I'm a big believer in practicing techniques before actually attempting them on location. In fact, I've been accused of spending too much time practicing, and not enough time with other aspects of the business. Sometimes you get lucky without practice, as I did with the Matterhorn pano from the previous blog posts, but in general, trying a new technique on location usually leads to small errors you don't realize until after you get home from the shoot.
I've always wanted to attempt a star trails shot over a famous landscape/monument, so I did some research on how to take a star trails picture. For those who aren't familiar with star trails, there's an example picture at the end of the post, but in essence, because of the earth's rotation, as time passes, the stars will appear to move in the night sky. Capturing this movement is what creates the trails in the sky.
There are two ways to do this - you could take one long picture, as a single exposure will capture the stars as they move. There is one big problem with this though - the longer you leave your camera shutter open, the more light that gets in, and as such, you can only have a relatively short exposure, otherwise your picture will be blown out. Why is this a problem? The shorter the exposure, the less movement you'll get from the stars.
The second method is generally the more accepted one of capturing star trails. You take a bunch of pictures consecutively for a certain amount of time (ie you can take 360 shots, with each one lasting 30 seconds, giving you 3 hours worth of star movement). You then take all the pictures into a program specifically designed for creating star trails (I used a program called StarStaX, which can be found at http://www.markus-enzweiler.de/software/software.html), and the program will stack all the images together, thus creating the trails in the sky.
That's the first picture I took. I outlined the Big Dipper in yellow, for reasons that will become obvious in the next picture, which is the final picture I took in the stack.
The first and last pictures were taken about 90 minutes apart, and you can see the movement of the stars in the night sky over that period of time, as evidenced by the Big Dipper. In total I took around 180 pictures, with each picture lasting 30 seconds. The second picture in the stack would have the stars slightly shifted from the first picture; the third picture would have the stars slightly shifted from the second picture, and so on. I took all the pictures into StarStaX, and it merged all 180 of them into the final star trails picture that can be seen below.
And that's basically how you create a star trails picture. It's amazing that Van Gogh was able to visualize this without any other aids when he created "Starry Night". In all honesty, it's a fairly simple technique, but the practice allowed me to become familiar with my camera remote, and the importance of keeping all the settings the same for each shot. The one thing I didn't account for was the possibility of airplanes affecting the final result, which can be seen in the above picture as the diagonal line, but there's not much you can do about planes flying by other than removing the line they create in Photoshop afterwards.
Anyhow, I felt pretty confident about trying to capture star trails in a more picturesque setting, and I'll talk about that in the next blog post =)