How phd2 does compute rms?

[Miguel G]

Hi all,

Is the rms calculation based on the star guide or mount displacements?

An extreme example, with one axis (ra). Having a completely horizontal RA line in -2'' will show a rms of 2 or 0?

Is RMS computed from absolute deviation from the lock point (guide star reference) or relative variation in the curve? I guess the former applies, but just to be sure.

Regards

[Bruce Waddington]

It's the standard deviation of the sample which means it's measuring the dispersion of points around the sample mean.  The points themselves are offsets from the lock-point, not the guide corrections.  In your edge-case example, the mean of the sample would be -2 and that would be used in the standard deviation calculation:

The fixed offset you were asking about - something the EE's sometimes call a DC offset - is intentionally ignored.  The purpose of the calculation in the context of guiding is to create a figure of merit that is correlated with guiding "quality", meaning taking into account errors that create elongated stars or smearing in an image.  A fixed offset doesn't do that, it's no different than a small pointing error arising from a slew to the target.  Of course, with a properly functioning system, the mean is typically near-zero and the discussion becomes moot.

Regards,

Bruce

[Miguel G]

Hi Bruce,

Thanks a lot for the kind reply and detailed explanation. All clear now.

Have a nice day.

Regards

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/0f562b57-af85-4d55-a8a8-9ac1ecf7892cn%40googlegroups.com.

[Miguel G]

Sorry Bruce but now reading twice; that means that in my edge case where the mean and every point is -2, the rms would be 0, right?

[bw_m...@earthlink.net]

Yes, that’s right.

 

Bruce

 

From: open-phd...@googlegroups.com <open-phd...@googlegroups.com> On Behalf Of Miguel G
Sent: Friday, August 29, 2025 12:53 AM
To: open-phd...@googlegroups.com
Subject: Re: [open-phd-guiding] Re: How phd2 does compute rms?

 

Sorry Bruce but now reading twice; that means that in my edge case where the mean and every point is -2, the rms would be 0, right?

 

El vie, 29 ago 2025 a las 8:15, Miguel G (<mguti...@gmail.com>) escribió:

Hi Bruce,

 

Thanks a lot for the kind reply and detailed explanation. All clear now.

 

Have a nice day.

 

Regards

 

El jue, 28 ago 2025 a las 23:58, Bruce Waddington (<bw_m...@earthlink.net>) escribió:

It's the standard deviation of the sample which means it's measuring the dispersion of points around the sample mean.  The points themselves are offsets from the lock-point, not the guide corrections.  In your edge-case example, the mean of the sample would be -2 and that would be used in the standard deviation calculation:

 

 

The fixed offset you were asking about - something the EE's sometimes call a DC offset - is intentionally ignored.  The purpose of the calculation in the context of guiding is to create a figure of merit that is correlated with guiding "quality", meaning taking into account errors that create elongated stars or smearing in an image.  A fixed offset doesn't do that, it's no different than a small pointing error arising from a slew to the target.  Of course, with a properly functioning system, the mean is typically near-zero and the discussion becomes moot.

 

Regards,

Bruce

 

On Thursday, August 28, 2025 at 2:30:34 AM UTC-7 mguti...@gmail.com wrote:

Hi all,

 

Is the rms calculation based on the star guide or mount displacements?

 

An extreme example, with one axis (ra). Having a completely horizontal RA line in -2'' will show a rms of 2 or 0?

 

Is RMS computed from absolute deviation from the lock point (guide star reference) or relative variation in the curve? I guess the former applies, but just to be sure.

 

Regards

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/0f562b57-af85-4d55-a8a8-9ac1ecf7892cn%40googlegroups.com.

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CABpRrO9MwHRBPnuzGEdHAABOBgOoW2Q_FFa6PNCs6HGj2EeB1w%40mail.gmail.com.

[Miguel G]

great, thanks again

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/002301dc18f8%2442837620%24c78a6260%24%40earthlink.net.