Easter Sunday Space Station


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

Springtime Cluster #2: Ancient M67


Poor old M67. Does anyone ever look at this cluster? I tend to ignore it, in favour of its brighter and bigger brother, the Beehive Cluster just to the north. Yet, this smaller cluster ranks with the best of the sky’s open star clusters for richness and brilliance. Only a few showpiece star clusters, like the Beehive and the Pleiades, beat M67.

Located in Cancer, M67 really deserves more respect – even a name! – as it stands out as one of the few prominent deep-sky objects in the otherwise sparse spring sky, at least sparse for bright targets for binoculars or a small telescope. Yes, if you love galaxies, the spring sky is heaven! There are thousands of galaxies to hunt down in spring, but most need a decent-sized telescope to do them justice. By contrast, M67 looks just fine in a small telescope. With a few hundred stars packed into an area the apparent size of the Full Moon this is one rich cluster.

M67 is called that because it is #67 in Charles Messier’s catalog of “things not to be confused as comets.” Messier came across this object in April 1780. Messier ‘s object #67 is one of the few open star clusters not embedded in the Milky Way. Like the Beehive, M67 sits well above the disk of our Galaxy’s spiral arms. We look up out of the plane of the Galaxy to view M67, sitting some 2600 light years away, over four times farther away than its neighbour in Cancer, the Beehive. Thus, M67 looks smaller than the Beehive because it is more distant.

M67 holds the distinction of being one of the oldest star clusters known. It’s been around for over 4 billion years. Its position well above the frenzied traffic jam of our Galaxy’s spiral arms helps M67 stay intact and together, an isolated island of stars in our spring sky.

This image was taken right after the M44 Beehive Cluster shot featured in my previous blog post, using the same gear. So the image scale is the same. You can see how much smaller M67 appears than M44. Because M67 was beginning to sink into the west when I took this, I bumped the camera up to ISO 1600 and used shorter 3 minute exposures and stacked five of them to smooth out noise. The telescope was the little 92mm TMB apo riding on the Astro-Physics 600E mount and flawlessly autoguided with the Santa Barbara Instruments SG-4 autoguider. I really love the SG-4 — just press one button and it’s guiding. True “Push Here Dummy” guiding!

— Alan Dyer, April 2011 / Image © 2011 Alan Dyer