Wednesday, October 21, 2009

Quincy Stars - Take #2

Circular star trails with the Quincy Number 2 shaft house in the center.
Another shot at star trails at the Quincy Mine.

After my first try at star trails over the Quincy Mine, I learned many things about taking long nighttime exposures of the sky. Last Saturday, I headed back up to Quincy to put those lessons into action. In this post, I'll analyze the results, figure out what worked, what didn't, and list some more lessons learned.

Technical Details: This photo was made by "stacking" 144 separate exposures, each taken with these parameters:
  • Focal length: 10mm, on my shiny new Nikon 10-24mm
  • Shutter speed: 30 seconds
  • Aperture: f/3.5 (which was the maximum aperture available)
  • ISO: 400
I took the photos using gphoto2 (installation and details), a PTP camera controller program, installed on my MacBook Pro. The laptop controlled the camera via a USB connection. I used the gphoto2 command:
    gphoto2 --capture-image-and-download -I 34
This instructs the camera to take a photo every 34 seconds. Why 34 seconds? Interestingly, this is actually the ideal spot for my computer and camera combination to take photos at the correct rate. There is a bit of a delay when the computer instructs the camera to take the first photo. After that, gphoto2 keeps track of how far behind schedule it is -- and tries to catch up. The result is that the computer instructs the camera to take a photo after about a 1 second delay, just enough to let the sensor cool down.

Improvements and Problems: The purpose of trying this same photo shoot again was to learn from my mistakes last time. Here's a rundown of what I like and dislike about this photo.
  • Batteries and time:In many ways, this photo shoot (and the resulting star trails photo) came out much better than the last try. The major change is that my batteries -- both camera and laptop -- were fully charged. This allowed me to get much more than the mere 20 minutes of exposures which I had last time. This time, I got around 72 minutes, leading to much longer star trails.
  • gphoto Timing: Also, as mentioned above, I significantly improved the timing of the image-taking. In my previous attempt, the camera eventually got out of control (perhaps because of a "backlog" of "take a photo now!" commands), and kept taking photos without the computer being able to download them. The result was "stutters" in the star trails, as I had to reset the camera.
  • Composition and star trails: The main thing I don't like in this photo is the angle. While the shaft-rockhouse is quite dramatic, the stars are moving in circles -- much less interesting than the big ovals in the original photo. In addition, the wider angle of the star trails in the original emphasized the movement of the stars and made them appear longer.
  • Inability to take more than 30 second exposures: I still haven't found a way to both control my camera via my laptop, and take photos longer than 30 seconds. No camera control software -- whether gphoto2, Nikon Camera Control Pro, or any others -- will let me operate the camera in "bulb" mode, which allows exposures longer than 30 seconds. The result is that I have to shoot at a very wide aperture (f/3.5) which reduces depth of field, and ISO 400 which adds a lot of noise. Anyone knowing of a reasonably cheap way to take time-lapse photos with a shutter speed of longer than 30 seconds, please let me know!
  • Colors and clouds: Finally, I don't like the colors as much in this photo: the moon was new, resulting in no reflections on the shafthouse and less light in the sky. As a result, the colors of the shaft house were mostly reflections of cars and streetlights, which resulted in much less pleasing colors. In addition, clouds running through the sky resulted in brown smears in the stacked photo -- be careful to find a truly clear night!

Lessons Learned: Here's a big summary for the aspiring star-trail photographer:
  • In gphoto2, use an interval slightly longer than your exposure. This lets the camera keep up with the computer's commands, and also lets the sensor cool (avoiding weird "purple clouds" in photos.

  • Charge your batteries! ... duh.

  • Use Incandescent white balance. Unless you feel like processing 100+ RAW files, you'll need to choose a good white balance ahead of time. Incandescent white balance makes the sky appear bluer and more natural.

  • Shoot when the moon is moderately full. This helps highlight objects in the photo and adds more light in the sky, helping get more natural photos with better light balance.

  • Get there early, set up, and focus. Another big problem was that I waited until the sun was mostly down to set up my rig. As a result, focusing was difficult (and sharpness wasn't helped by the large aperture). I also had a hard time composing properly, since the viewfinder was much too dark to see through. Get to your location early, set up your equipment, focus and compose. Then set everything to manual mode and leave it until you're ready to start.

If you have any questions, feel free to leave a comment!


Anonymous said...

what is the meaning of life?

Jay Balliet said...

I don't know, David. I really like the angle on this. Maybe try it again when the moon is out.

Also, wouldn't the ambient light of the moon blow out your pictures with a 30sec. exposure?

DC said...

@Anon: 42
@Jay: Hmm... well, I can see your point on the angle. I'll try it with a fuller moon. I'm going to try some totally different buildings this weekend, if the sky clears!

Also, my previous time-lapse (Take #1) was done during a full moon. It was much brighter, but nowhere near that bright. It wasn't in the frame, just providing reflected light on the structure.

Charles said...

Does your camera take a remote trigger? I just set my d300 up with the trigger locked and walk away. :)