Successful application of PSF-R techniques to the case of the globular cluster NGC 6121 (M 4)

Abstract

Context. Precise photometric and astrometric measurements on astronomical images require an accurate knowledge of the Point Spread Function (PSF). When the PSF cannot be modelled directly from the image, PSF-reconstruction techniques become the only viable solution. So far, however, their performance on real observations has rarely been quantified. Aims. In this Letter, we test the performance of a novel hybrid technique, called PRIME, on Adaptive Optics-assisted SPHERE/ZIMPOL observations of the Galactic globular cluster NGC 6121. Methods. PRIME couples PSF-reconstruction techniques, based on control-loop data and direct image fitting performed on the only bright point-like source available in the field of view of the ZIMPOL exposures, with the aim of building the PSF model. Results. By exploiting this model, the magnitudes and positions of the stars in the field can be measured with an unprecedented precision, which surpasses that obtained by more standard methods by at least a factor of four for on-axis stars and by up to a factor of two on fainter, off-axis stars.Conclusions. Our results demonstrate the power of PRIME in recovering precise magnitudes and positions when the information directly coming from astronomical images is limited to only a few point-like sources and, thus, paving the way for a proper analysis of future Extremely Large Telescope observations of sparse stellar fields or individual extragalactic objects.

Point spread function reconstruction for SOUL + LUCI LBT data

“…The PSF-R software we have used is able to reproduce the observed FWHM, EE, and SR with an accuracy of 2% to 4%, which is comparable to the level of accuracy reached by hybrid or calibrated techniques (see e.g., PRIME 29,30 ). Moreover, the reconstructed PSF provides a better fit to the observed data than the diffraction limited one.…”

Demonstration of an imaging technique for the measurement of PSF elongation caused by Atmospheric Dispersion

“…The post-observation reconstruction of the image is not easy, but significant effort is being put into developing the necessary tools for current and future instruments (e.g. Gilles, Wang & Boyer 2018 ;Jolissaint et al 2018 ;Massari et al 2020 ;Simioni et al 2020 ). The addition of (residual) atmospheric dispersion as a parameter in these tools could offer a good way to measure or monitor the dispersion at large observatories.…”

Review of PSF reconstruction methods and application to post-processing

“…al observed the core of the Galactic globular cluster NGC 6121 with the AO assisted camera SPHERE/ZIMPOL 64 in V-band, and under the program ID 60.A-9801(S). More details of the observation are given in 65,66 The observed field illustrated in Fig. 6 contains only five stars in a 3.5"×3.5" field of view, where the brightest star has a Vband magnitude of V'=10.9 mag, and has been used as the natural guide star for the AO system.…”