And here is how you test for and estimate field illumination falloff.
Use longer focal length eyepiece with a known field stop size.
Place a bright star at the center of the field and defocus the field slightly. Enough so that maybe five or six diffraction rings are visible.
Now, drift the pattern towards the edge of the field. At some point, you should observe a smooth, large arc intrude into the edge of the pattern. The point at where this starts, as measured from the center of the pattern, is the point where the field is being vignetted. Let's say you are using an eyepiece with a 27mm field stop (24mm Panoptic) and this occurred when you observed the center of the pattern to be about 1/3rd of the way to the edge of the field: since 1/3rd of the field size of an eyepiece with a 27mm field stop is 9mm, then that would be the size of the fully illuminated circle.
Now for severity... Continue drifting the pattern to the edge of the field. As it nears the field stop, more and more of the pattern will be covered by the vignetting source. Let's say that by the time the pattern is at the edge of the field, 35% of it (by area) is obscured. This would mean that you have lost about 35% of the illumination at the edge of the field to vignetting.
Now when the instrument is in focus, this small amount of dimming will usually be hard to see. The star test shows it, but it does not cause enough limiting magnitude loss to affect all but the very faintest stars, nebula, or galaxies near the edge of the field of view.
Most visual observers tend to choose eyepieces that follow a 1/3rd framing (the target occupies only the central third of the field) and unless their sky is unusually bright, they will not visually detect the sky darkening.
And observer in the city though will see that the outside area of the field does appear darker, and gradually lightens until the edge of the fully illuminated field is reached. From the city, more than about 30% soft vignetting is unpleasant to me personally, but many are oblivious to it.
For nebula, where the nebula often extends far past the edge of the field stop, 35% illumination falloff can cause some of this nebula to simply be missed. For my own use (image intensified), I try hard to keep the entire field 100% illuminated.
For normal visual use though, a 10mm fully illuminated circle is pretty standard. Most fast reflectors designed for visual use and most SCTs will be in this ball park.
Most refractors with 2" diagonal will do between a 20mm and 30mm fully illuminated field. Most Imaging refractors and reflectors will almost always do 30mm or more.
Bottom line though is that a 10mm fully illuminated circle is pretty common, and most people never see the effects of vignetting from sources far from the focal plane because the vignetting tends to be very soft (gradual).
Edited by Eddgie, 16 June 2019 - 11:06 AM.