# Thread: Caution: Boring Matter - Barn Door Tracker Calculation :)

1. ## Caution: Boring Matter - Barn Door Tracker Calculation :)

Hello. Main matter is: I want to build my barn door tracker .

I have a problem, i read about calculation, but i couldn't understand how its works.
Sideral time is 23 hours, 56 minutes, 4.0916 seconds. So tracker must rotate 360 degree per sidereal time. If we calculate it means tracker must rotate 15.0410682 degree per hour and 0.25068447 degree per minute and 0.0041780746 degree per second.

But at this point how can i calculate tracker arms length? And how can be occurred tracking error? What is the working mechanism, i couldn't understand.

Please explain me how it works?
I found calculate formula on internet but i couldn't understand. How its world?

Thank you so much.

2. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

You can make the arm any length you want. The important part is to match up the thread pitch of the drive screw, the rotation speed of the drive motor, and the length of the arm. The easiest one of the three to vary is the arm length, so you should choose your motor and drive screw first.

One turn of the motor will raise the arm by the height of one thread on the screw, in whatever length of time it takes the motor to turn once.

Don't worry about the difference between sidereal and solar time. For a barn door tracker, 24 hours or 15 degrees per hour is close enough.

3. But exaclty which point occure error and why? I want to make tracker better then barn door, but first of all i must learn and understand how is works.

4. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

Sorry, I don't understand the question. What error are you referring to?

5. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

Originally Posted by KeithBC
Sorry, I don't understand the question. What error are you referring to?
On wikipedia (Barn door tracker - Wikipedia, the free encyclopedia), barn door tracker errors mentioned below. They called tangent error.

Mount design alternatives
A simple single-arm barn door tracker can be made by attaching two pieces of wood together with a hinge. A camera is mounted on the top board, usually with some sort of ball joint to allow the camera to be pointed in any direction. The hinge is aligned with a celestial pole and the boards are then driven apart (or together) at a constant rate, usually by turning a threaded rod or bolt. This is called a tangent drive.

This type of mount is good for approximately 5–10 minutes before tracking errors become evident when using a 50 mm lens. This is due to the tangent error. That length of time can be increased to about 20 minutes when using an isosceles mount.

A curved drive bolt in lieu of either a straight tangent or isosceles mount will greatly extend the useful tracking time.

These designs were further improved upon by Dave Trott, whose designs were published in the February 1988 issue of Sky & Telescope. By using a second arm to drive the camera platform - thus adding complexity to the fabrication - tracking accuracy was greatly increased, and can lead to exposure times of up to one hour. The most accurate of these designs is the Type-4.

A modified double arm design minimizes tangent error by raising the point of rotation of the arm on which the camera is mounted. This has the effect of tilting the arc traced by the camera arm backwards causing it to follow a better path.

A basic geometrical analysis of the tangent error shows that it can be fully compensated for by inserting a specially shaped piece between the threaded rod and the top board. Such solution was already known for a long time before the original G. Haig publication.[4]

The most basic of these designs are manually operated, although some have added electric motors to automate and improve the accuracy of the tracking process.[5]

6. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

The articles linked from the Wikipedia article go into detail about the math involved in the various barn door designs.

7. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

So barn door tracker has errors or not?

8. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

Any mechanical device has errors. How much error a barn door tracker has depends on its design.

9. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

If you use a computer controlled stepper motor you can use a straight screw and compensate for the tangent effect in the software that controls the stepper delay. If you use a DC motor then use a circularly curved screw for perfect tracking. A dual hinge seems like more trouble than it's worth. If you are uncomfortable with the math I recommend looking for a low end motorized mount.

10. ## Re: Caution: Boring Matter - Barn Door Tracker Calculation :)

Originally Posted by CamelHat
If you use a computer controlled stepper motor you can use a straight screw and compensate for the tangent effect in the software that controls the stepper delay. If you use a DC motor then use a circularly curved screw for perfect tracking. A dual hinge seems like more trouble than it's worth. If you are uncomfortable with the math I recommend looking for a low end motorized mount.
I can drive stepper motor or dc motor via computer.
I'm software developer and electric engineer, i can write control program, and drive motors with pam signals and build circuits.
So i'm thinking make tracking equipment one step ahead barn door tracker. And this point, error handling (periodic error correction) is important i think.
What are you thinking?

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