A work in progress. These are notes:
Preparing to photograph star trails:
When: cloudless, moonless night, 90 minutes before/after sundown/sunrise.
Where: generally away from light pollution with interesting foregrounds
Gear:
Camera and charged batteries
Tripod
External Shutter Release
Compass or phone app such as Google Sky Map
Headlamp with a red LED
Flashlight - to paint foreground if desired
Dress accordingly to weather, be prepared.
On cold, damp night if your lens fogs up: try using anti-fog cloth on lens hand warmers wrapped around the lens miniature fan blowing on lens
Focus:
Manual focus on infinity.
Take a test shot to make sure your stars are sharp.
Aperture & Lens :
Ideally, use a fast lens and large aperture.
Largest aperture (smallest number) or not more than f- 5.6 is often recommended.
Lens Recommendation by Evan Sharboneau Copyright 2011
If you are using a 17mm lens, you will have to have your camera taking exposures for 4.5 hours to get a star trail that starts at one edge of the frame and ends at the other. If you are using a longer lens like a 480mm, it will take around 18 minutes to render an edge-to-edge star trail. 30mm lens would be about 48 minutes. I personally usually go as wide as I can when I'm doing star trails.
White Balance:
For file types other than RAW, Tungsten renders the sky blue, Shade will produce r a warm red / orange color. Anything between the two will render the sky purple.
Some photographers recommend avoiding auto-white balance.

A work in progress. These are notes:
File types:
RAW if possible, unless you are planning to use third party stacking software.
See the software guidelines for file formats.
Exposure, Time & ISO:
Single Long Exposure:
Calculating ISO for a Single long exposure: take one 30 second exposure at ISO
1600. If it looks good, multiply 30 X 16, or for ISO 3200, 30 x 32, and so on. Then change to ISO 100 (or 200 will be a little brighter). Some photographers use higher ISO. If you do you will have more noise.
Single long exposure: make a Dark Frame Exposure to subtract in Photoshop during post processing. Put on lens cap, cover viewfinder for the same exposure time.
Stacked exposures of 30 seconds:
Camera on Continuous-shooting mode - (sometimes called burst mode)
Change to Manual and set shutter speed to 30 sec.
Lock the shutter using a remote shutter release cable
Note: If your camera doesn't accept any remote shutter releases, you will have to wrap some rubber bands around the shutter button and wedge a small wad of paper or some other small object between the rubber band and the shutter button.
Advantages of multiple exposures: (according to Evan Sharboneau)
This simple method of using a cable release or rubber band is superior against the old fashioned single exposure method because:
•Taking a bunch of 30 second exposures and then combining them later on the computer will result in less noise in your photograph compared to taking a single 20+ minute single exposure.
•You don't have to worry about the camera running out of battery power and ending up with no shot like you would if you were taking the entire star trail photo in one exposure.
•You don't have to worry about calculating the exposure value. This method gives you freedom to take a few shots of the foreground being light painted and exposed properly, then move on to taking the star trails sequence.
•You don't have to worry about something bright going across the frame and ruining your photo such as car headlights beaming directly into the camera lens. If this happens, just delete the frame.

A work in progress. These are notes:
•You can make animations of the star trails moving across the sky in postproduction. Can't do this using a single exposure.
•You don't need to use an intervalometer (an intervalometer is used for taking shots at set timed intervals) that wouldn't do a good job of taking a star trails photo anyway. Some intervalometers require that the interval be set to 33 seconds instead of something like 31 or 30. This will produce a bunch of gaps in your star trail, rendering the trail more like a dotted line. Putting your camera on continuousshooting mode and then using an external shutter release cable or rubber bands ensures that you are capturing each photo with the shortest delay time possible.
This last comment above is disputed.
Combining Multiple Star Trail Exposures - Stacking Software
Do an internet search or check out my Web Resource page - at this time they are untested.
If you want to do this in Photoshop instead, there is a special Photoshop Action available.
You can also do this in Photoshop by going to File > Script > Load Files Into
Stack and then click OK. Wait for all the images to load, and then select all the layers and change the blending mode to Lighten. Try experimenting with different blending modes. Keep in mind that doing this in Photoshop might not come out as well if there is a ton of light pollution in the atmosphere. You might need to adjust the curves and make all the images darker in order for it to work.
For Subtracting Dark Frame exposure:
Use Dark Frame as a layer above your star trails image and change the blending mode of the dark frame to ‘difference to remove the noise for a cleaner final image.
Where to aim your camera (from our location in Northern Hemisphere):
Circular star pattern - aim at the North Star
Horizontal trials - aim South
Curved trails - aim East or West with extreme wide-angle lens
Turn the camera noise reduction OFF:
Takes a long time for the camera's processor to remove the noise and it can't take a picture during that time. Plus, it drains battery life.
Lens Choices:
If you are using a 17mm lens, you will have to have your camera taking exposures for 4.5 hours to get a star trail that starts at one edge of the frame and ends at the

A work in progress. These are notes: other. If you are using a longer lens like a 480mm, it will take around 18 minutes to render an edge-to-edge star trail. 30mm lens would be about 48 minutes. I personally usually go as wide as I can when I'm doing star trails.