Only by knowing a priori one of the two information it is possible to break such a degeneracy. A source might appear bright either because it is intrinsically so, or because of a higher SR at its location. Ii) the efficiency of the correction for the effects of the atmospheric turbulence, which is often quantified by the Strehl ratio (SR) parameter, varies across the FoV, producing a degeneracy with the magnitudes of the astronomical sources. In the worst case scenario, the natural guide star may be the only target available to model the PSF over the whole FoV, leading to markedly poor astrometric and photometric performances I) because of the PSF variability, fewer stars restricted to smaller portions of the FoV can be used as independent representations of the same PSF. This effect has two major drawbacks for standard methods of PSF modelling: In fact, these techniques enable diffraction-limited observations from the ground, at the cost of making the PSF variable across the field of view, over spatial scales the size of the isoplanatic angle. However, the assumption above does not hold for observations assisted by Adaptive Optics (AO) techniques.
In this case the PSF is modelled from the combination of the light profile of the brightest and most isolated stars, and is then used to measure the positions and the fluxes of all sources in the image. 1– 3) work, for example, to measure positions and magnitudes of stars in dense stellar fields. If the number of unresolved targets in the same image is large, the information from each of these can be combined together to obtain an accurate, well sampled model of the PSF. Under the assumption that each unresolved target (such as a star) that falls in the observed field of view (FoV) is a realisation of the same PSF, sampled at a different pixel-phase on the detector, the PSF shape can be modelled directly from the image itself. Astronomers have accomplished this objective by developing more and more refined astrometric and photometric techniques, which rely on the accurate knowledge of the Point Spread Function (PSF) in order to achieve high-quality performance. The aim of astronomical imaging is to measure how celestial bodies move and shine. These results thus pave the way for the exploitation of innovative techniques to investigate resolved stellar population science cases with the new generation of Adaptive Optics-assisted instrumentation at the ESO’s Very Large Telescope, Keck or the Extremely Large Telescopes. 2019, which is specifically designed to model AO-assisted data. A similar performance is also achieved when using the analytical PSF method described by F´etick et al. Compared to the results obtained using standard techniques, PRIME achieves improvement in precision by up to a factor of four, and ensures a photometric accuracy within ∼ 0.1 mag. Here we report on the successful use of PRIME, a new technique that combines both PSF-R and image fitting, to perform precise photometry and astrometry on real data of the Galactic globular cluster NGC6121, observed with SPHERE/ZIMPOL. Despite being theoretically well established, so far a-priori methods have never surpassed the performance obtained by standard methods when applied to real astronomical imaging. One alternative is to use a priori PSF-modelling techniques such as PSF-reconstruction (PSF-R), that rely on Adaptive Optics control loop data to determine the shape of the PSF at any spatial location. The recent advent of the Adaptive Optics technique makes this method more challenging, because of the strong PSF variations across the field of view.
To date, the best performances have been obtained when building the PSF a posteriori, meaning directly from the image of dense stellar fields, by exploiting the fact that each star represents a different realisation of the same PSF.
Precise stellar photometry and astrometry require the best possible modelling of the point spread function (PSF).
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