If L is the luminance of the object, A is its area, s = distance of the object then light falling on the objective is
The area of the image formed by the telescope (assuming that the image coincides with the object) is Γ2A and the area of the final image on the retina is
Where f = focal length of the eye lens. Thus the illuminance of the image on the retin (when the object is observed through the telescope) is
When the object is viewed directly, the illuminance is, similarly,