Tips and Pointers
For those wishing to try this technique, it is very helpful to use some sort of inter-valometer or timing circuit to automate the exposure times and intervals so you can leave the camera and not have to attend to each exposure.
It’s also important to keep pauses between exposures to a minimum (pauses will mean gaps in your star trails). With the Nikon D1x fitted with a 14-millimeter f/2.8 Nikkor lens, we were able to start the next exposure within about 1 to 2 seconds of the previous image–any longer than this and we would see a noticeable gap between exposures, even with such a short-focus lens (see page 130). Most of our experiments were done centered on the equatorial region of the sky; the exposure gaps, however, were minimized when we shot circumpolar star fields.
Digital cameras differ in their ability to take long exposures, but detector noise is an issue with all digital cameras once exposures go beyond a second or two. To overcome this, some cameras have an automatic “noise-reduction” option, which might help, but could be unreliable depending upon the exposure time. For star-trail exposures I suggest disabling any of the camera’s built-in noise-reduction features and taking a dark frame. It’s a good idea to take a dark frame at the start and end of each session since the noise characteristics of a CCD or CMOS chip can change with temperature.
Use flash-memory or microdrive cards with the largest capacity that you can afford. Some cameras will allow you to switch cards while an exposure is being taken (but not while it is saving an image, of course) so you can keep a sequence going without missing a frame to change cards.
Experiment with different exposure times, white-balance and ISO settings, lens f/stops, and so forth. Each camera’s silicon detector is different, so plan on doing some preliminary tests first to find the best combination.
When you’re taking images, it’s always best to capture them in a noncompressed format such as TIFF. Make sure that the dark frames used have the same image format.
If you have a portable power supply, use it. Digital cameras require a lot of power when you’re using the bulb (time-exposure) setting, and, as the ambient temperature drops, the camera battery’s performance will also suffer.
Autofocus digital SLRs might have trouble focusing at infinity, so set the focus manually. If you use a manual-focus camera, notice where its infinity setting is located, as it’s often just inside the lens’s focus stop. Do some tests first to find the best focus.
With our setup, we obtained optimal results with 45- to 60-second exposures, but we also obtained good results with exposures up to 2 minutes long. Our 60-second shots recorded stars down to about 7th magnitude.
Once you have obtained all your exposures, transfer them to a computer with image-processing software. I use Photoshop 7.0 on a Macintosh G4 running under OS X, but I suspect other programs would work. The following instructions will be for Photoshop users:
- Open the first image of your sequence and save it under a new name that you want to use for the final star-trail image.
- Open the next image in your sequence. Use the Select > All command and copy this image (Edit > Copy).
- Paste the copied image onto the first image–this is the beginning of the “stacking” process. You can now close the image that was copied to avoid confusion since you’re now done with that image.
- Open the Layers palette (Window > Show Layers) and select the new layer created in the previous step.
- Click on the Blend Mode menu in the Layers palette and select the Lighten mode. You should now see both images merged together as one.
- Repeat steps 2 to 5 until you’re done with all images. You will probably want to flatten (under the Layer menu bar) your star-trail images periodically to save on disk space and memory in your computer. When you flatten an image all of your existing layers are merged into one and the file size becomes much smaller.
- Once you’ve stacked and flattened all your images into a single one, you need to subtract the dark frame. To do this simply open the dark-frame image, click on Select > All, then copy the dark frame and paste it onto the star-trail image. At this point the image will go dark, so make sure that the new dark-frame layer is selected. Go to the Blend Mode menu again in the Layers palette and select the Difference mode. Now most of the noise on your image should be minimized, and you can flatten the image again and make final adjustments to it. Save the final image and you’re done!
Steps 2 to 6 can be tedious if you have a lot of images to stack. Photoshop has a very nice automation feature that can make processes like these much easier and quicker, so check your manual for detailed instructions on how to use it.
Depending on your camera’s resolution, lens, and sky location, if you look closely you might notice small gaps between your stacked images. The reason for this is that at the end of each exposure, the pixels that are being exposed are not (on average) getting the full duration of exposure. Then, when the next exposure begins, these same pixels are still not getting the full exposure. Since the Lighten blending mode is not additive, when you stack the images the brightest single pixel is selected in the stack and, on average, at each gap it’s only 50 percent as bright as the adjacent area.
Shooting at less than full resolution (by binning, or combining, the pixels) will often cause the gaps to close up. Also, if you shoot the polar regions (or use a very short lens), the problem is not as evident since the overlap is so great.
If you find the gaps unacceptable, there are several Photoshop techniques that you can use to correct them. The technique I’ve found to be most effective is to simply duplicate the completed star-trail image, copy and paste it onto the original with Lighten mode, and shift and/or rotate it slightly to fill in the gaps. However, for viewing on a computer screen and for making small to medium prints, these tiny gaps should not be objectionable or, in most cases, even noticeable.
I’d be interested to hear from readers who might come up with a more clever (and more elegant) solution on how to minimize exposure gaps.
I also discovered in my tests that if you take the images (and the dark frame) without using the camera’s “sharpening” feature, the dark frame subtracts out much cleaner. If you use sharpening you will notice an annoying black ring around bright pixels that goes away when you turn off sharpening. However, if you don’t use internal sharpening in the camera, you might find it necessary to do it with Photoshop (under Filter) once the final image is stacked and flattened.
Our experiments were all done with a fairly high-end Nikon digital SLR camera, but I suspect that most other professional digital SLRs could be made to work as well. Overall, I’ve been impressed with the power of digital photography, achieving results that I had previously thought were possible only with film. While our technique does require fairly good equipment and significantly more effort in the processing stage, the results are nothing short of spectacular, and I look forward to seeing what others can accomplish using this technique.