If you want to take your astrophotographs to new atmospheres, the gear you need is not necessarily a new camera or lens, it is a star tracker. Yes, a high-resolution camera and fine glass are a must for great astrophotos, but the star tracker is the tool that will help you ally yourself with the physics of our world and give you a fighting chance to create better astrophotography.
The earth is round (in fact, it’s an oblate spheroid) and it spins on its axis. This causes the moon, stars, and planets to appear to move through the night sky.
When you engage in night photography, the camera needs to operate at slower (longer) shutter speeds to allow the dim light of night (especially starlight when pointed at the sky) into the camera to create a decent exposure. The combination of a slow shutter speed and a rotating earth means that the stars in an astrophotograph will likely streak through the frame and not appear as points of light.
The Solution: The Star Tracker
If only there were a device that could counteract the movement of the earth so that the stars above would remain still through a long-duration nighttime camera exposure…
Enter the camera star tracker!
The star tracker is an electronic motor, mounted on the top of your tripod, that spins at the same speed at which the earth rotates—removing (or greatly reducing) the trailing of objects in the night sky as you photograph them.
Star trackers are what is referred to as “equatorial mount” and must be “polar aligned”—where you point the tracker’s axis to the immediate vicinity of Polaris (the current “North Star”)—usually with the help of a built-in scope. Because the earth is precessing (wobbling), you’ll tilt and spin the tracker to place the crosshairs of the scope at a point near Polaris, but not directly at it. For star-tracker users in the Southern Hemisphere, you’ll point your tracker’s scope near Sigma Octantis. Modern mobile apps that accompany most star trackers help you with the alignment.
Camera star trackers have different tracking speeds (yes, different objects in the sky move at different speeds) so that you can focus on stars (sideral), the moon (lunar), the sun (solar… with the proper solar viewing gear), or run the mounts at ½ sidereal speed if you wish to do a particular type of time lapse.
Mounting a Camera on a Star Tracker
There are several brands and styles of tracking mounts, so be sure to look closely at the ones you are considering to learn how to mount your camera and what else might be needed.
Two-Ball Head Mounting
Some star trackers are designed to mount atop your standard tripod head (be it a ball head, 3-way, or other type). This allows you to aim the tracker near the celestial poles as you’d aim your camera.
What is needed now is a second ball head to mount on the tracker itself and provide a place for the camera to attach to the rig. This will allow you to aim the camera in different directions while it is attached to the tracker. Yes, you can mount the camera directly on some trackers, but you won’t be able to reposition the camera if it is not mounted on the tracker on some sort of movable head. This is important, because many of us do not normally carry two ball heads into the field—or even own two ball heads!
Equatorial Wedge Mounting
For some trackers, an alternative to the double-tripod head mounting configuration is having the tracker mounted on an “equatorial wedge base” atop a level tripod. The wedge is a mounting base that allows you to dial-in the altitude (in degrees) of the tracker when the wedge and tracker are mounted on a tripod. In the northern hemisphere, conveniently, the degree of altitude corresponds to your latitude.
The camera is then attached to the star tracker on a ball head.
Standard camera star trackers are fairly portable and small. Because of this, they are not designed to hold huge telescopes or large telephoto lenses.
The operational payload capability of the star tracker is a huge buying/planning consideration because star trackers have differing maximum payload capacities. Mounting a heavier-than-specified rig on the star tracker may cause tracking errors or mechanical issues with the fine-toothed gears that turn the tracker.
Weigh your camera and lenses (and anything you might have stuck on them) to determine what payload capacity you need. You should also consider lenses or cameras you might acquire in the future.
Some camera star trackers allow for the addition of counterweights that will increase the stated payload capacity.
Focal Length Limitations/Considerations
Due to physics and mathematics, as your focal length gets longer, the faster stars will trail in a scene. If you are photographing a huge portion of the night sky with a wide-angle lens, it will take longer for movement of the stars to be noticed. But, if you point a soda straw at a star, you’ll find that that star moves out of the field of view relatively quickly because the star knows it has 24 hours to get back to near that same spot!
Because of this, camera star trackers are very good for taking astrophotographs with wide-angle lenses—perfect for big sky starscapes and Milky Way shots. (If you are envisioning a gorgeous landscape with the Milky Way hovering above it, remember, any terra firma in the frame, or things mounted on terra firma will get blurry as the tracker turns!)
With longer focal length lenses, you need to consider the weight of the lens, as well as the limitations to the tracking accuracy of the tracker. While camera star trackers are fairly precise, they are a long way from the precision of full-sized telescope trackers, and there you’ll find, in practice, a ceiling to the shutter speeds you can use with a given focal length lens. For example, we have found that a popular star tracker performs best with 60-second exposures through a 450mm lens.
Popular Star Trackers
There are three popular camera star trackers on the market these days from Sky-Watcher, iOptron, and Vixen Optics. Telescope fans are likely familiar with all three brands since they have been making telescopes and accessories for years.
The Sky-Watcher Star Adventurer is a long-time favorite for astrophotographers with its 11-lb payload capacity. A few Star Adventurers are bundled with an equatorial wedge mount and other accessories. The big sibling to this tracker is the Star Adventurer GTi, which has the same payload capacity, but connects to a mobile app and provides EQ tracking and go-to capabilities—where, after alignment, you can program the mount to point your lens at a number of stars or deep sky objects.
iOptron’s SkyGuider Pro EQ Camera Mount features the same 11-lb capacity as the Star Adventurers and many of the same features. The newer SkyGuider Pro Camera Mount with iPolar features an electronic polar scope to improve tracking alignment. The smaller sibling, the SkyTracker Pro EQ, features a 6.6-lb payload capacity. Both iOptrons come mounted standard on equatorial wedge bases.
Probably the most portable star tracker on this list is the Vixen Optics Polarie Star Tracker with its unique design and 7-lb capacity. The Polarie is about the size of a paperback book (remember those?) and weighs 22.4 oz without its 2 AA batteries. The newest version of the Polarie is the Polarie U Star Tracker that is also diminutive and has a 5.5-lb capacity. It is lighter than the standard Polarie.
As mentioned above, some trackers allow for the mounting of counterweights to balance heavier payloads more efficiently, and increase the payload capacity of the mount. You can find a selection of star tracker accessories that include these weights, ball heads (although any standard photo ball head is good), equatorial wedges, and more. One useful accessory not in that product category is the camera & tripod leveler that allows you to get a level surface for your star tracker without having to tinker with your tripod legs all night trying to get the top level.
Do you have any questions about camera star trackers or using them for astrophotography? Do you own one of these trackers and want to share your thoughts to help fellow B&H customers? Blast off in the Comments section, below!