Part II: Star gazing and astrophotography
After lunch, a nap, and dinner the sky was clearing nicely, with just a few clouds left over from a small weather system that had passed through during the day.
I’ve been an astronomy buff (geek?) since I was little, but living in a big light-polluted city doesn’t excite one to get out in the evening and stargaze! Whenever I’m out camping, however, the stunning beauty of the truly dark skies are always getting me out of the tent in the middle of the night to try and capture the wonder.
For the last few years, I’ve been using a fisheye lens on the SLR. The fisheye is fast (f2.8) and sharp, and I love the sense of vastness that the wide-angle coverage provides. Combining that lens with panoramic techniques gives an even larger field of view – in fact around 10 frames with that fisheye will actually capture the entire the sky – a full 360×180 hemisphere of stars. Here’s the best of the last few years.
I’ve been really happy with that lens, but the night sky is dark and the stars quite dim. Capturing as much as possible requires turning up the ISO on the camera as high as possible, before noise degrades the image. I push the new SLR (Canon 7D) to 2500ISO, and the fisheye allows a 15 second exposure before the stars show visible movement (trails), but the noise has always been a distraction.
Anyway, back to the weekend. As we were cleaning up dinner and the sky was darkening, we were treated to a very bright satellite cruising across overhead. It was the ISS! Brighter than any star or planet, it was amazing to realize that bright point of light sailing across the sky is a $100 billion space station weighing almost 1/2 million kg and moving at over 27,000 km/h. Wow!
We wander over to the Columbia Icefields interpretive center parking lot, where there’s a clear view of the Athabasca Glacier and surrounding mountains. Some nearby RVs and buildings have some lights on – next time we’ll find a better sight, but the view towards the SW, where the Milky Way is starting to appear over the glacier – is clear and untainted by artificial light. We take a few unguided shots – this one is a four-frame panorama covering about 100×100 degrees.
A little while ago, I picked up a lightweight sidereal drive made by Astrotrac, and this weekend was the first time I’ve really been able to make good use of it. The Astrotrac mounts on a tripod, and then the camera is mounted on top. The Astrotrac needs to be aligned to true north using a small telescope – it took less than 10 minutes – and the unit will slowly rotate the mounted camera to counter-act the rotation of the Earth using a battery powered motor drive.
By now (around midnight) the sky was fully dark, and after setting up and taking a few test shots, the moon started to rise! It wasn’t unexpected, but I was hoping to get another hour or so of very dark skies before the moon brightened things up. I wanted to take a guided panorama of the Milky Way over the Athabasca glacier, and had to make some quick decisions about ISO and exposure. I wanted more light than 15 second at 2500 ISO. I know the camera is very clean at 400 ISO, but did a “quick” (72 seconds) test shot at 800 to see what came out.
Impressed with that, I decided to drop to 400 ISO and double the exposure, then shoot a few frames to make up a panorama. Notice in above picture how the mountains are blurred – the tracker is following the stars as they rotate so of course the ground blurs out. Took an unguided photo to capture the mountains, which I planned on digitally compositing back into the sky panorama, to keep both parts of the image sharp.
I’m very happy with the result. While the rising moon washed out the deep black of the sky, and reduced the visibility of the Milky Way, the way it lit up the glaciated mountains was just beautiful! Because this is such a “digital mash-up” – three frames of 3-minute exposures for the sky, stitched together, plus a unguided exposure for the ground, I’m not sure you can call it a “photograph”, so let’s call it an image.
I’m also very pleased with the easy setup, portability and guiding accuracy of the Astrotrac. The longer exposures that are possible have much lower noise, higher natural contrast and better colors. More (sharper) astrophotography in the future!
Darren Foltinek, 2011