The impact of the point spread function fitting radius on photometric uncertainty based on the Fisher information matrix

Context . In point spread function (PSF) photometry, the selection of the fitting aperture radius plays a critical role in determining the precision of flux and background estimations. Traditional methods often rely on maximizing the signal-to-noise ratio (S/N) as a criterion for aperture selection. However, S/N-based approaches do not necessarily provide the optimal precision for joint estimation problems as they do not account for the statistical limits imposed by the Fisher information in the context of the Cramér-Rao lower bound (CRLB). Aims . This study aims to establish an alternative criterion for selecting the optimal fitting radius based on Fisher information rather than S/N. Fisher information serves as a fundamental measure of estimation precision, providing theoretical guarantees on the achievable accuracy for parameter estimation. By leveraging Fisher information, we seek to define an aperture selection strategy that minimizes the loss of precision. Methods . We conducted a series of numerical experiments that analyze the behavior of Fisher information and estimator performance as a function of the PSF aperture radius. Specifically, we revisited fundamental photometric models and explored the relationship between aperture size and information content. We compared the empirical variance of classical estimators, such as maximum likeli-hood and stochastic weighted least squares, against the theoretical CRLB derived from the Fisher information matrix. Results . Our results indicate that aperture selection based on the Fisher information provides a more robust framework for achieving optimal estimation precision. The findings reveal that S/N-based aperture selection may lead to significant discrepancies, with potential precision losses of up to 70%. In contrast, Fisher information-based selection allows a more accurate and consistent estimation process, ensuring that the empirical variance closely aligns with the theoretical limits.