That leads us to the next key concept: the entrance pupil.
And if you increase the aperture of the telescope, the galaxy will get brighter, right up to the point where the exit pupil matches the entrance pupil of your dilated eye.
Those include the basic lens model, radial distortion, tangential distortion and movement of the entrance pupil (Gennery, 2006).
In the case of wide-angle lenses the entrance pupil point varies with the off-axis angle of the incident ray.
Because of the short lines of sight (156 mm at zenith and 382 mm at horizon), the entrance pupil point position and its movement had to be taken into account.
In technical terms, this is called reducing the entrance pupil, or restricting the effective aperture.
It's calculated by dividing the entrance pupil (the diameter of the portion of objective lens that actually collects light) by the scope's magnification.
The exposure light from the illumination system is converted into a rectangular shape that illuminates only a portion of the reticle and images it through the entrance pupil
of the lens.
6) should read, "Think of the Meade objective as a very long focal-length 'eyepiece' that, just like any other eyepiece, forms a real image of the entrance pupil
of the telescope.
For visual observing through a telescope, the effective pupil size will be determined by your instrument's "exit pupil" or by the entrance pupil
of your eye, whichever is smaller.