Interested in astrophotography? Do you want to first try this hobby before investing on expensive computerized EQ mounts? A barn door tracker can be a good starting point after trying fixed tripod astrophotography. A well built barn door tracker at a fraction of the cost of other commercial options can produce photos which can stun you. It will surprise you that a simple device like this can actually track the stars and other celestial objects.
The barn door tracker was created by George Haig. It is also called as the scotch mount or Haig mount. The plans for his tracker were first published by the Sky & Telescope magazine in their April 1975 issue. The original design was was improved upon by Dave Trott who introduced a second arm in the design. This improved tracking accuracy over time. His new design was published by the Sky & Telescope magazine in their February 1988 issue. The original designs all involves manual actuation.
Today, a modern barn door tracker can be easily built with a stepper motor to automatically drive it.
The “smart” barn door tracker shield for Arduino
The features of the shield is listed below. The software for the shield is completely open source and you can modify it for your barn door. The software provided will work out of the box for the reference design provided here.
- DRV8825 based stepper motor controller
- Micro stepping of 1/32 steps, giving a single step granularity of 0.86 arc seconds for a 300mm arm / 8mm pitch.
- Open source
- Tangent error correction
- User controls
- Pause tracking
- Fast forward
- A keypad with 4 keys is provided to get started. User can replace the keypad if required.
- Automatic home detection through external limit switch or by manual overriding switch
- One LED to provide status information
- 12V 1A power input
- Reverse polarity protection
- Compatible with 12V NEMA 17 bipolar stepper motors
- Provides power to the Arduino. No need to power the Arduino separately.
Gallery of images shot with this controller
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Operation guide and steps
- For the operation of the tracker and tangent error correction, the controller needs to know the ‘home’ position (the lowermost, starting point). The operation of mount is suspended till the controller knows this home position. The controller will allow the use of the ‘rewind’ button to bring the tracker to the home position. Home position is detected through the external limit switch or manually by pressing the ‘home’ button on the controller. While waiting for home, the DX LED will be blinking. Every time the controller is switched on, this procedure must be done.
- After the barn door reaches home position, the ‘track’ button can be used to start tracking. Tracking can be stopped at any time by using the ‘stop’ button. ‘Forward’ and ‘Rewind’ buttons can be used to quickly move the camera arm up and down.
- The tracker will stop automatically when the predefined upper limit is reached. The tracker can be used again by using the ‘rewind’ button to bring it back to the home position.
Connecting the stepper motor
A NEMA 17 bipolar stepper motor is required for this controller. The stepper motor is connected to the controller through the screw terminals provided. Coil 1 is connected to A1,A2 terminals and Coil 2 is connected to the B1,B2 terminals. If the motor is rotating in the reverse direction, interchange the coils or swap the terminals.
Connecting the home limit switch
As mentioned earlier, the knowledge of the ‘home’ position is very important for the controller to perform the tangent error correction. There is a ‘home’ terminal in the controller which can be used to connect an external limit switch which will detect home position. If an external switch cannot be connected, an override button is provided on the controller itself to signal a home position.
Connecting the keypad
The controller has provision to connect four external switches/buttons for the play, stop, fast forward and rewind operations. The controller comes with a 4 button keypad which can be used for this purpose.
A 12V 1A external DC power supply is required for the controller. This power is internally routed to the Arduino and hence no separate power is required for the Arduino.
Detailed information on why do we need a tracker for photographing celestial objects and what is long exposure is explained in this blog post here.