My location for the photos below was the Laguna Mountains in San Diego county - at an elevation of something like 1600m to 2000m the haze in the atmosphere is less of an issue, and the spot is remote enough so that there's not that much light pollution from civilization. The nights out there were warm and pleasant and I found a nice and dark spot above the Big Laguna meadows (for the locals: where the Big Laguna Trail connects the Noble Canyon and Sunset Trail).
The Milky Way above Laguna Meadows (NIKON D700, 15s @ ISO 3200; f/4, 16 mm (in 35mm)
I found that these stills are actually fairly easy. I used a wide angle lens because I wanted to have a lot of sky in the frame. Using a wide angle lens also has the advantage that exposure times of 15, maybe 20 seconds are relatively safe (no motion blur of the stars will occur yet).
Addendum: just after I posted the initial version of this article, a similar discussion came up on Google+ and it included a "600 rule" for the exposure time. Basically, it says that 600 divided by your focal length is the maximum time you can expose without getting motion blur of the stars. For a 50mm lens, this would be 600/50 = 12 seconds. From my personal experience, that's too long. I have some 10 seconds exposures made with a 50mm lens and they already show motion blur. I'd make that a "400 rule" to be absolutely safe.
Also, remember the crop factor of your camera when you use that rule. With a 50mm on a DX sensor Nikon, it'll be like "600/(50*1.5)" = 8 seconds. (or with the "safe" approach of a "400 rule", just 5.3 seconds)
The real problem is of course capturing enough light within the maximum exposure time frame - and my f/4 wide angle zoom (the 16-35mm VR Nikkor) is probably a bit slow for that. The above photo was made while there was still some light left in the sky; for subsequent shots I had to raise the ISO to 6400, which of course introduces quite some amount of noise in the darker areas. Nikon's 14-24mm/2.8 (for full frame) or Tokina's 11-16mm/2.8 (for crop sensors) wide angle zooms are better lenses for that purpose. A 24mm/1.4 prime on a full frame camera of course will be fantastic, giving another 2 stops advantage - it comes at a price of course. ;)
With a single photo at a high sensitivity and relatively long exposure time, hot pixels are not that much of a problem, but things became quite different when I went for interval shooting to create a time lapse two nights later. Before I go into details, here's the resulting movie - please "click through" to the Vimeo site, click on "HD", switch to full screen, and depending on your screen size, turn the "scaling" option on or off:
Jeff Sullivan has just posted a nice general sum-up of creating time lapse movies on his blog so I just put down my observations from the night-sky time lapse making here.
- It's important to know where exactly the "infinity" focus is for the lens. As I already mentioned in my long-exposures post, most lenses focus "beyond infinity" (Buzz Lightyear would like it, I guess) - which will turn the stars from bright little spots into tiny little discs. Looks bad, not advisable. I also want to stress that it's important to check the focus twice - once when you set up the camera and frame the scene, and again just before you start the interval mode. It could happen that you accidentally touch the focus ring, and bam. I'm just saying. :P And bring a flashlight to check the distance scale of your lens in the dark.
- Operating mode of the camera. This should be manual for everything once you're done setting up the camera. This includes exposure (time, aperture, ISO) and focus as well as the White Balance. It's absolutely crucial to turn auto white balance OFF and set it manually. For a nice cool blue of the night sky, it's advisable to set it to Tungsten, or Fluorescent, or somewhere in between (less of a problem when using raw data, but with the amount of data to expect, I chose JPEG - see below.)
- Make sure the battery is really fully charged, or even better, use an additional battery grip. A 2 hour interval with 5 photos per minute drained my what-I-thought-pretty-full (yes, my bad) battery completely. At that time, clouds had moved in on the scene so it was not too bad as I had to end the session anyway, but for really long time lapses it will be a show stopper - there's just no way to swap the battery without changing the framing ever so lightly when the camera is on the tripod. Had I used long exposure noise reduction (see below) the battery would have been drained even faster I think.
- File format. I went for medium-sized JPEG files instead of raw data. After all, I wanted to create a relatively low-res movie out of the files. A medium-size JPEG of the D700 is still 6 megapixels (something like 3000x2000), in other words almost twice as high a vertical resolution as required for a "full HD" (1080) movie.
- The interval time. From a TV documentary I had remembered that the guy was making one photo every 12 seconds. With my relatively slow wide angle lens, that meant I could make a 10 second exposure at ISO 6400 - but couldn't use the long exposure noise reduction then. It turned out that one image every 12 seconds is too much, anyway. If I'd ever do that again, I'd probably go for one image every 20 seconds. Naturally, a long time-lapse movie will take even longer to create that way. On the other hand... the "majestic slow movement" of the night sky (the movie above runs at 20 frames per second) is a pretty nice feat of 5 images per minute.
- my tight interval timing and the lack of long exposure noise reduction of course caused a lot of hot pixels to appear. For the first time, I found out what the "sync" feature of Lightroom's Develop module is good for. :) I simply went to the last image of the sequence, removed all the hot pixels, and synced the setting over the entire sequence. In the above video, I have removed the red and white hot pixels. If you look closely, you'll still see quite some blue hot pixels (those are pretty hard to spot because they don't stand out so much in the stills).
- One extremely annoying problem that I ran into where the slightly varying exposure times even while using the lens wide open. Every now and then, a couple of photos in the sequence would be about 1/3 of a stop darker than it's neighbors. Everything was set to manual, so I have no explanation what might be causing that. Jeff's article that I linked above mentions a plugin for Virtual Dub that can average the exposures. I created my time lapse movie with Google's free Picasa (the "movie presentation" feature has a time-lapse mode that can create movies from 6 frames per second all the way up to 30 frames per second).
One interesting final bit: with the free software "StarStax" it's possible to stack a sequence of images, creating a star trail photo from the stills of the time lapse movie. Here's what it looks like, with all the ugliness of passing airplanes combined:
In the end... was it worth it? Oh yes. Being out in the dark at a location with little to no light pollution is an amazing experience all by itself. While the camera clicked along I was lying in grass, my head resting on my backpack, and simply gazed at the stars. After a while, the eyes adapt to the darkness, and it's quite surprising how bright a moonless but clear night sky can be.