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Quick introduction to the barn door tracker

Why do we need a tracker for photographing celestial objects?

Celestial objects baring the Sun, Moon and some bright stars are extremely dim and they require very long exposures to capture. The light from these objects are so faint it takes many seconds or minutes, even hours in some cases to capture them using a camera.

What is long exposure?

A digital camera works by collecting and recording the photons emitted by or reflected by the object we wish to capture. Under normal day time photography, the camera’s sensor requires only a few milliseconds to record these photons. For celestial objects which are very faint, the sensor requires many minutes or even hours in some cases to collect the photons. This is called long exposure.

What is the problem of long exposures?

The earth is rotating! The stars and other celestial objects in the sky are fixed. It is not the sun or stars that are rising in the east and setting in the west. Rather it is us rotating from west to east which makes it look as if they are rising from the east and setting in the west. Since the camera is sitting on earth, it is rotating along with it. If we have pointed the camera at a star, after a few minutes the camera will be pointing elsewhere and not at the star we had originally pointed it at.

What is the solution?

Tracking. Tracking is basically making the camera point at the same target by compensating for the earth’s rotation. The earth is rotating at approximately 15° per hour. The tracker will compensate for this by rotating at 15° per hour in the opposite direction. The earth rotates west to east, and the tracker rotates from east to west at the same speed to compensate from the movement.

Commercial, off the shelf options

  • Computerized equatorial mounts (German Equatorial Mounts, EQ mounts)
    Entry level photography capable mounts cost around $1,500. High end, good quality mounts cost upwards of $4,500.
  • Scissor mounts (Astrotrak etc.)
    Costs around $650.
  • Entry level tracking EQ mounts and clock drives
    Costs ranges from around $350 to $400 (Star adventurer, Polarie etc)

What is the option for a beginner? Or if you want to test your interest before investing on the commercial mounts?

The Barn Door tracker. Even a smart, reasonably accurate barn door tracker costs only around $60. With basic DIY skills you can build one yourself. Or, you can get help from a carpenter or local machine shop.

Checkout what is possible with a barn door mount:

 The Orion nebula, Running man nebula, Horsehead nebula and Flame nebula shot with a barn door tracker controlled by the smart barn door controller. The image is a stack of 120 x 15 second exposures taken with a 135 mm lens.

 

The orion constellation. Orion nebula, Flame and Horsehead nebula are also visible. This was shot with a barn door tracker controlled by the smart barn door controller. The image is a stack of 60 x 60 second exposures taken with a 50 mm lens.


History of the barn door tracker

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. Modern barn door trackers are motorized and hands free.

Basic working principle

 

The rotation of the screw rod (yellow) will make the top surface of the barn door to move upwards. By precisely controlling the speed, we can make the top surface rotate around the hinge at the same speed of the earth and thus achieving tracking. The camera is mounted on top surface (blue).

 


Basic components of the “smart” Barn door tracker

  • 2 pieces of wood which make the 2 arms of the barn door
  • 6 Hinges (for isosceles mounts)
  • 1 Bipolar stepper motor
  • Arduino (or clone) micro-controller
  • Stepper motor driver module
  • 8mm pitch screw rod and nut

It’s that simple, but there is a catch

The simple design of the isosceles barn door tracker has a design flaw known as “tangent error”. Applying trigonometry, we can see that by applying a constant linear rate of increase in the base of the isosceles triangle does not produce a constant linear increase in the included angle. The angle is what we care about as it is the one compensating for the earth’s rotation. After about 10 minutes of tracking, there will be drift in tracking and the target starts to drift.

Solving for “tangent error”

  • Traditional methods
    • The traditional solutions are from a time when micro-controllers were the toys of specialized engineers and technologists.
    • The solution was by adjusting the mechanics, introducing another arm. This is a beautiful solution but makes it more difficult to build.
    • Complicated to understand the math behind the solution.
  • Modern “smart” approach
    • Software code written in the Arduino microcontroller to adjust the speed of the motor driving the screw rod continuously and automatically to compensate for the geometry induced tangent error.

 

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