Re: PHD 2 Guiding "Good Enough"
See Slide 14 in Best Practices for PHD2
Also. from the Help file discussing the PPEC algorithm in PHD2 (note that PPEC guiding algorithm is NOT the same as PEC in the mount)
·Short-term: for high-frequency errors such as those caused by gear roughness or seeing
·Medium-term: for residual periodic errors, typically occurring at intervals less than or equal to the worm period
·Longer-term: for steady drift and for lower frequency (longer time interval) harmonics that can be caused by the interaction of multiple gears in the drive train
The short-term behavior is used to identify the unpredictable noise in the system, which is essentially filtered out in order to identify components that are predictable. For most mounts, the medium-term component is likely to be the most important. If you’re following best practices, you will have programmed periodic error correction in your mount (assuming that feature is available to you). Doing this reduces the amount of work that needs to be done by PHD2, and the PEC correction in the mount is normally saved permanently. This approach is preferable to having to measure and infer the periodic error behavior every time you set up your equipment.
That said, PEC in the mount is never perfect, and you will often see residual repetitive errors even when PEC is active. These often arise when the tracking errors occur with a frequency that is not a harmonic (integer fraction) of the mount’s worm period – most PEC implementations can’t deal with those. You can also get residual periodic errors if they are dependent on the mechanical loading of the mount or if the mount’s behavior has changed since the PEC was programmed..