M81 is a faint galaxy and rather a challenge to image in light-polluted skies. A single image is disappointing--showing a bright center and a fuzzy halo. Increasing integration time and histogram stretching is one way to bring out more detail, but if the data is not there in the first place, these techniques yield little, and increased sky glow is more likely to degrade the image. Stacking is the best way to capture this missing data and to pull detail out of a relatively featureless image.
The first image shows a stack of seven, 30-second integrations with some aggressive histogram stretching. The spiral structure of the galaxy is just visible above the sky greenish glow.
The single, "raw" image below shows what one frame looks like. The benefits of stacking (even such a small number of frames) are pretty clear.
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