Capturing star trails with a digital camera

Today’s digital SLR cameras and image-processing software are breathing new life into an old film-based technique.

Like many amateur astrophotographers who grew up using photographic film, I’ve been a bit reluctant to embrace the virtues of CCD imaging as a complete replacement for film. Sure, thermoelectrically cooled CCD cameras on telescopes play an important role at all levels of astronomy, but I’ve stubbornly (perhaps even a bit irrationally) held to the belief that some images can be obtained only by using good old-fashioned photographic emulsions.

One realm where I thought film would always reign supreme is in capturing star trails. One might ask why anyone would even consider taking something as simple and straightforward as star trails and adding the cost and complexity of digital photography to the mix. As I have recently discovered, however, it’s definitely worth the effort. The results presented in this article demonstrate the advantages of recording star trails digitally and show how this new realm of possibilities can breathe new life and creativity into your tired old star-trail photos.

Although I have to admit that working for Gemini Observatory with telescopes on Mauna Kea in Hawaii and Cerro Pachon in Chile gives me regular access to nearly perfect skies as backdrops, the technique described here can be adapted to urban sites as well. In fact, the improvements are even more dramatic in situations where light pollution or bright moonlight would prohibit shooting long-exposure star trails with film.

A Serendipitous Discovery

It all began one chilly night in early 2003 at the Gemini North Telescope, where two other astrophotographers and I were enjoying an evening of digital imaging. Our main purpose was to set up our Nikon D1x digital SLR (single-lens reflex) camera and to experiment with the possibility of creating a time-lapse movie of stars passing over Gemini’s dome from a sequence of 1-minute stills. Armed with an electronic timer, a portable power supply, cold-weather gear, and lots of hot chocolate, we began shooting images.

As often seems to happen when one experiments with new ideas and equipment, serendipity provided us with an opportunity to learn something new. In this case, the microdrive card that we were using could hold only 1 gigabyte of data. (A microdrive card is a miniature hard-disk drive that fits into the camera’s memory-card slot.) Every hour or so we bundled up and left the comfort of the observatory’s warm control room to stumble around in the darkness to replace the card. With time on our hands between downloading the cards’ contents, we experimented with the images we had obtained.

First we opened the individual images with Adobe Photoshop software and created an automated routine to subtract (remove) the electronic noise generated by the camera’s CCD detector using a dark frame. (A dark frame is an image of the same exposure duration and camera settings as the desired image, only with the camera lens covered.) This is done with Photoshop’s Difference blending mode, with the dark frame in one layer and the desired image in another. Each of these processed images was then flattened (combined) and saved and archived as a separate file on a CD.

After verifying that our image sequence would make a nice time-lapse movie, we began experimenting with a “stack” of images in Photoshop to see what other interesting things we could do. Inspiration struck one of our team members, Kevin Jones, when he began playing with the different blending-mode options used to combine the image layers. It turns out that the Lighten mode takes the selected image layer (the foreground) and compares its pixel values (in each color channel) with the underlying layer. If the foreground’s pixel values are the same as (or less than) the underlying layer, it leaves the resulting pixels unchanged. However, if the foreground’s values are higher (brighter), the Lighten mode uses the higher value of the two pixels in the composite. On the surface, this might not sound very profound, but the effect is that the sky brightness never increases beyond that present in the brightest single image. Meanwhile, the star images themselves continue to build up as the stars drift across the camera’s field.

The more images you add to the stack, the longer the resulting star trails. Since the sky brightness stays at a constant low level, the visual depth and contrast of a digital star-trail shot is more dramatic compared to a traditional long-exposure film photograph. (In the latter, the sky brightness accumulates during the entire exposure, reducing contrast between the stars and the background sky;)

Another advantage of the digital technique is that any foreground object that is illuminated will not become overexposed, as it would in a long-duration film exposure. This is especially important when you are attempting star trails with well lit foregrounds or under bright moonlight. Light pollution is also suppressed, allowing for dramatic star trails in relatively well lit areas that would otherwise quickly saturate film.

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. 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.

Image Processing

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:

1. 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.

2. Open the next image in your sequence. Use the Select > All command and copy this image (Edit > Copy).

3. 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.

4. Open the Layers palette (Window > Show Layers) and select the new layer created in the previous step.

5. 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.

6. 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.

7. 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.

Exposure Gaps

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.

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