guiding performance, residual periodic error due to main worm cycle

[Erquy]

Hi,

I am pretty happy with my PhD guiding my AVX mount /OAG with Lodestar/ either imaging at F6.3 with the C8 or with my WO GT81, using SX 694 camera: I manage 600 sec exposure with well round stars (some issues time to time with DEC guiding, but fixed when correcting only in north or in south).

 but by curiosity I loaded some of my guided PHD log into pecprep and it looks like there is residual PE due to the worm cycle (below is a guided log):

it is pretty small, but it looks like being still the main origin of RMS I have left to correct.

It is quite surprising as I would expect guiding  addressing easily and totally the main worm periodic error. Or will there will be always some residual PE from the main worm? Note that the problem is there whatsoever I use PEC or not and this comes in any of my log to about same extend. The PE of my AVX is about +/-16 arcsec and well reproducible and smooth. 

Is this normal, or not? If not, any tips to improve? My PHD settings are pretty standard ones, given directly from wizard, 2,5 sec with hysteris algorithm, (aggressiveness 70, hysteresis 15, min 0.25 pixel for the C8 at 6.3 (and smaller for the GT81).

No big deal, as anyhow my results  are all fine despite this, but I feel maybe borderline when imaging with the C8. So if I could further improve, that could be welcome.