I have provided this information in a great many threads on CN but I fear that it largely goes ub-believed or just gets dismissed out of hand but I will provide it to you and maybe based on your experience, you might be one of the few that actually accepts this point and it is a simple point.
There is the telescope, and there is the human eye. These dialogs almost always focus on things like seeing, obstruction, aperture, and on and on and on, but they almost never account for the behavior of the human eye.
The human eye has rods and cones. Five rods share one neuron, so this makes the rods quite good for detecting faint objects, but because the five rods cover more area for each neuron, they lack the resolution. Because of this, the fully scotopic eye only has a resolution of about 3 arc minutes.
Cones each get their own Neuron and because of this, they are far less likely to fire for a give number of photons hitting the retina, but because they each cover less area, this means they have far greater resolving power, and they also see more colors than the rods, which are mostly sensitive to green. Now in daylight, the cones are capable of resolving 1.1 arc minute detail.
When using a telescope to view planets, the eye is actually working somewhere between scoptopic mode and photopic mode. It is in a mode called mesopic mode, which is a combination of photopic and scotopic modes. It has better low light sensitivey than pure photopic, but it looses some amount of resolution because there are not as many photons available to fire the cones. This means that the eye is resolving better than scotopic though.
Enter the "Contrast Sensitivity Threshold." The contrast sensitivity threshold is the amount of contrast that has to be visible for the eye to be able to pick a feature out form the background. As the illumination of the retina gets progressively lower, the eye's contrast sensitivity gets worse. This means that you have to have more contrast to see a given detail. A detail with 2% contrast is resolvable with the photopic eye, but for the scotopic eye, it can be as high as 15%. Below this amount of contrast, and the eye simply will not be able to see it. This is dependent on the size of the detail (frequency) and different observers will have different contrast sensitivity threasholds.
The best way to increase contrast sensitivity threshold is to increase the illumination level. Now if you use a 4" scope at 150x you are already seeing a dim image, and this means fewer rods are firing, and because of this, you have poorer resolving power. In the much larger aperture working at the same power, you are firing you cones far more often, and as a result, your visual acuity is increased and your contrast sensitivity threshold is lowered so now, lower contrast details become easier to see.
See, there is resolution, and there is contrast. Your seeing might lower your resolution, but a brighter image might allow you to more easily resolve a bigger detail that would be below your contrast sensitivity threshold in the smaller scope.
With more aperture, you improve your visual resolution (for a given exit pupil) and you improve your contrast sensitivity threshold.
Again, these forums typically totally ignore the performance of the human eye when it comes to comparing smaller telescopes to larger telescopes, and that means that the dialog is far from complete. To really complete the picture, one has to factor in the behavior of the telescopic eye. It is after all, a vital component in your observations.
I have for a very long time now been adamant that the very best thing a dedicated planetary observer can do is to buy a bigger and better telescope because one of the most important parts of the game is firing as many cones as possible and a larger aperture will almost always give this advantage.
So, the larger aperture fires more cones, which increases the ability to resolve finer detail, and it raises the contrast sensitivity threshold so that larger lower contrast detail that is below that threshold in the smaller scope now exceeds that threshold in the larger scope.
And last but not least, illumination gives you c o l o r. The more cones you fire, the more colors, or shades of color you will see. A wide belt on Saturn will suddenly start to break down and be shown as two different color thinner bands next to one another. Rather than look like featureless streaks, the belts on Jupiter will start to reveal difference densities of red/brown, and the GRS, rather than appearing as pale grey brown spot, will start to show beautiful red and gold features.
I have said it over and over and over on this very forum but I keep repeating it because for the best planetary observation possible, one should recognize the role that illumination levels play on resolution and contrast detection.
While fine details may be lost to seeing, many structures on Jupiter, Saturn, and Mars are not so small as they are very low contrast. See the difference contrast makes? All of the letters are the same angular size, but note how some are very difficult to see. Turn down your computer monitor brightness and watch what happens as the luminance level falls!!!. Imagine doing the same thing to Saturn. Get the point? These are large details, but they are low contrast details, so even though the are more than big enough to resolve, if they fall below your contrast sensitivity threshold, they become hard to see, and when they fall below that level, they become invisible.
Bottom line? For low contrast detail on planets, bigger is almost always better because you are firing more cones and it is the cones that give use improved resolving power and contrast sensitivity. The simple act of raising the image brightness will almost always make the image better.
Edited by Eddgie, 12 October 2019 - 10:29 AM.