PEC correction
As Periodic Error Correction the ability of a control, the production-related error is called a worm gear to compensate. This is z. B. used in telescopes used for astrophotography when they are mounted on an equatorial mount . The repetition of the screw flaw has a relatively short period (typically in the range of 10 to 15 minutes).
functionality
description
In order to be able to operate astrophotography, it is necessary to be able to expose the mostly very faint objects as long as possible. For this purpose, the hour angle ( right ascension , abbreviation 'RA') of the telescopes used must be tracked in order to compensate for the earth's rotation . Because of the high gear ratio required (one revolution in RA takes 24 hours), a worm gear is usually used. However, the manufacturing tolerances of the worm mean that the hour angle cannot always be tracked exactly. The result is that the mount moves ahead or behind. This distorts the depicted stars from dots to lines.
calibration
Before use, a PEC must be calibrated on the respective mount. To do this, the mount of the telescope is precisely aligned (" incised "). With the RA tracking switched on, the position of a star must be corrected manually using a crosshair eyepiece for one complete revolution of the worm wheel . The correction values are recorded and saved by the control during training. The calibration is retained as long as the position of the worm wheel in relation to the drive is not changed.
Assessment of performance
- A PEC can be used to reduce the periodic error of a worm drive to an acceptable level, but it cannot replace a precisely manufactured worm wheel. If increased precision is required for tracking, you will not be able to avoid replacing the drive or purchasing a more precise mount in the long term.
- A PEC only corrects the periodic error of the worm drive. Tolerances of the driven gear and other sources of error cannot be compensated.
- The accuracy of the PEC depends on training that is as precise as possible (including the number of measurement data that the control is able to store and the magnification of the crosshair eyepiece used).
- In most control systems, the performance of the processors used is not very high. This has the consequence that the PEC occupies a high percentage of the available processor power at the expense of other functions of the control.
- The data of the PEC become unusable as soon as the positioning of the engine in relation to the transmission, which existed during the training, is changed. If the stepper motor 'loses' steps (e.g. if it slips due to overload or if the clamp is released), the PEC must be retrained every time.