[peter_4059 (Peter)]

In my quest for round stars I've spent a lot of time playing with the autoguiding. While I've tweaked the settings to get pretty consistent low RMS errors I was still seeing a peak in the RA error that was troubling me. This looked like a random peak and I've been trying to understand if there was some period to it that would allow me to track it down. This morning I tried a different approach - look line by line through the guide log and see if the error coincides with the dither command. Sure enough that's what it is - the PHD dither is a random amount of movement in the lock position up to a number of pixels set by a command, in my case issued by SGP that I've got set as 2 pixels. Because the magnitude of the dither is random this meant I could not spot the repeated pattern when looking at the guide errors - sometimes the dither is buried in the normal corrections.

The excel plots below show 500 dx values from the guide log - the upper plot includes the dither error while the lower one has the dither error removed. The second image shows the full session (2500 points)