Tracker Speed Calibration
The tracker’s ‘tracking speed’ needs to match the actual movement of the sky. The actual tracking rate adjustment is called calibration. Commercially-available trackers are factory-calibrated (someone was asked to test and calibrate the tracker during its prototyping stage). For DIY trackers, you will be the one to do the calibration. In the sketch loaded to the Arduino, you can simply adjust some values (like motor speed) to speed up and slow down the tracker. 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:
1. 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. If you are not familiar with the night sky, you may need to download a star map application for this, such as a freeware called Stellarium
2. Adjust the DSLR’s position so that the eastern side is a little heavier than the western side. This is important to make sure that the gears mesh properly (as if the load is slowly being lifted) and reduce any backlash.
3. 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).
4. 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. In this particular project, only one step of the stepper for every 6 seconds is required to match the movement of the sky–this value will differ depending on the geared stepper used. You may also use the ‘Live View’ feature to easily monitor the speed of your tracker by pointing the camera at a bright star and adjusting the tracker’s speed accordingly (use zoom for greater accuracy). Use the serial monitor to display the values for the tracking speed. At this point, 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 the ultra-portable tracker. This should also apply to any equatorial tracker.
1. 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 (see instructions above).
2. The goal here is to align the tracker’s main shaft (called the polar axis) with that of Earth’s polar axis. 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).
3. 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.
4. 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.
5. 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 mimimized. 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.
6. 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.
With practice, it is possible to drift align in 10 to 15 minutes. For queries about calibration and polar alignment, kindly leave a comment below.
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For a more advanced application of the drift alignment method (one that involves a telescope and tracking correction using a computer), click here.