modeling with a differentiable optical model

In astronomy, upcoming space telescopes with wide-field optical instruments have a spatially varying point spread function (PSF). Certain scientific goals require a high-fidelity estimation of the PSF at target positions where no direct measurement of the PSF is provided. Even though observations of the PSF are available at some positions of the field of view (FOV), they are undersampled, noisy, and integrated in wavelength in the instrument's passband. PSF modeling requires building a model from these observations that can infer a super-resolved PSF at any wavelength and any position in the FOV. Current data-driven PSF models can tackle spatial variations and super-resolution, but are not capable of capturing chromatic variations. Our model, coined WaveDiff, proposes a paradigm shift in the data-driven modeling of the point spread function field of telescopes. By adding a differentiable optical forward model into the modeling framework, we change the data-driven modeling space from the pixels to the wavefront. The proposed model relies on efficient automatic differentiation technology as well as modern stochastic first-order optimization techniques recently developed by the thriving machine-learning community. Our framework paves the way to building powerful models that are physically motivated and do not require special calibration data. This paper demonstrates the WaveDiff model on a simplified setting of a space telescope. The proposed framework represents a performance breakthrough with respect to existing data-driven approaches. The pixel reconstruction errors decrease 6-fold at observation resolution and 44-fold for a 3x super-resolution. The ellipticity errors are reduced by a factor of at least 20 and the size error by a factor of more than 250. By only using noisy broad-band in-focus observations, we successfully capture the PSF chromatic variations due to diffraction.

Rethinking the modeling of the instrumental response of telescopes with a differentiable optical model

We propose a paradigm shift in the data-driven modeling of the instrumental response field of telescopes. By adding a differentiable optical forward model into the modeling framework, we change the data-driven modeling space from the pixels to the wavefront. This allows to transfer a great deal of complexity from the instrumental response into the forward model while being able to adapt to the observations, remaining data-driven. Our framework allows a way forward to building powerful models that are physically motivated, interpretable, and that do not require special calibration data. We show that for a simplified setting of a space telescope, this framework represents a real performance breakthrough compared to existing data-driven approaches with reconstruction errors decreasing 5 fold at observation resolution and more than 10 fold for a 3x super-resolution. We successfully model chromatic variations of the instrument's response only using noisy broad-band in-focus observations.