[N1 (Mirko)]

Thanks for your post Steve. It helps illustrate my point. I'll try to explain.

Re issue 1: Assuming that the exit pupil of the 4" is at least that of the observer's eye, you are totally correct, you would not see a difference between the two scopes for brightness. That's exactly my point. If there was a penalty for allowing the exit pupil to become too large, the 8" should somehow suffer a loss in brightness, either compared to the 4" or compared to itself operating at higher power. It doesn't. Period. My arguments are being called irrelevant on here, when the only thing actually irrelevant is the light you are "throwing away". If one was to follow that logic, you'd also be "throwing resolution away" every time you use your scope below its highest practical power (seeing limits notwithstanding). You'd also be wasting all the light hitting the grass next to your scope, because you failed to bring a larger instrument. Of course you're not, it's just silly. As I said, "throwing light away" is a misleading argument, because it suggests you shouldn't be doing it*. That's what I take issue with.

Re issue 2. Yes, and that's why I said earlier it's of "approximately zero", not "zero" consequence. I acknowledge that the iris' edge is less than perfect and may introduce some diffraction artifacts. However, in practice the effect can be happily ignored. For example, when observing the Moon, the eye's pupil will be smaller than fully dilated, so it will encroach on the EP's exit pupil at a higher power than the telescope's "lowest practical power". If one was to follow your argument, full- disk, low power views of the Moon should be of mediocre quality at best in anything but small telescopes. They are not. They are actually quite good. Secondly, the iris stops down the light beam almost all of the time anyway. It's called life. If one was to follow your argument, people should be seeing said artefacts, and finding them obtrusive, whenever their pupils are the limiting factor to the amount of light their eyes receive, i.e. just about always. Yes, I do see some spikes on bright point like sources, but the are hardly obtrusive. Reduced contrast by using your eyes naturally? Gimme a break! For the avoidance of doubt, I'm not saying these artifacts don't exist, but I am saying that they are hardly a reason not to use as low a power as your eyepiece case permits. Importantly, we are talking about the effects of exceeding the maximum recommended exit pupil size here, not the difference between the optimum (2mm perhaps?) and say 10mm. Yes, that difference would be more than minor, especially for contrast and sharpness.

*The caveat to this, I think we all know and agree on, and it's what you appear to be wanting to avoid: Obstructed optics do have a low power limit. The effects introduced by the secondary absolutely dwarf anything I've seen myself having to discuss with you since my first post in this thread. I also note with interest that it has taken until now for anyone to produce the pupil-edge-diffraction argument. Not what I'd expect to see if it was the main, or even a significant, argument, sorry.

It's interesting how this discussion has evolved, considering my original argument. We've heard of digestion artifacts, been back to school, we've even gone travelling. However with the above in mind, I stand by every single word. Again, apologies to the OP for any unwanted diversion.

Further reading: Nagler, A. "Choosing Your Telescope's Magnification." Sky & Telescope (May 1991).