Cancelling Sidereal Rotation with a Stepper and a Lot of Gears

A project log for Astrophotography Camera Mount

A 3D-printed 10000 : 1 gear reducer and control hardware used for astrophotography

ChrisChris 02/05/2019 at 01:580 Comments

A stable long exposure shot in astrophotography requires cancelling out the sidereal rotation of the Earth. This is the rate at which the Earth rotates and is the reason for star trails. The idea is simple: if the Earth is rotating on its axis in one direction, rotate your camera on that same axis in the opposite direction at the same rate. Rotating on the same axis is easy. All you have to do is line up your mount's rotational axis to the North Star, Polaris, if you're in the northern hemisphere. If you're in the southern hemisphere there is a South Star, Sigma Octantis, but it is rather faint. Rotating in the opposite direction is also easy: counter-clockwise if you're in the northern hemisphere and clockwise if you're in the southern hemisphere.

The main problem is that the sidereal tracking rate is very very small.  ASCOM Standards has it at 15.041 arcseconds per second. In more useful-to-me terms that comes out to a minuscule 0.00069634 RPM - a little less than seven-ten-thousandths of a rotation per minute! Driving a stepper motor that slowly would be choppy at best.

In order to get a final output of about 7/10000 RPM, this astrophotography mount gears down a stepper motor by 10000 : 1 using six planetary gearsets: four 5 : 1 sets and two 4 : 1 sets. This lets me drive the motor at a reasonable rate of speed (6.96 RPM).