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Does the PMC-Eight provide Permanent Periodic Error Correction (PPEC)?


No, the PMC-Eight does not provide a PPEC function. 

One of the features that has not been included in the PMC-Eight system that some are claiming is a show-stopper (I think it is overblown), is Permanent Periodic Error Correction (PPEC). The architecture of the controller and its firmware is not intended to support this level of drive correction, and if implemented at all, would be implemented at the ASCOM Driver level, or in the ExploreStars application directly.

The primary reason that this feature was not included in the design of the PMC-Eight is that if you want the best performance and finest images using your mount then you are going to use an auto-guiding camera system to correct your tracking real-time. At best, PPEC can only correct 70% of the PEC of your mount, depending on the mechanical quality of the mount, and on the EXOS 2 mount would probably only correct to only about 50%. Also, the fixed correction provided by PPEC would not account for the variation in PE in your worm over tens of PE cycles during an imaging session. An auto-guiding system is designed to dynamically correct the tracking rate over your entire session.

The design decision was made that if the user is going to use an auto-guiding system anyway to get the best results, the inclusion of PPEC is somewhat redundant and just complicates the system unnecessarily.  Another issue is that our current mounts do not provide a hardware worm position index switch which would ensure that the correction curve is synchronized with the worm position. This is necessary to ensure the function works correctly. Relying on the user to manually synchronize the worm position can be problematic at best. 

The availability of our open-source code for the ASCOM driver through the OpenGOTO community provides an option for those users that wish to change the system/driver by including a home-grown PPEC function for their observatory system.