The Focus System

The ROTSE-III focus system consists of a stepper-motor and a movable diaphragm which pushes the secondary mirror. The diaphragm has a travel range of around 1 mm. The stepper motor has an arbitrary zero-point that can be set with a screwdriver. The stepper motor puts pressure on the diaphragm, and shifts the focus. When the stepper motor is moved back the pressure is released, and the focus shifts back.

The most important thing to note about the focus system is the focus motor position in milimeters does not directly describe the position of the secondary mirror. Depending on the position of the zero-point, there can be several milimeters of travel (typically ${\sim2 - 3}$ mm) before the motor applies pressure to the diaphragm. When pressure is applied, the focus changes. Around 1 mm from the initial pressure, the diaphragm cannot be stretched further, and the motor will stop. When the focus motor stops off-target, schierd will log an error and the system will shut down.

Once telescope alignment has been completed (if that is possible!) care should be taken to note both where the focus motor engages the diaphragm, and where the maximum focus position is located.