Motivation

Space - the final frontier. Spending a night under a starry sky offers a fascinating view into the past. To keep those images not only for one night, it's easy to set up an camera and take some photos.

Unfortunately (?) the earth keeps rotating, which makes it hard to get a picture of a star as a point. As shown in the picture below, this can also be a nice effect.

Balanced Rock with Star Trails (NPS/Jacob W. Frank, CC-BY 2.0)

I use a Pentax K-30, which has a crop factor of 1.5. Let's calculate the exposure times (all rounded down to full seconds) for different focal lengths, based on my lenses.

Tamron SP 10-24mm 1:3.5 - 4.5
10 mm = 33 sec
24 mm = 13 sec

Tamron 28-75mm 1:2.8
28 mm = 11 sec
75 mm = 4 sec

Asahi 50mm 1:2
50 mm = 6 sec

Introduction

If you would like to take photos with higher exposure times, you need to compensate the rotation of the earth. A simple solution is a barn door tracker, which is shown in the following picture.

Barn Door Mount (Rob Pattengrill, CC BY-NC-ND 2.0)

The mode of operation is quite simple. After the hinge is aligned the be parallel to the axis of the Earth, the camera is oriented to the desired area of the sky. To rotate in sync with the earth, the bolt needs to be turned at a defined speed. As the earth rotates 360° in 24 hours (or to be more correct 23 hours, 56 min, 4.1 sec), the included angle between the two sheets of wood needs to increase

• 360° in 24 hours
• 15° in 1 hour
• 3.75° in 5 minutes
• 0.25° in 1 minute

Pifalls

As you can imagine, its hard to rotate the bolt at a defined speed by hand, especially during a long time of exposure like 5 minutes.

Also this isosceles mount has a disadvantage, due to its construction. One turn of the bolt will change the included angle in a different way, depending on the size of the included angle. We will take a closer look at this on another post.

Own solutuion

During this project, i will construct a barn door mount, which is operated by a stepper motor. The stepper motor is controlled by a microcontroller, which will turn the bolt at a correct speed, which will be calculated depending on the opening angle.