[madbadgalaxyman (Robert)]

Paddy,

When you write "faint impression of spiral arms" and that this might just be your imagination, this illustrates a common problem with visual observations which are made at or near the extreme limit of the eye's sensitivity to low surface brightness diffuse light;
given that the eye/brain system's detection of an object is intermittent, at this low level of surface brightness, with a feature or an object flashing on and off, how to decide if an "observed" feature is really there or not?

In my bygone days of regular visual observation (I observed mainly galaxies and diffuse nebulae for some 26 years), I used a few techniques to help me decide if my eye/brain system's "detection" of a vanishingly faint object or feature actually corresponds to something in the real universe rather than just something that my brain has made up:

(1) I would keep observing the faint apparent object or feature for a prolonged period of time, and I would count the number of times that I could see it, even if it only flashed on for a second each time I did see it. If I could glimpse the apparent "feature" at least 6 times, I would estimate a high probability that the feature is actually real, even if afterwards my tired eye could not see it any more.

(2) Given the notorious "suggestability" of the human brain, when it responds to images from the eye which are at the extreme observational limit (e.g. the canals on Mars), we do not want to be in the position where a feature is deemed to be real by the observer simply because it was briefly "glimpsed" or perhaps only imagined to exist, due to the tendency of the eye+brain system to believe it has seen something despite an absence of hard evidence.
So what I do is to try to negate this obvious bias of "really wanting to see something" which then causes me to "see" something which does not exist in the real universe;
I try to convince myself that I am not seeing the apparent object which is "seen" at the extreme limit of detectablity (thereby setting up a bias in the opposite direction), and if I still see an object or a feature, despite trying hard to convince myself that it is not there, the observation is much more likely to be an actual detection of light from the object than merely my imagination playing tricks.

(3) Come back to the object later on in the night, or wait for that "perfect" night, in order to confirm or deny an existing tentative observation.
There were many cases in which I had to repeatedly come back to an object in order to figure out what I was seeing (or not seeing) ; for instance it took me a very long time with a 10 inch Newt to figure out the complex structures observable within NGC 1313 and NGC 6744

(4) Rest the eye and body and mind, at various times during a prolonged and arduous observing session. A tired eye+mind+body is not a good instrument for detecting faint features in galaxies - so go and have an cup of coffee, or lie on an air mattress with binoculars...which gives the eye the opportunity to relax. When I observe, I always have some periods with binoculars, lying on my back, between the periods with the telescope....it refreshes mind and body and eye, most amazingly.
You see more afterwards!

[of course, other Deep Sky observing tricks include averted vision, and moving the vanishingly faint object just slightly (the eye sometimes responds more strongly to a moving object) ]