RA drift on the EQ8-R mount with PHD2 - Forums

Topic starter

3 hours ago

Dear star friends,

For over a year, I've been trying to find the cause of the RA drift on my EQ8-R mount by using PHD2 and I need your help.

📷 20260406_220956.jpgMy EQ8-R is equipped with a mounting plate including two clamps for attaching two telescope tubes in parallel. A C14 and a RASA11 are mounted there in parallel. The total weight is certainly close to the maximum load limit, but it doesn't exceed it. Furthermore, the system is so well balanced that it's always in equilibrium and doesn't exert any uneven pressure on the stepper motors.

When I use FireCapture to guide a planet at a long focal length with the simple, uncalibrated tracking tool, the EQ8-R has absolutely no problem keeping the planet centered in the frame for hours.

However, when I use a reducer with a much larger camera and guide with PHD2, a considerable positive RA drift occurs after only 5 minutes (see first picture above), even though the guiding values according to PHD2 seem to be within a good range:

📷 20260406_220655.jpg📷 20260406_221606.jpgIn the declination direction, I noticed a backlash with the SynScan control and corrected this mechanically on the mount.

📷 20260406_221632.jpgHowever, this error had no effect on the guiding and doesn't help me with the RA problem. I haven't detected any backlash in RA, so a mechanical correction on the mount shouldn't help, right?

I'm wondering why PHD2 isn't able to keep the guide star in its starting position and doesn't correct until the starting point is reached again. FireCapture manages this without a complex multi-star method.

Does anyone have any idea what I need to change in PHD2 to prevent the drift? Or is there a problem with the EQ-8R that I've overlooked?

CS Henning

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Tony Gondola

Frequent contributor

Exemplary contributor

1 hour ago

Are you dithering? Does Fire Cap keep the object centered when you turn its tracking tool off? Is your PA perfect? Are your scopes perfectly orthogonal to the mechanical axis of the mount? Are you sure you have zero deferential flexture?

I find that over time my framing will change slightly for a number of reasons. I use the “center after drift” command in NINA’s advanced sequencer to correct that. Its a part of the imaging loop and checks positioning with a plate solve after a number of frames that I specify. If the change in framing shifts beyond a given distance, it recenters.

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andrea tasselli

Frequent contributor

41 minutes ago

when you guide with multi-star you are effectively following the frame and if you have field rotation (that is, if your PA isn’t perfect, literally) then you would experience both RA and Dec drift whose magnitude depends on both the PA error and your image scale (which I get is pretty high). The rate depends on the distance from the pole.

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Henning Schmidt

Topic starter

36 minutes ago

Thanks for your feedback, Tony!

I'm not consciously using dithering in PHD2. I hope it's not automatically or by default enabled somewhere in PHD2. But even if dithering were active, shouldn't the movement be circular?

In FireCapture, the object is usually kept centered for up to 30 seconds with tracking disabled.

The polar alignment is almost perfect; the object is nearly centered in the field of view after every GoTo movement. The telescopes should also be perpendicular to the mount's mechanical axis.

To what extent do you think lateral curvature could affect the guiding?

Is there a "Center After Drift" command in PHD2?

My problem is already the drift within a 5-minute exposure.

I would appreciate your feedback.

Clear skies, Henning

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Henning Schmidt

Topic starter

29 minutes ago

andrea tasselli · Apr 11, 2026, 01:55 PM
when you guide with multi-star you are effectively following the frame and if you have field rotation (that is, if your PA isn’t perfect, literally) then you would experience both RA and Dec drift whose magnitude depends on both the PA error and your image scale (which I get is pretty high). The rate depends on the distance from the pole.

Thank you for your feedback, Andrea!

Since the polar alignment is relatively precise and the exposure time relatively short, I don't believe field rotation is the cause here. For me, it only becomes clearly noticeable after several hours.

CS Henning

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Tobiasz

20 minutes ago

Whats your guiding setup? I guess you’re using a guidescope instead of an OAG because of the higher number of stars available in PHD2. If you’re using a guidescope it will be differential flexure most probably.

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