Night Sky in Focus

Due to Earth’s rotation, objects in the sky appear to move from east to west. Taking a long-exposure photo of stars using a camera on a non-tracking mount will produce trails. To compensate for the Earth’s rotation, a tracker as simple as a geared stepper motor can be used. This tracker is controller by a simple Arduino-based stepper controller.

Any geared stepper motor with sufficient torque can be used as a drive mechanism. For this project, I used a stepper motor with a built-in 1:500 gearbox.

I simply attached an aluminum plate to the end shaft of the stepper. A ball head mount was then used to connect a DSLR to the plate. All the components can fit easily in a small camera bag. It is designed to carry only a very light payload such as a DSLR with a wide-field lens.

To test it further, I also tried it with a more demanding lens: 55 mm. Without tracking, stars appear as streaks, but with tracking, stars remain as fixed points.

Even with a 55 mm lens, the tracker is capable of accurate tracking up to 120 seconds, which should be accurate enough for Milky Way shots.

Calibrating the DIY Tracker

The tracker’s ‘tracking speed’ needs to match the actual movement of the sky. Calibrate your own tracker by making sure that the stepper does not rotate a bit too fast nor too slow. 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). 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 reveal the position of stars). By looking at the live view images or photos taken, you should be able to tell whether or not the tracker is moving too fast or too slow.

Polar 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. When pointed to a star in the east, minimize the north-south drift by adjusting the polar axis higher or lower (altitude adjustment). When pointed to a star in the celestial equator (near meridian), minimize the north-south drift by adjusting the polar axis to the left or to the right (azimuth adjustment). The east-west drift is corrected by adjusting the tracker’s speed.

This project has been featured in HACKADAY.

Related links:
DIY Telescope Clock Drive
DIY OnStep Go-To Telescope Controller
View all home-brewed DIY astronomy equipment

Night Sky in Focus 
© Anthony Urbano | Bacoor, Philippines

Clock drives are simple tracking mechanism that move a telescope’s RA axis one rotation every one sidereal day (23 hours and 56 minutes) to effectively compensate for the Earth’s rotation. It allows precise tracking of planets, galaxies, nebula, and other sky targets.

An inexpensive Arduino Uno board and L293D-based stepper motor driver can be used to control a telescope. Attach a stepper motor on a telescope’s RA adjustment knob, then find the correct motor speed that will match the movement of the sky. Below is a sample sketch for a simple telescope clock drive controller.

//Simple clock drive controller by Anthony Urbano 06 September 2021. It uses an Arduino Uno and an L293D.

#include <AFMotor.h>                  //Go to SKETCH > INCLUDE LIBRARY > then lookup "Adafruit Motor Shield Library"
AF_Stepper motor1RA(24, 1);           //Initializing motor's steps per one full rotation; Connect the motor to M1 port
void setup() 
{
}

void loop()
{
  motor1RA.setSpeed(100);              //Change the value to speed up or slow down the tracker
  motor1RA.step(1, FORWARD, DOUBLE);   //Motor takes 1 step forward; to reverse direction, replace FORWARD with BACKWARD
}
 

With proper polar alignment, a simple clock drive is capable of imaging deep-sky objects, such as the Flame and Horsehead Nebula. This image was taken with a telescope at 565 mm focal length.

Related links:
OnStep Go-To Telescope Controller
View all home-brewed DIY astronomy equipment

Night Sky in Focus 
© Anthony Urbano | Bacoor, Philippines