What a picture perfect pass this was, on Easter Sunday evening, April 24, 2011. The International Space Station (ISS) rose out of the west right on time, passed almost directly overhead, the flew off to the east, fading out just as it approached the horizon. The sky was a deep blue in the late twilight, with the spring stars beginning to appear. As it usually does, the Space Station outshone them all, including Arcturus, the brightest streak at right. The Station also passed just north of the handle of the Big Dipper at the top of the frame.
I took this with the 15mm lens that takes in a field of view from the horizon to almost straight up.
Because the sky was still bright, one long exposure wouldn’t work with this shot. The Station took nearly 3 minutes to traverse the frame. The only way to avoid overexposing the sky would have been to stop the lens way down or use a very slow ISO speed, either of which would have meant the Station and stars might not have recorded very well, especially the fast-moving Station.
So instead, I used what turned out to be thirteen short 15-second exposures at f/4 and ISO 400, taken 1 second apart. Each one is exposed correctly, and the aperture and ISO speed are fast enough to pick up stars and the moving satellite. The trick is to then stack the images in Photoshop. To do this I import the images first from the memory card using Adobe Bridge, then process them in Adobe Camera Raw. ACR provides most of the processing necessary for shots like these – sharpening, noise reduction, some contrast boost, and the usual Vibrance and Clarity. Once processed, I use Bridge’s “Import into Photoshop Layers” command to automatically create a single image with all the processed frames stacked on top of each other in layers. Then it’s a matter of turning the Blend mode of each layer from Normal to Lighten, and voila! – the frames turn transparent and appear merged together, all properly exposed. The result is a single exposure effectively 3 minutes long, long enough that the stars also trail.
In this shot, the 1-second interval between exposures creates the gaps in the ISS trail. I could do a little Photoshop trickery to eliminate those but I think they give a sense of the speed of the ISS as it flies overhead quite quickly when it is closest, then slows down in apparent motion as it flies away into the distance.
A great help in planning a shot like this is Starry Night software – it loads in the latest satellite orbit data and shows the exact path of the satellite across your local sky. So I was ready and waiting with the camera all framed up, knowing just where it would appear, then started the camera firing as the ISS came overhead and into the field of the camera. It was a fine sight on an Easter Sunday.
– Alan, April 2011 / Image © 2011 Alan Dyer