Star Trails, Digital Style; Exposure And Stacking Techniques:
Page 2 Shoot as far from city lights as possible, avoid traffic, and avoid curious strangers with titanic flashlights. Once, near the end of a painstaking three-hour exposure, a nearby camper ruined it by coming over with a flashlight the size of a 10-gallon drum. In spite of my frantic signaling, he pointed his mini-klieg light down the barrel of my lens and asked, innocently, “Hey, watcha up to?” Most city light grids use orange sodium vapor lights that give the sky a muddy red glow as far as 50 miles away. For this, there is no cure but color temperature sliders in raw conversion, or color adjustment in Photoshop.

For All Layers

This is the total of nine exposures, blended by the stacking technique in Photoshop. Throughout the shot, I was using a red LED flashlight to accumulate illumination for the foreground. I was also walking through the house with a flashlight, shining it into the rooms for 20 or 30 seconds at a time. Most of the light in the living room was provided by the 7w bulb in my aquarium.

Let’s think about composition for a moment. Star trail shots can be like shots of the Grand Canyon. Glorious, magnificent; but so what? It’s been done before. What can you do to make this something fresh and interesting? Think about foreground elements. Find unusual ways to frame the sky, like shooting through, under, over, or around natural formations like rocks, trees, mountains, and lakes. Do some “painting with light,” discreetly illuminating the foreground with 20-second bursts of flashlight, or colored light. Don’t lock yourself into imaging the circle that rounds Polaris. Aim at different segments of the sky to compose shots with star trails arching over scenic elements.

Now we’re ready to go. You’ve framed your shots, camera’s mounted and focused, aperture’s set, shutter’s on Bulb. Maybe you’ve even got one of those precious labor-saving devices, a timer switch, otherwise known as an intervalometer. You’ve got a plan: 10 shots of 6 minutes each. Now you can start your exposures.

Wait, there is one more thing! Did I mention that exposures have to be very close together? A second or two between exposures is okay. Anything longer will show up as gaps in the trails. Most D-SLR buffers can handle this load. If you’re using a digicam, find out your buffer’s capacity and shot to shot recycle time before going after a night of star trail shooting. After a certain number of shots the camera can lock up while it writes to the memory card. You could shoot a lot of very short exposures, say, 60 shots at 90 seconds each. The stacking can become tedious, so I want to steer you toward an ingenious shareware program called Image Stacker, available at: www.tawbaware.com.

Final Screen Capture

Here is a screen capture showing nine layers that comprised almost an hour’s exposure time. Raw color temperature adjustment was used throughout to compensate for the orange glare of a sodium light grid that begins 10 miles from my house.

Time To Stack
I shoot raw and do the stacking in Photoshop. This gives me maximum control, enabling me to use various tools such as the Opacity slider to even out differences in brightness of the various images. After converting the raw images, with color temperature corrections, I make a folder of TIFF files.

The stack procedure is simplicity itself. Keep the first exposure as the base exposure. Each image must be placed in the order it was taken. Take the second exposure, go to Select All (Ctrl +A). Then, holding down the Shift key, use the Move tool (V) to place the second exposure on the first. All that’s left is to go to the Layers palette and change the Blending mode to Lighten. Holding the Shift key ensures that the photos are aligned. As you repeat this operation, you will see the trails accumulate.

Each time you change the Blending mode to Lighten, the lengthening trails pop into view. The file size can grow quite large, so it’s wise to duplicate the image at various stages and go to Layers>Flatten Image. If you want to preserve an original document with every layer intact, save the final version as a PSD or a TIFF.

Now that you have stacked your images you can do a little color balance and a little sharpening; season to taste.

To see more astronomy and Photoshop-inspired sky landscapes, visit my website at: www.artsdigitalphoto.com.