The Mk2 Tracker has permanent Periodic Error Correction ( PEC ). This feature uses a look-up table in an EEPROM to store correction values for the speed of rotation of the RA drive motor. The worm shaft has a 128 position absolute encoder attached to it, which is used to time the reading of the correction values. These corrections are used directly in the drive electronics to set the speed of the stepper motor. Once the PEC is programmed, it can be used to compensate for repeatable errors which occur once per rev of the worm shaft but cannot correct for non-repeatable or random errors.
The data obtained from the first autoguiding test runs indicate the Right Ascension (RA) tracking error is typically ±3 arc seconds. When I designed the PEC circuitry I expected that it would be used to correct errors that were an order of magnitude greater than this. Fortunately this has not been the case.
The chart below shows two sets of data from two different regions of the main 180 tooth gear. The data is plotted with the worm shaft encoder position plotted along the x-axis. (The worm shaft encoder has 128 positions which are displayed on a LED display as the worm rotates.) A sinusoid with an amplitude of 1.3 arc seconds has been overlaid on the data to show a close representation of the repeatable periodic error in the RA tracking. This is an insignificant error, as is at level that most manufacturers might expect after Periodic Error Correction has been used! (To put this in perspective, 1 arc second is the angle subtended by a golf ball at a distance of approximately 4 miles).
When I get an opportunity I will program the PEC function on the tracker and see how effective it is at removing the ±1.3 repeatable error.
