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Very interesting thread. As a visual deep-sky observer for over 40 years, I have been constantly aware of this issue for a very long time. When I first started making sketches of deep-sky objects (in the mid-1970s), I deliberately would not look at photographs of the object beforehand, in an attempt to defeat this I-know-it's-there bias.
These days, with the huge flood of high quality images, it's harder to avoid seeing an image of an object before viewing it. I do not, however, inspect images shortly before observing in the hope that I can see something.
I observe carefully, recording what I can see. Sometimes I suspect detail that I'm not sure about. I will note the suspicion, but not make it part of the sketch. I think we *are* biased by our preconceptions (and our previous experience, e.g. of seeing an image of what we're looking at).
Equally, I think it's something we can only avoid with great difficulty. It's human nature.
I agree with Alexander that knowledge informs our perceptions. It has to. It's how we survive at a basic level. Knowing that a tiger can eat you will prevent you behaving towards it as if it was just a large pussycat.
FYI, the sketch of M31 was drawn by French astronomer Etienne Leopold Trouvelot at Harvard Observatory in 1874. Continuing the theme of us being influenced by what we have already seen (what we know, if you like), here's an observation of M31 made by American astronomer George Philips Bond, again at Harvard, but this time in 1847.
To me, it is very clear that Trouvelot had seen this drawing before he made his own. To a modern observer, both appear 'wrong' in exactly the same way.
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