So, I have noticed that there is a lot of discussion about exit pupil and what size range is usable/ideal. While reading these discussions I have come to the conclusion that I must be misunderstanding some key aspect. When I think about exit pupil I imagine a little disc hovering in space on which the image is projected. Of course, only if something such as an eye is at that location to receive the photons is the disc realized. Otherwise the photons keep moving and spreading out from the exit pupil.When people speak of too large an exit pupil, they speak as though the "excess" photons are distributed across the disc, thus dimming the image compared to what it would be if able to receive all the photons. But why would they be distributed like this? Why doesn't the unusable ring of photons result in a reduction of the view while keeping the remaining view at the same brightness?Hopefully this makes sense. And hopefully I'm not overlooking anythingtoo obvious. Thanks!

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm

The focal plane is a disk where it all reaches focus. Focal planes can be thought of as disks.The exit pupil is a cone streaming out of the scope. One slice of it is the focal plane. Your mission is to place your head where your pupil intersects the exit pupil and then to adjust focus so that it best suits the need of your eye.If the exit pupil is so large it exceeds the size of your pupil, the light splashes uselessly on your iris. If the exit pupil is very narrow it is harder to acquire.Wide exit pupils and narrow exit pupils each have ways of revealing different eye defects.Exit pupil allows us to talk about magnification relative to the instrument without reference to the scope. 0.5 mm exit pupil is high magnification on any scope. It is 200x on my 4 inch refractor and and 700x on my c14, i.e. it standardizes magnification per inch of aperture. 200x is high magnification on the refractor and medium low on the c14.

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...

QuoteDeb:Your question is a good one and it's a question that is fundamental. You are wondering why the field of view does not change if your eye does not admit all the light from the objective.The key to understanding this is to realize that each pencil, each tiny part of the exit pupil contains the entire field of view. From a practical point of view, you can see this by simply placing your hand in the light path, you still see the same field of view, it is just dimmer. Analytically you can see that it must be true, field of view calculations include the eyepiece parameters and the focal length of the telescope but never the aperture of the telescope. A 100mm F/10 and a 200mm F/5 provide the same field of view with the same eyepiece because they have the same 1000mm focal length, the difference is that the 200mm scope provides an exit pupil that is twice as large. If that light does not enter the eye, it has the same effect as masking the aperture.Imagine you were looking through the two scopes in question with a 40mm eyepiece and that your dark adapted eye only opened to 4mm. The 100mm would provide a 4mm exit pupil and all the light would enter your eye. The 200mm scope would provide an 8mm exit pupil (40mm F/5) and only 4 mm of the exit pupil would enter your eye. The images would be indistinguishable from one another. Conceptually, one can bundle these thoughts together. Any part of the objective (assuming no vignetting) provides an image of the entire field of view at the focal plane. If you block part of that the objective, there will actually be a hole in the exit pupil, you can see this easily by inspecting the exit pupil of a scope with a central obstruction. But that does not affect the field of view, it affects the image brightness, the effective aperture.I hope this helps.Jon

Deborah,I admire your desire to understand this beyond the merely superficial!The exit pupil is basically the image of the objective as projected by the eyepiece a small distance behind the eyepiece. If you place a piece of translucent material (wax paper, or 'frosted' Scotch tape) exactly at the exit pupil, you see a sharply focused image of the objective. If the scope has a secondary obstruction, it too will be sharply focused with the objective. (To be strictly correct, the secondary will be focused at a slightly different distance than that of the objective, but this is a small difference and we can ignore it for now.) If you move the translucent screen inside or outside the actual exit pupil distance, the disk of light has an unfocused edge, becoming ever more blurred the farther from the exit pupil it's located.For every one image point contained in the field of view (and we can say there are of order a million), there is one cylindrical bundle of light exiting the eyepiece whose diameter equals that of the exit pupil, and which, like every other cylindrical bundle, passes through the exit pupil. Indeed, the exit pupil is the place where all those cylindrical beams overlap as they cross each other.If the eyepiece has an apparent FOV of, say, 60 degrees, the cylinders of light for image points at the very edge of the FOV exit the eyepiece at a 30 degree angle with respect to the optical axis. For image points ever nearer to the field center, their cylindrical bundles exit the eyepiece at progressively smaller angles with respect to the optical axis.There is no image as such located at the exit pupil. Because the eyepiece must send essentially parallel, unfocused light to your eye in order that your eye's lens do the actual focusing upon the retina, all light streaming through the exit pupil is merely the superposition of a multitude of cylindrical bundles of parallel or nearly parallel light.The eyepiece is really an optical coupler which effectively 'compresses' the larger objective down to the size of your eye's iris, plus or minus. Equally correctly, the eyepiece expands your smaller iris to the size of the objective, plus or minus. Just as your hand placed on or near the objective will have its image focused upon the exit pupil along with the objective, so will some object located at the exit pupil be projected upon the objective.For instance, if the objective aperture is 200mm, your hand width is 100mm, and the exit pupil is 4mm, your hand placed just in front of the objective will be projected at a width of 2mm in that 4mm exit pupil.Conversely, for the same objective aperture of 200mm and exit pupil of 4mm, if a 2mm wide stick is placed across and at the exit pupil, that stick's image will be projected upon the objective to a width of 100mm.In the foregoing examples, the 200mm aperture and the 4mm exit pupil tells us the magnification is 50X. Note that the 'shrinking' of the hand held before the objective is a de-magnification of 50X, and the 'expanding' of the stick at the exit pupil is also 50X. Beautiful symmetry, via proportionality of the relevant triangles.If the exit pupil exceeds your iris, the outer annulus of light not passing through your smaller iris only illuminates the iris. In such case, your iris edge would be projected upon the objective as smaller in the same proportion as the iris : exit pupil diameter.Well, that should be quite enough background material of the more fundamental kind for now, which I hope facilitates a deeper understanding.

When outside at night, the human eye becomes dark-adapted over a span of time, and the eye's pupil enlarges, allowing more light to enter, with the maximum diameter ranging from 7-8mm in youth, to 4-5mm in old age. Dark-adapted pupil diameter decreases as we age.To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm...or more simply, 25 ÷ 6 = 4.2mmYour 34mm has an exit-pupil of 5.7mm. If the diameter of your dark-adapted pupil meets or exceeds 5.7mm, then you're good. If not, then the part of the light that exits the eyepiece will fall onto the iris instead of into the pupil; thereby light is wasted, and in the pupil not receiving all of the light gathered by the primary mirror of the telescope.Take this 2" 50mm eyepiece, for example... http://agenaastro.co...w-eyepiece.html50 ÷ 6 = 8.3mmThat's a considerably large exit-pupil, and also a very low power. A 50mm would give a low magnification of only 24x with your XT8, but if the diameter of your dark-adapted pupil is less than 8.3mm, then some light will be lost; again, falling onto the iris instead of entering the pupil. It would be like observing with a 6" or 7" telescope even though one has an 8".However, some don't mind wasting a little light, if they can have that very low power in its place.People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.As one goes up in magnification, like with your 10mm(120x), the smaller the exit-pupil becomes(1.7mm), and everybody's happy.