The previous post showcased one image taken last Saturday night at Patricia Lake. This is a composite of 98 such frames, producing an image of stars circling the sky.

This is the motion of the northern sky over 75 minutes, as the Big Dipper and other circumpolar stars arc around the celestial pole, just off camera here. A few faint meteors streak at left. And the makings of an aurora appears at right.

Each exposure was 45 seconds long. I used a Photoshop Action to automatically select each frame in turn and stack it on top of the previous image, then change the blend mode to Lighten and flatten the layers. The end result of the computer crunching away is an image that recreates what we used to achieve with film, by stopping down the lens and exposing a slow ISO film for an hour or more onto one frame.

I last shot this same scene a decade ago with just that technique and Fuji Velvia film, a favourite of mine back then for star trails. But these days shooting multiple short exposures digitally provides the advantage of also netting a folder-full of images suitable for a time-lapse movie, something we could never do with film cameras, unless they were modified movie cameras. I like DSLRs better.

— Alan, August 1, 2012 / © 2012 Alan Dyer