  Accurate measurements with implications in many branches in Physics have been
accessed using Penning traps and conventional techniques within a temperature
regime where each eigenmotion of a charged particle is still a classical
harmonic oscillator. Cooling the particle directly or indirectly with lasers
allows reaching the quantum regime of each oscillator, controlling subtle
effects in the precision frontier by detecting photons instead of electric
current. In this paper, we present a new in-vacuum optical system designed to
detecting 397-nm fluorescence photons from individual calcium ions and Coulomb
crystals in a 7-T Penning trap. Based on the outcome of computer simulations,
our design shows diffraction-limited performance. The system has been
characterized using a single laser-cooled ion as a point-like source, reaching
a final resolution of 3.69(3) $\mu$m and 2.75(3) $\mu$m for the trap's axial
and radial directions, respectively, after correcting aberrations.
