Last night we gave astrophotography a whirl out in the Cape Hatteras National Seashore. We had mixed results, but I learned a great deal about technique and improving my planning and preparation. In addition to Julie, my brother and sister (Shawn and Colleen) came along to watch and get a chance to see the stars against a really dark sky.
Location
In selecting the location there were two main factors. First, we looked at the darksitefinder.com map for a dark location. Second, we looked for someplace close to the Outer Banks because, what the heck, that’s where we were. What we found was a spot south of Salvo on NC 12 that had very dark skies with little light pollution, so that’s where we headed.
Equipment
You probably need a digital SLR for reasonable results with astrophotography. Capturing enough light to record a good image means a longer exposure and higher light sensitivity. For my Nikon D5100, I attached a Sigma 10-20mm f3.5 lens. That seemed like a good choice for astrophotography because it’s pretty fast (has an f3.5 aperture) and the wide-angle lets me capture more of the sky and horizon.
For astrophotography, the D5100 is very flexible. I set the focus to manual and initially pushed the ISO (light sensitivity) up to 1600. That captures the limited light faster. Then I played around with exposures ranging from 10-25 seconds. Fortunately, I’d packed a small, light tripod that gave me a stable platform for the photos. Okay, I guess we’re all set.
Work in Progress
What I discovered about astrophotography is that working at the seashore is a lot different than in the mountains. Julie and I made an unsophisticated attempt several years back in New Mexico. Outside of Albuquerque, the skies were awesome, but the pictures suffered from poor equipment and our clumsy approach. As the skies darkened near Cape Hatteras we realized that sea level combined with the humidity in the air conspired against us. The sky was fantastic, but not as sharp and clear as we’d seen in the West. When you think about it, at sea level you’re looking through a lot more atmosphere and the increased humidity definitely impacts clarity.
Regardless of the humidity, the sky was dark and I thought the stars looked pretty clear. After a few initial attempts, I really had to push the light sensitivity of the camera up to keep the exposure duration relatively short. The pictures were shot at f3.5 for 10-25 second exposures. To get anything close to what I was hoping for, I had to set the ISO (sensitivity) to 3200, but that resulted in a lot more noise in the photos that I wanted.
Results – First Pass
The last issue was camera related. With astrophotography, we’re dealing with points of light in the dark sky. Focusing accurately was turning out to be a problem. First I tried just setting the focus to infinity but I got some blurring. Trying to manually focus gave me some success, but the results weren’t very good either. We left the beach that first night with some decent shots, but there was still too much blurring for me. Here’s one of the photos that came out pretty good – Julie is pointing out the constellation Orion to my sister.
The second photo is more interesting. My brother Shawn saw these faint red lights moving in the sky to the west so we quickly repositioned the tripod and took a 15-second exposure. We’re 90% sure that they were military aircraft, but my brother is still pushing the UFO theory. You can see the red streaks in the lower third of the captured image. That bright spot near the bottom is Venus setting to the west. The cluster of stars left of center is the constellation Pleiades or the Seven Sisters.
Astrophotography Research
Sitting at home the following day looking at the photos I tried to figure out how I could do better. The first thing I figured out was that I’d forgotten to remove the polarizing filter from the lens. In hindsight, I wonder how I’d missed that. I’m not sure how much light gets blocked by the filter, but looking around on the internet I see estimates in the 1.5 – 2.5 f stops range for decreased light. No matter how you look at it, that’s a significant reduction in light getting to the camera sensor. That filter came off right away – first problem solved.
The next problem I looked at was focusing. Back to the internet! At first, I was hopeful about articles I was finding on focusing baffles, but that turned out to be for photography through a telescope. Then I came across references to ‘live view’ on Canon cameras. I checked on the Nikon site and found that the Nikon D5100 had a ‘live view‘ feature as well. After experimenting with this a few times in daylight I felt ready for the next nighttime opportunity.
Another Night
Rather than driving an hour south into the Cape Hatteras National Seashore, we set the camera up on the deck in Nags Head. There is plenty of light pollution here and lots of cars drive down a nearby highway, but I just wanted to test out the fixes I’d come up with. I was very happy with the results.
First, removing the polarizing filter had an obvious impact on the brightness of the photos and the clarity. Even with the less than ideal sky, I was able to get much better photos, even with the ISO set at 1600. Next, I tinkered around with the focusing issue. That turned out to be a little trickier.
Live View
Looking at the ‘live view’, I was initially presented with what I thought was a blank, black screen. It turned out that finding a star in a dark sky to focus on can be more challenging than you might think. After a bit of tinkering, I was able to find and center a bright star on the display screen of the camera. Next, I manually focused the image and then zoomed using the ‘live view’. Using that display screen I was quickly able to manually adjust the focus to get a really sharp and crisp image! Nice! Now let’s take some photos.
I only took a few photos because the sky in Nags Head was not great for astrophotography, I was more than happy with the results. The stars in the images were finally clear and crisp and the lower ISO resulted in less noise in the resulting images. When I zoomed in on the image from the 25-second exposures, I could now clearly see star trails from their rotation in the sky. That’s not an issue because my goal is for the broader effect of photographing large portions of the sky. All in all, the results were a great improvement and I’m now confident that the next time I get out for nighttime astrophotography I’ll be ready.
Raw images can be viewed here.