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.

What every musician should know about photography?

So your band is recording an album, and you’re starting to think about how you’re going to promote it to the industry and public. One of the key items you need to consider is decent photography. Musicians need photography to use on their CD cover/liner notes, and possibly more important, for promotion purposes. This photo is a representation of the artist or the band, making it necessary to put a lot of thought and creativity into it. Unfortunately, hiring a professional may not come cheap.

While speaking to four photographers from Halifax, Montreal, Toronto and Vancouver the prices vary for bandphotography, but the advice is similar.

When choosing a photographer, it is necessary to research what the photographer has done in the past and choose one that fits your budget. Bands that are signed under a major label have their photo shoots paid for by the label, enabling them a larger budget to work with. Independent bands have a more difficult time. When finding different prices for your band photography, make sure you know what is included in all the packages. Ask what it will cost for additional supplies such as contact sheets, film, processing, final prints, etc. Ask a photographer to explain clearly any procedures you don’t understand. As well, make sure you ask about photo credit. Many photographers keep the rights to the photos and want to be credited every time it is used. Like the rest of the music industry there is negotiation involved resulting in some form of an agreement.

Don Bird, a music consultant from Bird’s Word Productions Ltd., in Toronto, ON, suggests that if a band can’t afford a first rate photographer, then they should go to some of the art colleges and schools because there will be students who will have fresh ideas music wise.

John Leighton a photographer in Halifax, charges $100 an hour plus supplies. Leighton prefers that when a band comes to him for an album shot or promo picture, they have pre-planned what they want as a band. They should have an idea of what will work for them and decide on one member of the band to discuss with him what they want. “When I have six different band members coming to me and telling me different things, it gets frustrating.”

Johanne Mernmercier of Montreal, PQ. says, “I listen to the music to see what kind of mood they want. Sometimes they want photos from their live shows, in the studio or just outside. We just try to have fun. I think the artists should do research to see what has been done and find pictures of what they like.” Mernmercier charges $1,600-$1,800 for a one-day photo shoot. This does not include make-up artists, wardrobe or supplies. For a black and white Press Kit photo, she charges $300 and it belongs to the artist to do what he or she wants with it.

According to Jim Dawson of Fotowork in Toronto, ON, “if the portfolio looks good and the photographer is above board in explaining your options and his/her charges, then it’s a go.” Dawson’s prices are in the hundreds, “it all depends on the package,” he said. “Most clients are more interested in who you’ve photographed, more than what you’ve done with the people you’ve shot.” Musicians should focus more on the photographer’s style than who they’ve photographed before. Just because a photographer has shot some famous musicians doesn’t mean they’re going to be great for you or your band.

Photographer Ashely Maile from Vancouver, BC says, “I will cut a deal with an independent band and I hope they will help me along the way. It has happened that bands I have photographed, that are now signed, do help me out. My rates are less expensive for an indie band because they don’t have a lot of money and are not backed by a label.”

The band should be telling the photographer what they would be using the picture for. This will help the photographer understand what kind of photo is needed. “If you are looking for an album cover or design, then you should go for something darker, but if you’re just need an 8×10 glossy photo, you want it to be bright so it can reproduce really well. You have to picture it being reproduced in a little community newspaper or something like that,” says Maile.

150 years of photography

It is a curious but tidy coincidence that the two earliest surviving photographic images – the saintly relics of the medium – were both taken through a window. In 1826, the inventor Joseph-Nicephore Niepce pointed a primitive camera out of the dormer of his house at Gras, France. It took eight hours to record, on a polished pewter plate coated with asphalt and lavender oil, a crude schematic view of Niepce’s backyard. But the result is the unmistakable forerunner of the delicate, silvery daguerreotype, the invention of Niepce’s collaborator, Louis-Jacques Daguerre. Nine years after Niepce’s experiment, a patrician Englishman named William Henry Fox Talbot aimed a tiny 2.5-inch square camera at a latticed window of his family seat, Lacock Abbey, in Wiltshire. He succeeded in making a paper negative on which it was possible, with the aid of a magnifying lens, to count 200 panes of glass. There was the birth of photography as it is known today.

Joseph Nicéphore Niépce

For both men, the window was probably little more than a convenience, the closest source of light to a makeshift laboratory. But in a way, the inventors’ subjects proved oddly prophetic. In no time at all, photography itself came to be viewed as a kind of window, a transparent medium whose subject matter was the world itself. The first newspaper report on Daguerre’s invention seized on the fact that the photograph differed in some profound way from the work of visual artists. As La Gazette de France proclaimed in 1839, “Let not the draftsman and the painter despair: M. Daguerre’s results are something else from their work, and in many cases cannot replace it.” Quite what constitutes that “something else” is still being debated, although an astute definition was provided as early as 1857 by the Victorian writer Lady Elizabeth Eastlake. Photography, she wrote, is “that new form of communication between man and man – neither letter, message, or picture.”

The most important invention of the 19th century may have been the idea of the invention itself. Even so, there was something momentous about the introduction of photography. The Paris crowd that waited 150 years ago, on Aug. 19, 1839, outside a joint meeting of France’s Academy of Sciences and the Academy of Fine Arts, where Daguerre’s process was revealed, was tense with excitement. The promise of the new medium was that for the first time in human history, visual information could be conveyed without having to rely on handmade marks. And although the Victorians could not be aware of it, photography changed man’s relationship to the fleeting and the ephemeral.Photography succeeded in freezing the present tense forever.

The perceptive Lady Eastlake was quick to understand many of the new medium’s characteristics, especially its sheer ubiquitousness. Even by 1857, photography had become, as she put it, “a household word and a household want; it is used alike by art and science, by love, business and justice, is found in the pocket of the detective, in the cell of the convict and on the cold brave breast of the battlefield.” She also understood, with great clarity, the profoundly time-bound nature of the photograph, the way it bears the impress of a particular hour or age – as well as the way it can capture a telling detail. Photographs of children, she wrote, may fail on the level of art, “yet minor things – the very shoes of one, the inseparable toy of the other – are given with a strength of identity which art does not even seek.” As for the vexing question of whether photography was an art, she found the new medium wanting – although it could relieve artists from the humbler tasks of depiction, such as the miniature portrait.

The problem of photography’s status as art – a recurring, even tiresome, question in the medium’s history – seems never to be entirely laid to rest. In the beginning, the audience was prepared to marvel at the sheer fidelity of a daguerreotype – to count window panes or chimneys – and to leave it at that. (Such a sense of open-mouthed wonder can still be observed now in those who encounter for the first time the eerie, spectral presence of a hologram, a flat but three-dimensional image.) In the 19th century, a few who worked self-consciously as artists stand out – a photographer like the Englishwoman Julia Margaret Cameron, whose monumental close-up portraits flouted the polite, standoffish conventions of the day. But in general, the century may be said to belong to photographers of a more utilitarian cast – to men who, like Carleton Watkins or William Henry Jackson, recorded the opening up of the pristine landscape of the American West, or to that talented group of camera operators who, during the American Civil War, worked under the name of their taskmaster, Mathew Brady.

In the 20th century, the achievement of art photography is less equivocal. By 1939 – the centenary of the medium –photography had a rudimentary history and the sense of a usable tradition. There were already bodies of work that were distinguished not only by a certain visual grace and intelligence, but also by the light that they cast on their own civilization. In the 1920s, the Cologne photographer August Sander embarked on a vast collective portrait of the German people that to this day remains a model of sociological shrewdness and psychological insight. His 1928 Boxers is an ironic contrast to Lewis Foote’s 1927 image of the Prince of Wales and his brother Prince George in Winnipeg’s railway station. And it is hard to think of 1930s America without recalling the lyrical, emblematic images of Walker Evans and his colleagues who worked for the federal Farm Security Administration.

There was a period – most notably in the 1940s and 1950s – when photographers who considered themselves artists had access to the great mass-circulation magazines. That era reached some sort of climax – and perhaps came to an end – with the famous 1955 Family of Man exhibition organized by Edward Steichen of New York City’s Museum ofModern Art. The show was an enormous success with a worldwide audience, although even then there were many photographers who felt that Steichen’s editorial call for a brotherhood of man colored the work included in the exhibition. Certainly a younger generation of artist/photographers today regards the humanistic pieties of The Family of Man as threadbare, if not thoroughly suspect.

Today, a certain knowing skepticism reigns in contemporary photography. Indeed, it seems at times as if the “real” world has been used up, and all that remains is a vast netherworld of images. In vanguard circles, the dominant tendency is less to make new photographs than to use appropriate existing ones in an effort to grapple with a society that appears to be terminally saturated with images. The other route for photographers who want to be noticed in an increasingly frantic art world is to see how far they can push back the borders of the acceptable.

No one better exemplified that trend than the American Robert Mapplethorpe, who died earlier this year of AIDS, and who was, without any doubt, the most visible photographer of the decade. His subject matter ranged from impeccable still lifes of flowers to scenes that, in their polymorphous perversity, seem to have been taken from the last five minutes of the Roman Empire. Photography, which most people once thought of as a neutral window on the world, now appears more like the distorting mirror of our own perceptions. It may even be a distant early-warning system, telling us that the age of innocence is finally and irrevocably over.