By pure chance I had photographed M81 and M82 and their surroundings on 20 January, the day before the discovery. I tried again on the 25th, to show the supernova. It turns out I do have it at a lower brightness on the pre-discovery photo too (I read that they have picked it up on photos back to 15 January).
My photos were stacks of 10-second exposures at 600mm f/4, ISO6400 on a 5DII (on an Astrotrac). The earlier one was 24 lights (= 4 mins), and the later one was over 110 (= nearly 20 mins), both used darks, flats, dark flats and bias too. The processing obviously wasn't quite the same, but these were just the first go at each because I was impatient to have a look. Here is M82 at 100% from the two images.
The problem was that by bringing up the galaxy brightness I was masking the supernova, so I went back to one single 10-second frame and processed that a bit, to make this image, where the galaxy is dimmer, so you can see the SN much more clearly.
