You're about to become better at post-processing! Raiatea Arcuri, a landscape photographer from Hawaii, has an impressive portfolio. I was pleased to learn that he also shares some of his secrets to processing his landscapes. Arcuri teaches you how to process a stack of images shot at night to create a wonderful star trail nightscape using Lightroom and Photoshop, and I will share some additional tips to help you achieve stunning star trails.
1. Interval Timer
Arcuri set the camera on interval timer to effectively capture a time lapse. After the initial processing of one of the images in Lightroom, you can simply synchronize the images and have the settings from that image applied to all of the images in the stack. Once you have them exported and opened up in Photoshop, it's a good idea to clone out the trails of both airplanes and satellites, as they'll take away from the central composition Arcuri shows us.
The "lighten" blending mode in Photoshop works by showing pixels which are lighter in the selected layer than in the layer below. If the pixels in the selected layer are darker, they are replaced with pixels on the layers below. So, darker pixels will become transparent to the selected layer. Note that this effect is applied to each of the 3 channels in RGB colour-mode. Small star trails from each subsequent 30-second exposure will add to the total length of the apparent star trail. The more exposures you have, the longer the trail.
3. Thermal Noise
Why wouldn't you just take out most of the processing effort, set up a tent and comfy outdoor sleeping arrangements, and expose just one image for a couple of hours? The sensor of your camera will be constantly on during the entire length of the exposure, so there would be an electric current flowing through the sensor for the better part of the night. This current generates heat through resistance of the electronics. During long exposures, this heat affects the amount of electrons collected in the sensor. All the electrons that do not contribute to the actual signal of what you're imaging create a false signal. This is called the "thermal dark current," or "thermal noise" for short. In your image, this kind of noise will show up as hot pixels: red, blue, or green specks that seem randomly placed. While a 30-second exposure in a modern digital camera may have one or two such artifacts, the effect is cumulative. That means that a longer exposure will add more thermal noise. In-camera noise reduction will not be an option here, because this will lock up the camera for another couple of hours. It also doesn't work that well with exposures reaching one or several hours.
4. We Need More Power
Several hours of exposure need several sources of power or a constant power source. A battery grip will help you out here, but I like to use an uninterruptable power supply (UPS) for my D750. I can then swap batteries or attach a 27,000 mAh power bank that will keep me going through the night and at least the sunrise after that.
5. Round She Goes
By day, the sun rises in the east and sets in the west. But the sun progresses through the south in the northern hemisphere, while she progresses through the north in the southern hemisphere. At night, a similar difference is noticeable, but before I point you to Polaris or Crux, I'll teach you something that makes you either very popular at birthday parties or the outcast of the social group, depending on the audience.
You might have learned in school that the Earth spins on an axis we call the geographic pole. One spin around this axis takes about 24 hours to complete. Yes, about. Our 24-hour clock isn't that precise. One sidereal rotation period is actually 23h 56m 4.1s long.
In slightly more than a year (it's why we have complex leap years), we complete one lap around the sun.
Precession of the Equinoxes
Now, the fun part. In roughly 26.000 years, the Earth completes a gravity-induced wobble of the axis of rotation. Think of this a spinning top that wobbles along the floor or table. Now, think of the part of the top pointing up as the geographic north pole. That north pole is pointing to some part of the ceiling, but as the wobble goes on, it constantly points to a different part. Now, think of the ceiling as the night sky. Let's talk astrology. Yes, the
pseudo-science study of "the movements and relative positions of celestial objects as a means for divining information about human affairs and terrestrial events." Ok, I got that from Wikipedia. But if you read further in that entry, you’ll learn that there are several kinds of astrology. Western astrology, with which we are most familiar, is based upon divining a horoscope for an exact moment, such as a person’s birth. But the tropical zodiac, from which horoscopes are derived, doesn’t account for this precession.
The dates of the ‘star signs’ were fixed over 2,000 years ago, when the zodiac was first devised. Then, they corresponded to the constellation of stars that appeared behind the Sun on the day you were born. But an astronomical phenomenon known as 'precession' means that the constellations have drifted since then. So, which stars were you actually born under?
The classical argument in defense of astrology would be: the position of the sun in the sign of the zodiac on the date on which you were born only accounts for 1/12th of the total chart. Well, yes. But that 1/12th might be wrong, as your true sign may not be what you think it is. Take the test at the BBC and find out.
6. Alignment and Composition
For star trail photography, this means that we have to find the alignment of the geographic pole with the corresponding celestial pole. In the northern hemisphere, it’s rather easy for these kind of wide-field nightscapes if you know where to find the Plough (Big Dipper). Just follow an imaginary line from the two stars at the end of the pan, Merak through Dubhe, and extend that line until you notice a lone star in the north. That point of light is actually the combined light of at least two stars in a binary system called Polaris. Today, the celestial north pole is so close to Polaris, that its precision (not precession) doesn’t matter. As long as you know that the stars seemingly rotate around Polaris, you can place this system accordingly in your frame to create a pleasing composition.
In the southern hemisphere, we’ve got the Crux (Southern Cross) constellation. It’s a little more complicated than saying it’s in the middle of the cross. Follow the longer axis that makes up the cross from top (Gacrux) to bottom (Acrux). Strike an imaginary line through Gacrux and Acrux and follow that 3 1/2 times the distance between those stars. Directly below that point is south, near a dim star called Sigma Octantis. But be creative with any or all placement of the celestial poles in your composition. You can always point your wide-angle anywhere and witness the celestial splendor with your own eyes as the stars streak by. Be sure to share your pics on Fstoppers!
Check out the video by Raiatea Arcuri; it will be worth your time because you're about to become better at post-processing your nightscape shots! More of his stunning work can be admired on his website.
Images used with permission of Raiatea Arcuri.