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How is PHD2 RMS actually calculated?


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I always assumed that PHD2 RMS is root mean square of guide star displacement - as in (expected position - measured position), but this seems not to be so.

Here is quote from PHD2 manual:

Quote

The major portion of the window shows the detailed displacements of the guide star for each guide exposure, plotted left-to-right. Normally, one line shows displacements in right
ascension while the second line shows declination displacements. However, you can use the 'Settings' button to the left of the graph to switch to camera (X/Y) axes if you prefer (not
recommended). You can also use the 'Settings' button to switch between display units of arc-seconds vs. camera pixels or to change the colors of the two graph lines. The range of
the vertical axis is controlled by the second button fromthe top, labelled y:+/-4" in this example. The range of the horizontal axis - the number of guide exposures being plotted - is
controlled by the topmost button, labelled x:50 in this example. This scale also controls the sample size used for calculating the statistics you see in the lower left part of the graph
window. These values show the root-mean-square (RMS or standard deviation) of the motions in each axis along with the total for both axes. These are your best estimators of
guiding performance because they can be directly compared to star sizes and seeing conditions

First part of quote is what I expect - graphs are actually difference between expected and measured star position, but then (my emphasis) sentence goes to explain that RMS value is relation to motions of the mount rather than star position.

It looks like RMS is RMS of corrections rather than star position?

Another piece of data to corroborate this is screen shot of my guiding that I have:

image.png.b98a15507bb9f9a411dc5ae7db159c77.png

So calculated RMS is 0.36", but if we look at this graph:

image.png.2d86f81fb09a681ca1e21824dde78e1d.png

It appears that 0.5" mark divides hits in 1:2  (around 50 and something inside 0.5 and 27 outside) - or ~67% of fall within 0.5" and not 0.36". Single RMS contains ~68% samples (68.27% to be more precise).

This would mean that 68.27 hits should be inside 0.36" diameter - but there is much less (if only 67% is within 0.5).

Why is this important?

Well, because RMS can't then be used as accurate measure of guide performance. It shows just how much you made corrections - but if your guide settings are such that you don't make corrections when you should - it will look like good guiding rather than bad. Take for example min mo parameter.

Set it too high and mount will not make corrections at all - guide RMS will be 0 but hits (expected vs measured star position) will be all over the place

 

 

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As I understand it the centroid of the star movement back to the center is your RMS, taken .. rms is the average, peak to peak is the total movement

I'd have thought you of all people would be in arc secs rather than pixels

Skipper Billy mesu makes no corrections but the rms is there  

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