Gary, Alex, and all other contributors: thanks very much for your explanations. I feel I really learned something today. It almost seems like the more obvious reaction of the eye (pupil dilation) is actually the less important. It would be interesting if a setup like the above could measure the impact of each. Am I correct in assuming that seeing or not seeing certain stars e.g. in open clusters (such as M45) is less suitable for this than say nebulae, due to the way stars and extended objects are "processed" differently by the instrument?
Re movement - Alex, again I'm completely with you there. My opinion is reaffirmed that my scopes too should remain hand-operated as long as they are only used for visual work.
I guess seeing the two bright DSOs relatively unchanged by dinner the other day really was an optical illusion (i.e. brain failure according to Neil Degrasse Tyson
)