tracker

Telescope right ascension drive controller

version 0.0.2

Supported architectures

• Arduino Nano rev 3 [Verified]

Overview

tracker is the command and control code that is intended to permit an Arduino Nano rev3 drive a stepper motor at a rate that will slew a telescope right ascension drive at the correct rate to track objects with reasonable accuracy for visual astronomy.

The Earth rotates at a rate of 1˚ every four minutes on its axis, which means that a telescope that has been correctly Polar Aligned is able to track celestial objects by rotating around its Right Ascension axis opposite to the direction of Earth's rotation at the same angular rate.

1˚ every four minutes translates to a rotation rate of 0.00416(6) degrees per second on the RA axis.

Given the variability of stepper motors and the inability to provide an exhaustive set, configuration values are used intead.

Configuration

• The control program requires the setting of a threshold value (3 seconds in the default setup)

Control circuit

D2 and D3 switch inputs toggle the control system between fast-forward and reverse modes respectively. This is interrupt-driven. Due to the limitations of the Nano (only two interruptable digital channels), D9 is used to turn the drive system on and off.

D6-D8 are the system state output pins and drive LEDs to indicate whether the control system is in states OFF, TRACKING, REVERSE or FAST_FORWARD.

Compilation: 
Sketch uses 3596 bytes (11%) of program storage space. Maximum is 30720 bytes.
Global variables use 272 bytes (13%) of dynamic memory, leaving 1776 bytes for local variables. Maximum is 2048 bytes.

RA drive hardware

|| Component || Name                                    || Reduction ratio || Rotation ˚ per step ||
| Motor      | NEMA 17 stepper motor, 1.8˚ per step     | 1                | 1.8˚                 |
| Gearbox    | 1:51 planetary gearbox, 8mm output shaft | 0.01960784314    | 0.03529411765        |
| Pinion     | 16T5 11mm width pulley                   | 1                | 0.03529411765        |
| Drive      | 45T5 11mm width pulley                   | 0.35(5)          | 0.01254901961        |

Given the figures above, it is possible to achieve the required rotation of 0.00416(6) degrees per second at a step frequency of approximately 1/3 Hz.

This means that the controller needs to drive the stepper motor one step forward every 3 seconds approximately.

Tripod layout indicating major components