This is an image from a few years ago, but it shows what kind of surprises can pop up when doing astrophotography. When doing a 40 minute tracked exposure of a cabin near Ottawa, I noticed a faint fog on the horizon. I decided to push through with the exposure anyway, and because this was done on film I didn't actually know what was captured until days later.
You can imagine my surprise to find out that I had in fact captured very diffused auroral activity. The trouble was that it was so faint that the colour sensing portions of my retina (cones) were not able to pick up the green hue and only my monochrome (rods) could see it at all. For that reason it was grey in appearance and I simply suspected fog.
Note that due to the tracking by my mount the log structure would have been blurred (see the roofline). Fortunately it was very dark at the site and I light painted it halfway through the exposure for a brief moment to freeze the motion.
Did you spot any constellations? Here is the secret decoder picture:
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