Star Trails

How to photograph Star Trails

Star trail of Polaris, the North star.

I find the night sky fascinating and spend a good amount of time each year observing it. After reading an article on star trails, I decided to try photographing them. This is not only fun, but goes along with my amateur astronomy hobby and does not require all the special tracking equipment of regular astrophotography. I already had all of the items I needed (except the stacking software) to do this correctly because of my photography hobby. Their is no end to the creativity that you can accomplish, from simple star trails around the North Star to including star trails within a foreground scene. There are many good tutorials on how to do this (Google or Bing – star trails). I will concentrate on what I do and let you develop your own creative techniques from there. Below is a list of things you will need to start.

 

  • DSLR or mirrorless camera with manual mode for ISO, Aperture and Shutter Speed.
  • Tripod or way to hold the camera study horizontally or vertically for long periods.
  • Cable release or an Intervalometer or way to hold the Shutter Button down.
  • StarStaX software or equivalent to stack a group of images into one photograph.

What kind of camera works best? If you have a DSLR (digital single lens reflex) or MILC (mirrorless interchangeable lens camera), you’re probably set. I use a Nikon D80 or D5100 DSLR, but some point and shoot camera’s also have a manual mode which will cover the ISO, Aperture and Shutter Speed and may work ok for star trails. Most people like to use 30 seconds for each exposure when stacking, so see if your camera can do that in M mode. Start with your widest angle lens and the aperture in Star trail camera with intervalometer, battery grip and L-bracket.the wide open setting. If you can get down to f/2 or lower, than you can use ISO’s in the 100 or 200 range (don’t use auto iso), thus lowering the chance of any noise. Turn off long exposure noise reduction and also turn off image preview on the LCD screen. Your camera manual is the best guide for all this information. While you’re at it, look up frame limit (buffer capacity) and see how many frames you can shoot with the shutter pressed down. Mine is 100, other camera’s may be different. At 30 seconds, 100 images equals 50 minutes of time. I adjust my camera quality setting for JPEG, fine, small. This way the pictures are only about 600 kb’s each. By using fine/small, I get the highest quality JPEG at a 1936 x 1296 pixel size. The problem with this of course is if you are going to print a large image of it, you need to go with a larger pixel size and probably larger than a 10 megapixel camera. Many camera’s today are 20 megapixel’s or above.

What other things do I need? I use either a wired cable release with a hold button on it or an intervalometer. Many of the newer camera’s have built in intervalometers (interval timers, see below) and you may be able to use that, but keep in mind that your goal is a minimum delay between shots. This is important as you move further away from the North stars toward 0 declination (celestial equator) in the sky. The stars move faster, often giving a dotted line look to the star trail. Think of the earth as a spinning top. At the equator, stars appear to travel a lot faster than near the North or South poles. When you measure the circumference of a top, it may only be 1″ near the poles, but 12″ at the equator. Bottom line is, if your photographing star trails near the North Star you normally will have nice solid trails, if near the celestial equator you may see dotted line trails. This also becomes more important as you use a longer (telephoto) lens and less important as you use a shorter (wide angle) lens. I also use an L-bracket for quick change from horizontal to vertical pictures.

How about my camera’s frame limit? You have probably already noticed my camera can only take a 50 minute set at 30 seconds per shot (100 shots). I have a work around for taking longer sets. After 45 minutes, I simply unlock and re-lock the hold button on my remote and it re-sets for another 100 shots. A simple digital kitchen timer reminds me when to do this. I can keep doing this until the battery goes dead (about 240 images). I’ve added a battery grip that holds two batteries, so I get twice the battery life (about 4 hours, 480 images).

With an intervalometer, you do not need to worry about this. I simply set the shot for 30 seconds with a 1 second delay for a total of 31 seconds for each picture and set the shutter speed to bulb. You can also set the number of pictures you want to take. Remember, the longer the exposure time, the longer the star trail. The further from the North Star, the longer the star trail. The longer the lens, the longer the star trail. You have lots of ways to be creative.

What if I have an interval timer (intervalometer) in my camera? My Nikon D5100 has one and here is how I set it up. Other Nikon cameras should be similar.

Camera preset settings:

  1. Image Quality = Jpeg Fine (choose small for web, large for prints)
  2. Shutter Speed = 30 seconds
  3. Aperture = Wide Open (lowest f number)
  4. ISO = 100 or higher (start at 100 for f/1.8 or 500 for f3.5)
  5. Mode dial set to M (manual mode)
  6. Lens focus set to M (manual focusing) (pre-focus before dark on infinity)

Setting up the interval timer. Select interval timer shooting from the Shooting Menu (last item listed), follow these 4 steps:

  1. Choose start time = Now.
  2. Interval = 33″ This is a 30 second exposure + 3 seconds required for processing.
  3. Number of times = 109 for 1 hour, 218 for 2 hours, 327 for 3  hours, etc.
  4. Start = On then push OK.  Camera will start imaging in about 3 seconds.

You should get a 1, 2 or 3 hour star trail with only slightly more width between each image of the trail because of the three second delay between shots. With a wide angle lens they will not be visible unless you enlarge them to 100%. This is a simple way to do star trails without a cable remote or intervalometer hanging from your camera. Then just run the images through StarStaX and you’re done.

What about focusing and dew? I like to focus ahead of time on a far away mountain, light or a bright star if it’s dark. Most camera’s will not auto focus at night, so once you have focus lock, switch to manual focus and leave it alone. I can’t over emphasize how important it is to have a sharp focus during the star trail picture sequence. Be careful that you do not move the focus ring during setup and make sure the tripod is on solid ground. Setting up on a wood deck or unstable areas can cause movement as you walk or the wind blows. On nights that dew or humidity are a problem, attach a chemical hand warmer (hot-pak) around the lens hood. These usually last for 4 or 5 hours and put out enough heat to keep the lens element clear and dry. I use a special sock my wife made and wrap it around the lens. Do this gently so you don’t disturb the camera’s focus or final check the focus on a bright star before starting your star trail.

How do I know when it’s totally dark? The best way is to use the U. S. Naval Observatory data services. Pick the table for Twilight Times and put in your State and City. Be sure to specify the type of table that you want. If you want a totally dark sky with lots of stars, use the table for Astronomical Twilight. If you want a dark blue sky with less stars, use Nautical Twilight. You can experiment with starting up to 15 minutes before nautical twilight for a lighter shade of blue and dimmer star trails, but more detail in foreground images. I print out both tables for the year and have them handy. Believe me, this is very useful information to have to help you become more creative. Also print out the Phases of the Moon chart while your at the USNO site. Moon glow can play havoc with your dark sky plans. Best times for evening star trails are about 10 days before new Moon to a couple days after. Remember the darker the sky, the more stars you see. So, a trip to the mountains can be fun and rewarding.

What about airplanes passing overhead? If you happen to setup near the flight path of commercial airlines, you could be in for a long night. Their powerful strobe lights can ruin a star trail picture quickly. Your best bet is to adjust your field of view of the area you plan to photograph, so as not to include commercial airplane flight paths. One nice thing is most commercial planes fly on these designated flight routes and you just need to determine where they are once you have picked your location and not include them in your star trail. You will still sometimes pick up small private planes that don’t use these flight paths

Star trail with airplanes and the International Space Station.What if I think airplanes are neat? You may find airplane trails add to your composition of the picture, and if so, just point your camera at a commercial flight path. Many excellent pictures have been taken of car lights streaming down a road or freeway, so why not airplane trails in the sky? This picture is of the International Space Station (lower) and eight airplane (upper) trails, along with the bright stars in the handle of the big dipper. There’s some pretty neat stuff in the night sky.

What else can I photograph besides airplanes? Try to get a meteor in the picture (this is shear luck) or set up your camera to capture a star trail with the International Space Station making a pass through it. Click here to see when it makes a pass over your area. The best time to try and photograph a meteor is during one of the annual meteor showers, but it’s still not a given that you will capture one. Your best chance is by using a wide angle lens. You can also make a comet trail, but it should be naked eye visible to be bright enough to make a good trail. Adding landscapes or foreground objects to your star trail is a fantastic way of being creative and the options here are endless. Keep in mind you are shooting with a wide open aperture and shallow depth of field focused at infinity, so foreground objects need to be far enough away to be in focus, otherwise use them as a dark silhouette. Beware of trees or objects moving in the breeze.

How do I process the pictures after I take them? When taking the pictures, I use the best quality and smallest size JPEG the camera will do if for web use. If your plan to print them to a large size, you will need to use the largest JPEG available or RAW. If using StarStaX, the RAW images will need to be converted to JPEG before stacking the images. I use StarStaX (free) software to stack the images and save as a TIF. This is your master image, so make backups of it for safe keeping. You can then get rid of all the small JPEG’s you took and work from the TIF. Use your favorite post processing software to crop and re-size the image to your needs. Irfanview (free) is a good one if you don’t have one. I like to add some contrast to darken the sky and brighten the star trails. You can also brighten or darken the image to suite your taste. Then add text if you want and save it as a JPEG. Remember the TIF is your master or digital negative, so you can always come back to it in the future. That’s all there is to it.

Things to think about – Have everything ready before you setup your tripod and have a plan ahead of time on how you want to compose your picture. Will it be a vertical or horizontal image? What lens will you use, wide angle or telephoto, what ISO and f stop? Figure this all out ahead of time, unless you want over or under exposed images and the field of view is to large or to small for what you had in mind. Nothing is worse than trying to fumble around in the dark trying to get your gear level and ready to shoot and the camera adjusted. Plan everything in advance from a walk around the area if you’re going to include some of the local landscape to composition of the picture itself. Take some test shots if needed in the daytime for composition or at night for correct exposure and focus at a 30 second shutter speed, before starting a 3 hour star trail.

Photographing star trails is fun and the compositions you can come up with are limited only by your imagination and camera. It also gets me out to the telescope to observe while I’m waiting for the timer to run down, which is a good thing. If you haven’t done so, visit my Cowiche Astronomer webpage and enjoy the night sky – Bruce Perrault

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To view the star trail gallery, click on a thumbnail to view larger images and descriptions of each star trail. Click or press the right or left arrows to navigate images. Hit Esc or click the image to close it and return to thumbnails.