Question

what is the telescopic retinal illuminance of a star?

Answer 1

This answer is necessary to compare the astronomer's illuminance values to the human visual research in resolution, contrast and flicker sensitivity functions at what are probably low mesopic and scotopic (not foveal) stimulus intensities in double star astronomy.

As baseline values, the illuminance of a 0 magnitude star (vega) is about 2.4E−06 lux, and the area of a 250 mm aperture, which is fully illuminated by the source, is about 490 cm2. The angular extent of a diffracted star image, at a magnification equal to the aperture, is about 4 arc minutes on the retina.

The fully illuminated aperture and the diffracted image are finite areas, not "point sources". In particular, I am unsure what role magnification and/or exit pupil play in reducing the apparent brightness of the star.

Answer 2

illumination is independent of the angular area of the source, so the system of stellar magnitudes is based on the retinal.