This project has been featured in HACKADAY.
Due to Earth’s rotation, objects in the sky appear to move from east to west. Taking a long-exposure photo of stars using a camera on a non-tracking mount will produce trails. To compensate for the Earth’s rotation (and minimize trailing), a tracker as simple as a geared stepper controlled can be used.
The tracker was built so that I could still capture the Milky Way in remote locations where traveling with a heavy telescope mount is not an option.
Any geared stepper motor with sufficient torque can be used as a drive mechanism. For this project, I used a stepper motor with a built-in 1:500 gearbox.
I simply attached an aluminum plate to the end shaft of the stepper. A ball head mount was then used to connect a DSLR to the plate. The photo below shows how the parts are connected together.
All the components can fit easily in a small camera bag. It is designed to carry only a very light payload such as a DSLR with a wide-field lens.
To test the accuracy of the tracker, I tried to capture the Milky Way using an 18-55 mm lens, set at 18 mm (wide field). The point of the test is to determine if the tracker is capable of producing pin-point stars.
To test it further, I also tried it with a more demanding lens: 55 mm. Without tracking, stars appear as streaks, but with tracking, stars remain as fixed points.
Even with a 55 mm lens, the tracker is capable of accurate tracking up to 120 seconds, which should be more than enough for Milky Way shots.
The tracker’s ‘tracking speed’ needs to match the actual movement of the sky. To calibrate your own tracker and make sure that the stepper does not rotate a bit too fast nor too slow, follow the procedure below:
Align the tracker’s axis of rotation (or what is called the polar axis, which in this case, the stepper’s main shaft) with the north star Polaris (for observers in the southern hemisphere, point the tracker’s polar axis in the general direction of the Earth’s southern polar axis).
Trackers suffer from two kinds of drift: (1) east-west drift and (2) north-south drift. A star drifting in the east-west direction indicates an error in the tracking rate. A star drifting in the north-south direction indicates an error in polar alignment (this will be discussed later in the Polar Alignment part).
Point the camera to any bright star. Turn the tracker on and start tracking the sky. Take a series of shots (with just enough exposure to capture the position of stars). By looking at the photos, you can readily tell whether or not the tracker is moving too fast, or too slow. In this particular project, I started with a very slow tracking rate (that is, tracker is moving too slow). I then took some photos, and then increase the speed a bit. With each adjustment, I found out that the trails are slowly reduced until it is minimized. You may also notice a north-south drift. Disregard that error since the tracker speed calibration can only correct an east-west drift. The north-south drift indicates a poor polar alignment (even the best mounts in the world will suffer from a north-south drift if not properly polar-aligned). If performed properly, this procedure should only be done once.
Astrophotography demands a lot of precision. All equatorial trackers, from the simplest DIYs to the most complex observatory-grade ones, must be properly polar-aligned before satisfying results can be obtained. There are no easy ways of doing the alignment. The following procedure details the steps on how to polar align any equatorial tracker, such as the DIY tracker.
To align the DIY tracker, use a polar-alignment method called drift alignment. Before attempting this method, make sure that you have already calibrated the tracker, that is, you’ve managed to achieve a correct tracking rate.
Perform a rough polar alignment first by simply pointing the main shaft of the stepper to Polaris (for observers in the southern hemisphere, point the tracker’s polar axis in the general direction of the Earth’s southern pole star).
Proceed with the drift alignment method. This part requires adjustment of the polar axis in two directions: (a) altitude and (b) azimuth. To adjust the altitude, move the polar axis up or down (higher or lower). To adjust the azimuth, you need to move the polar axis to the left and to the right. The adjustments needed to precisely align the polar axis will be determined by observing the movement (or drift) of two stars: (a) one star in the eastern horizon–a rising star in the celestial equator and (b) another star at the zenith–a transiting star in the celestial equator. This method should also work for observers in the southern hemisphere.
Altitude adjustment: With the tracker roughly polar-aligned, and the ‘top’ of your camera pointed to the north (or south for observers in the southern hemisphere), point the camera to a rising star in the east. Take a series of shots. The star is expected to drift northward or southward. The goal is to minimize this drift (since this drift will show up as streaks in your shots). Move the tracker’s polar axis higher or lower until this drift is minimized. If say, you moved the polar axis a little higher, and upon testing, the drift has worsen, then you know that you must move instead in the opposite direction. Through a series of small adjustments, it is possible to find the correct altitude of the polar axis.
Azimuth adjustment: With the tracker roughly polar-aligned, and the ‘top’ of your camera pointed to the north (or south for observers in the southern hemisphere), point the camera to a star at the zenith. Take a series of shots. Again, the star is expected to drift northward or southward. The goal is to minimize this drift. This time, move the tracker’s polar axis to the left or to the right until the drift is minimized. If say, you moved the polar axis a little to the left, and upon testing, the drift has worsen, then you know that you must move instead in the opposite direction. Through a series of small adjustments, it is possible to find the correct azimuth of the polar axis.
Repeat the steps several times to achieve better alignment with each iteration. Observatories around the world are likely polar-aligned in this manner, so you have to be patient if you want really good results.
Remember, when pointed to a star in the east, minimize the north-south drift by moving the polar axis higher or lower (altitude adjustment). When pointed to a star in the zenith, minimize the north-south drift by moving the polar axis to the left or to the right (azimuth adjustment). The east-west drift is corrected by adjusting the tracker’s speed.
Related article: Arduino Stepper Motor Controller
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Astronomy and Amateur Radio
© Anthony Urbano (Manila, Philippines)