Learning how to photograph star trails

I'm taking a photography course through the Harvard Extension School, and it was quite fitting that star trail photos were suggested for this week's project, since one of my recent photographic goals was to take star trail photos.  I'm a space exploration enthusiast, and star trails have always amazed me.  Living in Boston makes it very hard to take such pictures, due to all the light pollution from the city.  I recently took a trip to central NY, and figured that a weekend in the country would be the perfect time to take star trail photos.

Preparation:  After doing some online research about star trails, I purchased a wired remote for my Canon 60D.  This allowed me to hold the shutter open for longer than 30 seconds, as well as take multiple shorter photos for stacking.  I also purchased a tripod for this shoot.

Conditions:  I had two consecutive evenings (10/8 and 10/9) to take star trail photos.  I knew that the first evening would be full of trial and error photos.  The conditions were near perfect for taking star trails photos.  There weren't any clouds in the sky, there was minimal light pollution from populated areas, and the temperature was in the 60's, so I really didn't run into any condensation on my lens.  Even though I was out in central NY, away from heavily populated areas, light from houses and street lights still washed out sightings of stars right along the horizon.  The timing of the moon phase couldn't have been worse though, since it was nearly a full moon.  There was quite a bit of reflection of light off the moon lighting up the night.  I could almost read a book out there with light reflecting off the moon. 

Positioning:  After the fact, I figured out that it would have been best to drive way out into the country, far away from any houses or street lights.  Instead, for this shoot, I was in the backyard of a house in a populated neighborhood.  That added a bit of light pollution, washing out some star sightings.  I set up my tripod with my back to the moon, to make sure that it wouldn't enter my shot as it moved across the sky.  The moon is way too bright for these types of long exposures.  I made sure to get the north star in my shot, so I would have star trails circling around it.  Not essential, but a cool looking effect.  

First night:  After setting everything up in the backyard, I manually focused using Jupiter, since it was the brightest item in the sky that I could see on my LCD screen.  I zoomed in my LCD screen on Jupiter, and set my focus to infinity.  This wasn't at the very end of the focus ring, since (as I later researched) lens manufactures have a focal point past infinity, to account for slight variations in the lens due to temperature and other conditions.  My first attempt was simply a three minute test shot using bulb mode and wired remote with the following settings:  50mm, 180 sec, f/1.8, ISO 100.  This was unsuccessful, since there was so much light captured by the sensor and it resulted in a pure white image.  I closed my aperture down and proceeded to take my first full length star trails photo using the following settings:  50mm, 2025 sec, f/22, ISO 100:

50mm, 2025 sec, f/22, ISO 100

I was happy to capture some successful star trails, but disappointed in the photo for a few reasons:  not many stars were captured, I didn't have anything in the foreground to provide a point of reference, there was lots of digital noise in the image and I wasn't positioned properly to show the north star.  I didn't capture many stars because I was forced to use a large f stop to properly expose the 34 minute image.  The digital noise was a result of leaving the shutter open for almost 34 straight minutes.  I had read about another method of taking star trail photos called "stacking".  By taking a series of shorter exposure photos, and later stacking them using Photoshop or other programs, you cut down on the digital noise and are able to take shots with a lower f stop, resulting in capturing more stars.  My final shot of the night was taken using the following settings:  18mm, 29 sec, f/3.5, ISO 100 and I took 60 consecutive shots, effectively giving me total of 30 minutes of photos.  These shots were taken 1:30-2am.  Using a setting on the remote, I left one second between each shot to give the camera time to save the 29 second image it just captured.  I then used a program called "Startrails" to stack all 60 images on top of each other, using the lighten blending mode.  This could also be accomplished in Photoshop using layers.  I was very happy with the result:

18mm, 29 sec, f/3.5, ISO 100, 60 consecutive shots

I was able to capture many more stars due to the wider aperture, my digital noise was very low, I now positioned a tree in the frame for some reference, and I positioned the shot to capture the north star.  Other than some slight blurring in the tree (the wind was slightly blowing) and some whisps of clouds on the bottom right, I was very pleased with the result.  Stacking consecutive images definitely seems like the best method of taking star trails photos.  

Second night:  Now that I had one successful star trail photo using the stacking method, I wanted to try taking more shots (to get longer trails) and increase the ISO (to capture more stars.)  I used these settings for my second shot:  18mm, 29 sec, f/3.5, ISO 200 and I took 239 consecutive shots, capturing almost two hours of star trails.  After stacking the images, I noticed the problem of airplane trails all over my photos:

18mm, 29 sec, f/3.5, ISO 200, 239 consecutive shots

Compared to the previous night, I took these photos much earlier: 9-11pm.  This resulted in many more airplanes flying through my frame.  To be exact, 70 of the 239 photos had airplane trails in them, and some of the planes took three photos (a minute and a half) to get through my frame.  I tried simply removing the 70 frames from my stacked photo, but as you can see, that just resulted in broken star trails and a horrible Morse code effect:

70 (out of 240) photos removed due to airplane trails

I finally went through the tedious process of cleaning out only the airplane trails from each of the 70 affected photos using the clone stamp tool in Photoshop.  It took a couple of hours, but eventually I had a great star trails photo:

Airplane trails edited out of 70 photos using Photoshop

My favorite part of this photo is a shooting star that I captured.  If you find the highest point of the left tree, and move up the image you can see the shooting star as a small diagonal trail.  It was easy to tell that shooting star apart from the airplane trails, since it started and stopped over only an inch of my image, rather than travel through the entire image.

Conclusions:  After trying a shot simply leaving the shutter open for 30 minutes, and trying the method of stacking consecutive shorter exposure shots, I would highly recommend the stacking method.  There was much less digital noise and I was able to capture many more stars due to the wider aperture.  I also liked the look of increasing the ISO to 200 for my second evening's shot.  The increased sensor sensitivity allowed me to capture more stars and also gave the sky a slightly bluer look to it, rather than dark black.  In the future I look forward to taking more star trails photos.  I hope to try taking four hours worth of shots and try something a bit more exciting in the foreground, like a building.