Startrails

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Having expended significant energy to prevent the apparent movement of the stars in the picture, we now aim to utilize them as an actively designed element of the image. As we have learned, the stars already begin to leave a trace of their apparent circular motion after six to eight seconds of exposure time, although this trace is not yet visible to us. Therefore, all we need to do is choose a section of the sky that is well-interspersed with visible stars, then take a long exposure of 30 to 180 minutes (the longer, the more complete the image of the circular motion), using an open aperture of 2.0 or 2.8 on low-sensitivity material (ISO 50 or 100). With higher-sensitivity material, feel free to stop down a stop or two. The natural offset of light points ensures a well-filled frame, and as long as it remains completely dark, overexposure is not a concern. Since the startrails are usually a rounding, decorative element of the shot, a wide-angle focal length between 18 and 35 mm will usually be used, which leaves enough room for foreground elements and also sharpens them with its own relatively large depth of field. Naturally, you should set the lens to infinity. To prevent blackouts, the camera should either mechanically operate the shutter for extended exposure times, depending on the situation, or utilize a battery-saving „T“ setting. Digital camera models are much more critical in this regard, by the way, because their image sensors are real energy guzzlers and consume more the longer the exposure. Use the mirror lock option on your camera to avoid vibrations when it flips up during exposure. That covers the fundamentals, but as usual, we want to delve deeper into specifics.

Exposure and technique

To first determine how long we need to keep the shutter open to achieve a certain length of star trail, we can consult a formula that again applies to stars at the celestial horizon, i.e., a declination of 0°, where the motion and resulting arc are greatest:

Formula 3

For other declinations, multiply the targel length and divide the exposure time by the following cosines of the corresponding angles:

Angle	Factor
10°	0,98
20°	0,93
30°	0,86
40°	0,75
50°	0,64
60°	0,50
70°	0,34
80°	0,18
85°	0,10

As we already know from the previous section, the aperture setting determines the brightness of the stars, the width of each individual line trace, and the color of the sky. Large apertures (1.8 to 3.5) result in a trace with good width but rather soft edge and a more blue or, near larger cities, also an orange-yellow sky. Medium apertures (4.0 and 5.6) result in less width but a sharper rendition of the star trail and provide an overall quite dark sky.

The color and brightness of the sky lead us at this point to an issue equally relevant to all of our nighttime work: light pollution. A multitude of different light sources accompany every urban activity today. Their rays like a diffuse bell of fog over our cities, sometimes illuminating the night sky beyond recognition. This light pollution poses a significant threat to our photography, as it amplifies the brightness of dimly lit objects in the sky, diminishes contrast, and produces unsightly color casts due to the Schwarzschild effect of analog film. Since we can hardly flip the main switch of a big city to ensure good shooting conditions, we have to choose another avoidance strategy. The simplest strategy is to find a shooting position that is reasonably far from the nearest city, work during a time close to the new moon, or aim the camera at a patch of sky where the moon won’t appear (depending on the time of year and the moon’s phase, which rises between the northeast and the southeast and sets between the northwest and the southwest). This is because moonlight illuminates the sky. However, if the chosen subject necessitates a location near civilization, we must utilize technology to stop the aperture down or shorten the exposure, which may compromise the clarity of the stars and their trails. The most uncompromising solution is to use a so-called broadband Light Pollution Reduction (LPR) filter from observational astronomy. Such filters, though very expensive, block the light from conventional mercury and sodium vapor lamps, which as street lights are responsible for the bulk of urban light pollution. Specialist dealers like Astrocom GmbH (www.astrocom.de) or Lumicon (https://www.lumiconinc.com/) sell them.

In addition to the man-made light pollution described above, we also have to deal with a natural variation, the so-called “sky glow”.This is due to the fact that the atoms in the denser lower air layers are excited by sunlight during the day and release the energy absorbed in this way during the night in the form of visible light. We can also limit the interference from sky glow with special SkyGlow filters.

However, the night should always meet one basic requirement: in any case, the sky should be starry, because passing clouds cause holes in the star fields and the star arcs.

Another problem with long exposure night photography is condensation of dew on the equipment, especially on the front lens of the optics, as the equipment releases heat into the surrounding air. However, a fogged lens will no longer pick up sharp points or arcs of light. At best, it will produce washed-out images. To prevent condensation, we can remove and wipe off the UV filter screwed on just one turn in between, but this usually involves shaking the camera, or we can prevent it by taking advantage of physics. Since the emitted heat tends to rise upward, we must cover the recording device from above. This can be done either with a dew cap (a long and very wide tube made of cardboard, styrofoam or other insulating material, again available from telescope dealers), which under unfavorable circumstances can shade the field of view of the lens, or (don’t laugh now!) by putting up an umbrella and holding it over the camera and providing a certain amount of air circulation. For those who prefer a more technical approach, there are also electric heating systems available. The Kendrick company (http://www.kendrickastro.com/astro/index.html), for example, makes such systems as accessories for telescopes, but I would only use them on purely manual cameras since the weak voltage applied could possibly interfere with the on-board electronics of digital cameras.

Not all arcs are the same – appearance and design of the startrails

The excursions into astronomy necessary to round off the topic have already made clear to us that the stars north of the celestial equator orbit around the celestial north pole and those south of the celestial equator orbit around the celestial south pole and do this in opposite directions, i.e., from east to west or west to east. Thus, a north-facing image in the northern hemisphere shows counterclockwise rotating upward or n-shaped startrails around the celestial north pole, and a south-facing one consequently shows clockwise rotating downward arcs that write the „n“ in the other direction. If the view goes to the west or east, the orbits of the stars present themselves as more or less flat, straight lines. The closer they are to the celestial equator, the straighter or less curved they are. If you aim the camera to the northeast or northwest so that you don’t have the celestial north pole in the picture, you will get start trails in the shape of a „u,“ whose open part points in the direction of the north pole. The same is true for the southeast and southwest, except that the aperture points to the south pole. In order to capture images of star trails rotating around both the North and South Poles, where the arcs are open to the left and right and separated by straight lines, it is necessary to be close to the equator, ideally within 30° to 40° north/south latitude, and to aim the camera with a wide-angle lens at the celestial equator. But because a picture is worth a thousand words, we summarize this section again in the form of the following graphs.

To visualize the directions of movement for the latitude of your location, you can create a „sky strip“ by panning the camera from the northern horizon over the zenith to the southern horizon at the distance of individual pictures with identically long exposure times, and then mount the pictures to each other.

We have frequently discussed the celestial poles, but I have not yet disclosed their exact location. I want to make up for that now. The celestial north pole, around which the stars visible from the northern hemisphere seem to turn left, is marked in our age almost exactly by the polar star (Polaris or North Star) and this is the bright end star at the drawbar of the constellation Little Dipper, which is often also called Little Bear (Ursa Minor). We can best locate it by orienting ourselves toward the more conspicuous Big Dipper, also known as Ursa Major. Fortunately, both are circumpolar, meaning they remain visible in the sky year-round in northern latitudes without setting. The upward open side of the chariot always points in the direction of Polaris, and the fivefold extension of the imaginary connecting line between the two rear bright chariot stars gives pretty much the distance. A little help for finding it in the seasons (the earth circles the sun and changes its position in relation to the stars): In spring the Big Dipper is upside down above Polaris; in summer it is to the left of it with its drawbar pointing upwards; in autumn it is below Polaris, this time the right way around; and in winter it is to the right of it with its drawbar pointing downwards. Moreover, the constellations not only depict the apparent seasonal shift in position caused by the earth’s rotation but also the continuous rotational motion that guides us, ensuring that the Big Dipper passes through these positions once every night.

Unfortunately, the celestial south pole is not so easy to find because it is not marked by any prominent stars, and of course it can only be recognized from the southern hemisphere. Nevertheless, there are three helps: the Southern Cross in the bright band of the Milky Way, the Small Magellanic Cloud (KMW) and Canopus, the second brightest star in the southern sky. To find the pole in the middle of the dark part of the sky surrounding it, we extend the vertical bar of the cross a good five times, forming an imaginary line to the KMW. The imaginary perpendicular from Canopus to this straight line then intersects at the position of the South Pole. In other words, the pole is a good three fist widths from the Southern Cross, about one and a half from the KMW, and four fist widths from Canopus. Of course, this constellation shares the same alignment and rotation as the previously mentioned ones, but because it populates the southern sky, it rotates to the right.

And something else is important for finding the poles: They are always at a height above the horizon, which corresponds to the latitude of the observation position. So near the equator this is low, and if we point the camera at a pole and take a long exposure, the image will show a large number of semicircular startrails grouped quite flat and almost vertically around this central point. Further away from the equator, in Central Europe or in Australia, for example, the arcs give a really circular impression.

To locate the aforementioned constellations, astronomical novices can use a rotatable star chart, and for dry runs on the PC at home, I recommend the free program AstroViewer (www.astroviewer.de).

Creative approaches and a bit of cheating

Since no foreground object can be correctly exposed under the technical conditions of our night photography, we have to reach into the bag of tricks from time to time to be creative with our possibilities. Let’s see what we can do.

Underexposed black silhouettes, the more jagged the better, are a good way to add tension to the foreground of a star or startrail shot. A range of hills or trees, a prominent building such as an observatory, or perhaps the skyline of a major city are all possibilities. Even the just-recognizable rock formation in front of the dimly red illuminated evening sky is a grateful motif. Particularly for startrails, a larger water surface is an excellent choice for reflecting the star arcs on a windless night. Unfortunately, however, the reflection is already lost with the advent of any light breeze.

If, on the other hand, we want the foreground object to be clearly visible against the night sky, we basically have three options. First, we can insert it in a double exposure; second, it can be „sandwiched“ or digitally mounted from a separate exposure; and third, it can be „lined up“ with additional light. Option one: To do this, orient the exposure of the first shot to the brightest point just above the horizon and underexpose by one step. As soon as the stars are visible, take the second exposure as a long one. Option two: In the good old days of pure analog photography, two images were „sandwiched“ to insert something. That is, two separate images were taken from identical positions. One during the day with a recognizable foreground and a sky overexposed by a good two stops so that it no longer showed any recognizable details, and another at night showing the starry sky of whatever kind. Both images were then superimposed in a frame and copied together, or sandwiched. Today’s digital Photoshop era eliminates the need to manipulate real photographs, yet the process stays largely unchanged. Option three: You can brighten up smaller subjects with the low beams of a car turned on for a minute or two, or with the beam of a good flashlight, which is easier to sweep across the subject. For a more extended foreground, on the other hand, the flash unit is a good choice, fired several times from different positions at 100% power and five to ten meters away to provide soft natural lighting. In any case, experimentation is a must!

Next Let’s let geometry and the big plan work for us

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