Radial mapping of the camera, relating the camera-bound colatitude δ_c of a sky object, and its projected radial distance r on the camera image, measured in pixels. In the case of stars (red diamonds) the measured radial distance comes with the discretization error bar of size (see Appendix B) more visible on the lower panel. The black dots show a simple equidistant mapping r ∝ δ_c . The magenta dashes show the equisolid projection of equation (5b) expected for the fisheye and focal reducer lenses used here; the green solid line show the best fit distorted radial model of equation (5a); while the blue dotted-dashed line illustrate the impact of atmospheric refraction on observations with a perfect equisolid optical system (see Appendix A). The upper panel shows the raw radial distance r, while the lower panel shows the difference between r and the equisolid model.