hy wrote: This is what Analyze currently does re RMS guide error. Please let me know if you think I got something wrong.

Internal guider, see
invent.kde.org/education/kstars/-/blob/m...uide/gmath.cpp#L1242
SquareError(t) = ((RA(t) - RA_target(t))**2 + (DEC(t) - DEC_target(t))**2)

In the PHD2 case, the RA(t) - RA_target(t) or diff_ra_arcsecs is taken straight from PHD2 (PHD2 calculates that and sends it to Ekos).
See invent.kde.org/education/kstars/-/blob/m...lguide/phd2.cpp#L487 for PHD2 part.
So, in the PHD2 case, Ekos is taking as the RA or DEC error, what PHD2 says is the RA or DEC error.

Analyze implements an IIR filter with an approximate time constant of 40 samples (nothing magic about 40 samples == about 2 minutes with 3s samples)

// initialize
filteredRMS = 0;
constexpr double timeConstant = 40.0;
rmsFilterAlpha = 1.0 / pow(timeConstant, 0.865);

// at every time sample (x below is SquareError(t) above)
filteredRMS = rmsFilterAlpha * x + (1.0 - rmsFilterAlpha) * filteredRMS;
invent.kde.org/education/kstars/-/blob/m...yze/analyze.cpp#L485

and takes the sqrt of the (poorly named) filteredRMS to get the rms value you see. (I guess I should have named it filteredSquareError).
invent.kde.org/education/kstars/-/blob/m...yze/analyze.cpp#L542

It does not take the mean of the RA or DEC samples. I understand your logic, though, why that might be nice.

All that said, that's not quite true for the case of RMS error printed in the Details box, e.g. when you click on a guide timeline session or a capture timeline session.
There, it does not use an approximate IIR filter, but rather computes exactly the sqrt of the sum of the square error in the interval.

In any event, just to be clear, this is what is currently implemented.
Happy to entertain suggestions,

Hy

This should be a really useful tool. Great work!
What would be really neat would be the RMS over the course of each capture to identify which ones might be sub-optimal due to poor guiding.
For info: The PHD2 remote interface returns the simple distance from the star centroid to the lock point. But in the Log Viewer (and I think also in the application but I havent checked) the RMS calculation subtracts the mean of the set from each sample before then calculating the RMS. So what is shown on the PHD2 screen is the RMS about the mean offset. What I think of as the "DC offset".

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