They were combined from layers in Photoshop with 'lighten' blend mode between them
The process if fairly simple.
1. Setup camera on tripod.
2. Search for the polar north to point the camera at if you want to get concentric the circles. I used the star walk app on the iPhone and searched for Polaris.
3. Compose the shot. I used a torch to illuminate the trees in order to see them through the viewfinder and get them framed properly. This example is a pretty lame composition but it was in a controlled environment (my garden!)
4. Turn IS off on your lens if it has it. You are on a tripod, IS could only interfere with the result.
5. Set your camera's exposure setting to 'bulb' mode.
6. Choose your focal length, aperture and ISO. I went as wide as possible with the maximum aperture available in this example. Since I used a Canon EF-S 10-22mm I went with 10mm wide, f3.5 on and I choose ISO 200 to keep noise down.
7. Set the lens focus to infinity or try focusing on a strong star. You don't want the focus to change from here on so now turn AF off.
8. Take a test shot with your settings at the expected exposure time in bulb mode. Examine the shot o make sure you can see all the stars you can at least see with your own eyes. With the settings above I went for a 2 minute exposure.
9. Shoot the required set/number of shots. I took 20 x 2 minute exposures in this example. You can do this with a cable/manual shutter release if you have the patience. I used a electronic Intervalometer/timer to do the donkey work for me (a wireless Hahnel Giga T Pro II). There are wired options on the bay for around £20.
10. Import the shots into lightroom. Open them in Photoshop as layers. Change blending mode in each of the layers but the last one to 'lighten'. There is free software out there to do this too but I haven't tried them. There are also addons for Photoshop to automate this task.
11. Voila! I saved the composed as a TIF file and adjusted brightness / contrast, added a bit of clarity and detail in Lightroom. I also adjusted the temperature towards blue since there was a lot of orange light from the street on the other side of the house in the shot.
What I learnt:
- If you are trying for the first time. take 2 / 3 exposures, load them up on the computer and make sure they are good. Play with your settings and when you are happy write them down.
- Check and double check your settings when you come to shoot. It's going to be long time to run this so make sure everything is right.
- I had a UV filter / protector on my lens. At about 1 hour in, the temperature must have changed as this started to fog the inside of the filter and the lens. I guess the camera could also be warming up, don't know but the fog ruined my last 7 exposures. No loss but next time I'll do this with no filter attached.
