Posted 07 February 2013 - 07:49 AM
Some of the explination may be related to the energy distribution and the emergence of astigmatism.
The book Telescope Optics has an excellent 3D diagram of the light distribution in a comatic/astigmatic star pattern.
While the comatic/astigmatic tail can expand quite far from the abberated Airy Disk, it is shaped almost like the root flare of a tree. The trunk represents the Airy Disk, and a couple big roots coming out at an angle represent the exetnsion of the comatic tails.
As you first move away from the center of the field, most of the energy is still encircled in a very small area. Until it extends more than a couple of arc minutes of apparent field (the amount required for the dark adapbted eye to actually resolve a shape) we see it is a point.
The further we go out, the more energy is transferred from the Airy Disk into the comatic and astitmatic fans. The Airy Disk grows dimmer but the fan itself doesn't necessarily grow brigher because as the fan developes, the way the energy is distributed further and further into the area away from the Airy Disk that the extensions themselves become dimmer and dimmer.
They are bigger and bigger, but because the energy is spread over a far larger area, the very ends of the extension will simply be below the eye's dark adapted ability to detect.
The dark adapted eye's contast sensitivity threshold is only between 5% and 15% depending on the size of the target and the observer's own contrast sensitivity, which does apparently vary from observer to observer.
Anyway, the comatic blur is always much larger than we can see visually.
But put a camera on it and take a long exposure picture, and you see that the coma is far larger than what we see visually because the amount of energy way out in the fan is too low for our eyes to easily detect.
And this. Most reflectors are only fully illuminated at the center of the field. It is not at all unusual for the designer to let the illumination fall off by 30% in a telescope designed for visual use.
So, once again, as you move further from the optical axis, the energy in the fan is groing and the energy in the Airy Disk is draining away, and the more spread out the fan is, the harder the comatic/astigmatic extensions become to detect at the limits of their extension, but at the same time, they are also growing dimmer because of the off axis illumination of the telescope itself.
I believe some of the reason for the descrepencies lie in these to explinations. The energy distribution is growing larger and larger so the tails are growing dimmer at the tips while they are growing ever larger, and the off axis illumination of the scope itself may be further reduceing the brigtness of that blur at low powers so that we never really see the full extension of the comatic/astigmatic blur.
Again, a long exposure image would show the true extension.
As you use eyepecies with narrower field stops, the comatic/astagmitic blur is smaller, but the light intensity distribution makes it much brighter near the Airy Disk.
An easy experiment to kind of explain this is to drift a star near limiting magnitude towards the field stop of an eyepiece with a very wide true field.
If there is bad coma in the system and the field is not fully illuminated, you will see that the star will literally disappear before it gets even close to the field stop.
Studying some 3D plottings for energy distribution for an abberated star and you quickly realize that spot diagrams doe a poor job of telling you the intensity of the extensions. They can get very long, but that energy gets more and more diffused.
In other words, it is very complex, and much depends on the individual's contrast sensitiviy threshold (how faint against the background do the ends of the tail get before they drop out of visibility), the visual acuity of the individual, and the off axis illumination of the scope used.