Magnus -?

Noise adds in quadrature. The reported RMS numbers are standard deviations, comprised of different effects (eg. noise, mount mechanics) each with their own individual RMS values or standard deviations. If the effects are independent (eg. seeing and mount mechanics) then the overall effect can be approximated by squaring the standard deviations of the individual effects and adding them, then taking the square root. That is what I've done here.

I think also that it is important to differentiate between high frequency noise - which is things like vibrations and seeing - and the longer period oscillation from worm movement, which is typically what we talk about when we discuss periodic error correctable by PEC or PHD2. Bruce Waddington covers this very nicely in his video that was shared here recently. Things like vibrations and high frequency oscillations from seeing are not corrected by guiding software. I like to think of the high frequency star motion RMS as the absolute best guiding you can get on any given night. It is basically everything that PHD2 cannot correct!

Arun