Updated: Apr 8
Modern DSLRs and advanced mirrorless cameras are so good in low light that they can make images of the Milky Way from a stationary tripod that show stars as points (well, almost as points). Film cameras and early DSLRs required such long exposures that Earth’s rotation turned the stars into obvious streaks. Despite recent advances in technology, however, shooting the Milky Way with a stationary camera still requires using a high ISO. Even with today’s best cameras, using a high ISO generates noise, reduces resolution, and compresses dynamic range (the difference in brightness between the darkest detailed shadow and the brightest detailed highlight). And while the stars may appear to be round in a JPEG sized for the web, close examination of a big print reveals that the stars are actually rendered as short streaks.
For decades, astronomers have used equatorial mounts that moved their telescopes in time with the stars, counter-acting the Earth’s rotation so that the stars were rendered as points. Until fairly recently, however, equatorial mounts were big, heavy, expensive, and designed to work with telescopes. Few landscape photographers used them. Now the explosion of interest in wide-field landscape photography at night has led several companies to make relatively small, lightweight, and affordable star-tracking devices designed for use with DSLRs.
As part of my research for my book Dusk to Dawn: a Guide to Landscape Photography at Night (Rocky Nook, spring 2018), I bought a compact equatorial mount from iOptron called the SkyTracker Pro. (Writing a book is always a great excuse to buy new toys…) The mount has two parts, the battery-driven, motorized tracking device itself and the alt-azimuth base, a piece of hardware that allows you to make precise adjustments of the altitude (orientation in the vertical plane) and azimuth (orientation in the horizontal plane) of the tracking device.
An iOptron SkyTracker Pro and counterweight, mounted on a Really Right Stuff BH-55 ballhead. An Acratech Ultimate Ballhead supports a Canon 5D Mark IV and Canon 16-35mm f/2.8 L III lens.
Theoretically, you could mount the tracking device directly to your tripod head with a standard ¼-20 mounting screw. As a practical matter, however, using the alt-azimuth base makes setup much easier. Equatorial mounts can counteract the movement of the stars only if one axis of the mount is aligned with the axis of Earth’s rotation—in other words, if the axis of the mount points to Polaris, the North Star. Without the alt-azimuth base, you must perform a precise adjustment of the SkyTracker Pro’s orientation using only the controls on your tripod head. Achieving that level of precision is quite difficult. The alt-azimuth base simplifies that task. In addition, the base includes a bubble level. The altitude of Polaris is always equal to your latitude. A topographic map or a smart-phone app like Sun Surveyor will give you the latitude of your current position. Leveling the base, then setting the altitude of the SkyTracker Pro to your latitude is a big help in the most difficult part of using an equatorial mount: polar alignment.
The SkyTracker Pro comes with a small polar scope. Inside the polar scope is a reticle, a series of concentric circles marked with hours like an old-fashioned clock face. Aligning the SkyTracker Pro with Earth’s axis requires locating Polaris and positioning it correctly within the field of view of the polar scope. Polaris is not located precisely on Earth’s axis of rotation. For best accuracy, you need to position Polaris on one of the reticle’s concentric circles. The exact position depends on the time of night and time of year. A phone app called PolarFinder or a cheat sheet available from iOptron will give you that information.
The mount itself, with its alt-azimuth base, weighs just 2 lbs. 11.5 oz., but that isn’t the full weight of the kit in your pack. For starters, you’ll need a second tripod head. The Acratech Ultimate Ballhead I use weighs about a pound. If you’re using a DSLR, you’ll also need the counterweight kit, another 4 lbs. 12.5 oz. The total weight for my kit is 8 lbs. 9.5 oz. Throw in some kind of case and the heavy-duty tripod you’ll need to support all that weight, and it’s clear you’re not going to be carrying the SkyTracker Pro too far from the road.
Although the counterweight kit is essential, the correct position for the counterweight along the shaft is not obvious. One solution to finding the correct position is to remove the mounting block from the tracker, assemble the remaining parts, and move the counterweight back and forth until the assembly balances on a pen set atop a 2x4 to keep the counterweight from resting on the table. Measure the position of the counterweight in relation to the end of the counterweight shaft, write the position on a cheat sheet, and stash the sheet in your camera bag. This is obviously a procedure best done at home. Fortunately, it only needs to be done once so long as the camera is pointing in roughly the same direction each time you set it up. That's likely to be true if you are shooting single-camera-position images of the Milky Way, since the galactic center, the best part of the Milky Way, is always found in an arc between southeast and southwest at the mid-latitudes of the Northern Hemisphere. Shooting the Milky Way is the most common use of star-tracking mounts for photographers making wide-field images of the night sky. If you point the camera in directions other than south, you'll need to recalculate the position of the counterweight.
To determine the correct position of the counterweight, assemble the tracker, attach your tripod head, and mount your camera pointing in the direction you intend to shoot. Place a pen atop a 2x4 and place the center of the mounting block directly on top of the pen. Adjust the counterweight until the assembly balances. Note the distance from the left end of the shaft to the lefthand side of the counterweight. For my Canon 5D Mark III and IV with a Canon 16-35mm f/2.8 L III lens, that distance is three inches.
Adjust the position of the counterweight by loosening the knob and sliding the counterweight along the shaft. For my Canon 5D Mark III and IV, the distance from the end of the shaft to the near surface of the counterweight is three inches if the camera is pointing south.
After writing the original version of this blog post in November 2017, I worked out a method of finding the correct position for the counterweight in the field. The key was filing a notch in the bottom of the mounting block, as shown in the images below. That notch fits over the top edge of the SkyTracker Pro once you've tilted the SkyTracker to the correct altitude and keeps the mounting block from slipping off the top of the SkyTracker. To use this approach, start by fully assembling the SkyTracker Pro and mounting your camera on your tripod head. Compose your shot. The position of the counterweight will differ depending on which direction the camera is pointing. Then remove the mounting block from the SkyTracker. Place the notch in the mounting block atop the SkyTracker, and move the counterweight back and forth until the counterweight balances the camera and lens. If you look closely at the two photos below, you can see that the counterweight is not in the same position along the shaft because the camera is pointing in different directions.
The reward for packing the weight and dealing with the complex setup required to use the SkyTracker Pro is the ability to make photographs of the night sky at much lower ISOs than are required when using a stationary camera. That, in turn, allows you to create the cleanest, most detailed images of the night sky that can be achieved with a DSLR. The SkyTracker Pro costs $299; the counterweight kit costs $79.
The Milky Way over the spires of the Needles District, Canyonlands National Park, Utah, shot with an iOptron SkyTracker Pro and Canon 35mm f/1.4 lens mounted on a Canon 5D Mark III. After shooting the sky with the tracker on, I shot the land with the tracker off, then merged the two images in Photoshop.
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