Firstly what are Star Trails - in essence they are streaks of light left behind on the sensor or film from the stars as they travel across the sky in front of an open camera shutter. In fact, it only seems this way - in reality the stars are stationary and it's the rotation of the earth that makes them appear to streak and spin.
... OK first the end result in all it's glory... would love any feedback/suggestions/etc...
Technical Details:
Nikon D7000 + Tokina 12-24mm f/4.0 Lens @ 12 mm
Twilight Image: 58 seconds at f/11, ISO 100
Star Trails Images: 210 x 60 second exposures (3.5 hours) at f/4.0, ISO 1250
For good star trails you need a couple environmental conditions:
- clear skies (clouds obscure the stars)
- minimal light pollution (lights from the ground shining up into the sky washing out all but the brightest stars - cities are bad)
- minimal moon (anything above about a quarter moon will both light the sky and obscure the stars (and overexpose the frame if it includes the moon))
As luck would have it the first night we were on the farm met these conditions - which was lucky since the nights which followed did not (very very cloudy).
We'd been driving for half the day to get there and we arrived late in the afternoon and by the time we got settled I only had a very short time to find a suitable location close to the farmhouse (within a few minutes walk) and setup my tripod before sunset.
I knew if I wanted to have the trails appear as a circular pattern around a central point in the sky I'd need to point roughly south (actually they spin around the Southern Celestial Pole (in the southern hemisphere)). So I found this old dead tree which would provide my foreground interest. Then I fired up an IOS (iPhone/iPad) app called SkySafari on my iPad so I could locate where Sigma Octantis (the closest thing to the southern pole star) was going to be in the sky and arranged the composition appropriately.
What do you need to take star trails:
- a camera (with the ability to specify manual settings and ideally with the ability to select a bulb mode for long exposures)
- a sturdy tripod
- a remote cable release (with an intervalometer function)
- you may want to keep the lens warm and protected to prevent dew formation (lens hood and I used some woollen socks wrapped around the lens)
- enough battery power (single large battery, spare batteries or battery grip) to last the desired number of frames (remember that batteries last less in the cold)
- a large enough memory card to hold all the images
- software to stack the resulting images (Photoshop, Startrails.de, DeepSkyStacker, StarStax, etc)
What is the technique/camera settings:
- lock your camera down on a tripod (I used a sand bag hanging from the centre column hook to weigh it down)
- set manual focus for the scene (on the foreground interest or about a third the way into the scene)
- take a suitable twilight image (this will form the basis of the non sky components of your final image) using normal landscape settings (aperture stopped down, low ISO, grad filters, etc)
- do not move the camera - change to star capturing settings:
- do NOT change focus
- put the camera in bulb mode
- wide open aperture (in my case it was f/4)
- higher ISO (I used ISO 1250) - aperture and ISO determine exposure for stars only
- daylight white balance
- select an appropriate shutter speed (in my case it was 60 seconds but something in the range of 30 to 120 seconds is likely the go) - shutter controls the ambient exposure - don't overexpose the sky or any light pollution
- setup the intervalometer to take the number of continuous frames you need (in my case 60 second exposures with a one second interval between them)
- if you want lots of circles in the image you'll need at least two and a half hours worth of images (in my case I had three and a half hours worth)
- start the sequence once the sky is completely dark
Once I'd started the star captures I actually left the camera in the paddock and went back to the farmhouse to wait it out in more comfort. My only concern was that some of the bulls might come over to where I'd put the tripod and knock it or worse still trample the camera, but the camera gods must have been smiling on me that night and all was good when I checked after the first hour and a half and then again at the end after three and a half hours.
What I hadn't realised at the time was the town over the hills in the distance which was almost under the celestial pole which polluted the sky just above the horizon and also the highway over the ridge line in the distance which every now and then had headlights pointing my way - this all required a bit of post processing to fix (which I had to learn
). Still I was very happy with the end result for my first attempt.Also after I'd finished shooting the star trail interval shots I took some dark frame images (which I didn't end up using) and I also took some single shot images of the stars in the sky.
To do this you basically just frame a composition pointing the camera up into the sky and use camera settings like:
- wide open aperture (in my case it was f/4)
- higher ISO (I used ISO 2500) - enough to expose the stars
- select an appropriate shutter speed - in this case you want a shutter speed slow enough to not have motion blur of the stars moving in the sky - the value is actually a factor of
- lens focal length (the longer the focal length the shorter the shutter needs to be since the relative motion of the stars is increased)
- direction you're pointing at (the stars "move" slower closer to the celestial pole and faster further away)
- in my case I chose 25 seconds (but 15-30 seconds would be a good starting point)
