Detectability limits for low-redshift Einstein rings from ground-based 1.5-m imaging: a case study of NGC 6505

This repository contains the complete analysis pipeline used to produce the scientific results in the paper "Detectability limits for low-redshift Einstein rings from ground-based 1.5-m imaging: a case study of NGC 6505". The code is organised as eight sequential Jupyter notebooks that reproduce every measurement and figure in the paper, starting from the stacked observations of NGC 6505 obtained with the 1.5-m T150 telescope at the Sierra Nevada Observatory (OSN): Image stacking and alignment of the two observing nights (8.03 h total). PSF characterisation (FWHM = 2.47 arcsec from six field stars). Photometric calibration against Pan-STARRS DR1 (ZP_r = 16.15). Galaxy light modelling (elliptical isophotes, 1D and 2D Sérsic fits with bootstrap uncertainties). Residual analysis and 3-sigma surface-brightness upper limits. Forward-modelling of the Euclid-measured Einstein ring injected into the data. Generation of all publication-quality figures. PSF robustness test comparing Gaussian, Moffat and empirical PSF models. The central result is that the Einstein ring of NGC 6505, resolved by Euclid, would not be detectable in these ground-based data because the Einstein radius is comparable to the seeing (θ_E ≈ FWHM). The forward-modelling procedure provides a reproducible criterion (θ_E/FWHM ≳ 1.5) for assessing the ground-based recoverability of future Euclid low-redshift lens candidates. Input data are two stacked FITS frames from the OSN observing campaign. Dependencies: numpy, scipy, matplotlib, astropy, astroquery, astroalign, photutils. See the included README and INDICE_MAESTRO for the execution order and the mapping between notebooks, results and paper sections.