check out saturn as of 6/4/2010, 9 30 pm

[Canopus Lim]

One thing perhaps worth considering is whether or not the response of the rods is linear. Even if it they are, there is a fair amount of image processing going on in the retina itself before the signals reach the brain (e.g. edge detection in the ganglion cells), where there is presumably even more processing going on.

Anyway, if there is any non-linearity, a relative drop in available light (say 1/2) might appear as a more significant drop (1/8) depending on the absolute light level. This might explain any perceived sudden "cut off" at a certain exit pupil.

Some quick googling reveals sensitivity for the cones, but not for rods:
http://hyperphysics.phy-astr.gsu.edu/hb ... dcone.html
http://en.wikipedia.org/wiki/Rod_cell

I didn't know the integration time for rods could be up to 100ms!

BTW one way to get and keep your observing eye dark-adapted is to use an eyepatch. You look like some weird astro-pirate, but it works! Only take it off when observing and cup your eye to ensure no local light pollution gets it. Compare it with your non dark-adapted eye - you'll see it can make quite a difference.

Whenever I observe, I always use my eye patch to cover my non-observing eye and it is like second nature. Yes perhaps one day I can try to cover my observing eye for awhile and then observe and see if I can see more subtle details on the planets.

Regarding cut-off, yes at certain exit pupil there will be a bigger diminishing of returns. It sounds crazy, but I actually did months of experimentation (using calibrated monitor and self design contrast targets) of my eye on Contrast Threshold performance and found out that through experiment the best exit pupil to use is 0.8 to 1mm. This corresponded to field use of my telescope when I observe the planets and it also corresponded to what experience planetary observers say about 0.8mm being around the best exit pupil for observing planets.

[Canopus Lim]

From my experiments with contrast resolution targets I created, I found out that my eye/brain ability to discern contrast is pretty close between 1mm and 0.8mm and progressively gets worse at 0.7mm and gets much worse at 0.5mm. It is 'non linear' in fashion and maybe a log function.

If you are interested, you can search the net on Contrast Sensitivity Function which plots the eye's ability to discern different level of contrast vs different frequencies (or resolution). From this charts it clearly shows the eye/ brain behaving like a low pass filter, having better contrast on lower frequencies (low resolution) and poorer contrast at high frequencies (higher resolution). Also brightness will shift the curve and alter the curve if the brightness is too high (saturation) and if too low. That is why when observing planets at too low a magnification (large exit pupil), the low contrast and low resolution features are also hard to be seen because of saturation (being too bright to the eye).