Unexpected long term drifts in position after each dither is applied

[David Wilden]

Hi,

 This is my first posting, and apologies for its length...

Even though I've polar aligned to about 1 arc min overall error (using Sharpcap's tool) and each sub has circular (not elongated) stars , I'm still seeing a sizable systematic drift superimposed on the random dithering applied by PHD2. 

 I’ve done some analysis of the PHD2 log files to try to reconcile the image shift between subs as measured in Deep Sky Stacker with the dither movements recorded in the PHD2 log files but cant work out where the discrepancy is coming from, so any help understanding this would be greatly appreciated.

 My imaging system is a skywatcher 72ED refractor (FL 420 mm) with sv605CC camera, (pixel size 3.76 um) with an Orion Guide scope (FL 62 mm) with sv305 camera (pixel size 2.9um).  This is mounted on skywatcher AZ-GTI mount on an equatorial wedge. I believe the system is well balanced on both axes. 

 I've attached two picture files each containing a graphical plot of the data recorded for a different run which show the problem. In the first run, dithering was applied after each 4 min exposure, and in the other run, dithering was applied after each third 2 min exposure.

 Each plot shows:

In blue, the (X.XXX, Y.YYY) dither values from the lines in the log which say “ INFO: DITHER by -X.XXX, Y.YYY, new lock pos = AAA.AAA, BBB.BBB

In green, the  lock position values (AAA.AA, BBB.BBB) values from the same log lines.

In red, the image offsets calculated by Deep Sky Stacker when it aligns the subs.

Note: In order to be able to compare the three data sets on the same axes, I have subtracted fixed offsets from each data set to make the first data point in each series equal to (0,0).

Also I have scaled the PHD2 data sets by multiplying them by 2 to convert from guide scope pixels to imaging scope pixels.   (Pixel ratio =   420 x 2.9/162/3.76 = 1.9996)

Data set  1 (2025-10-11)

Each sub was 4 mins with dithering applied after each sub with range (+/- 10 guide pixels = +/- 20 imaging pixels). In total there are 17 subs and 17 dithers.

The PHD2 log analyser calculates that the polar alignment error was only 1.6 arc mins for this run and that the rms error reported by PHD2 over the 17 exposures was  0.60 guide pixels (1.2 imaging pixels).

The blue dots (dither positions) in the figure show the expected range of dithering values, with pseudo random dithers within a +/-20  imaging pixel range.

The green line (PHD2 lock positions)  and the red line (DSS offsets) both show a large  systematic drift during the run which is is not what I  expect given that the dither steps are roughly random in direction and magnitude.

Overall the drift over a period of 17 subs was more than 90 image pixels which equates to an average of more than 5 pixels per sub. Yet the star images in the subs show no significant visible elongation, which means that there cannot be a significant drift while each 4 min exposures was being made.

Data set  2 (2025-09-25)

Each subs was 2 min exposure and dithering of +/-5 guide pixels (+/-10 image pixels) was applied after every third sub. In total there are 60 subs and 20 dithers.

The blue dots show the expected range of random dither values.

However the green and red lines again show a large   systematic drifty during the run.

The DSS data shows another feature which demonstrates that the systematic shift only occurs when dithering occurs. The DSS data points appear as closely spaced triplets. Within each group of three no dithers have been applied and the points are close together suggest very small drift.  Then there is a large jump in position between each triplet when the next dither is applied.  The mean shift between exposures within each triplet is only 1.0 imaging pixels, which confirms that no significant drift is occurring while the exposures are being made.

Conclusion

The captured PHD2 and DSS data for both runs show very large (up to 10 pixels or more) steps in position when each dither is applied and this seems to result in a systematic (i.e non-random) drift of up to 90 pixels over a 1 hour time period. This does not seem to be consistent with the fact that captured dither values do appear to be roughly random in magnitude and direction.

The data and the quality of the star images and the measured polar alignment accuracy all indicate that any drifts in star position during the  actual 2 to 4 min exposure times+ were negligible  ( less than 1 pixel). This means that polar alignment accuracy and periodic errors in the mount cannot be the cause of the systematic jumps seen after each dither.

I am not sure how the new lock positions are meant to be calculated when each dither is applied (there is no explanation or even a definition of what lock position means in the PHD2 manual) . I expected that each new lock position would just be the old position plus the new dither but that is not what the log files show.

I have noticed that PHD2 is reporting settling errors after many of the dithers were applied. Is it possible this is causing the drift? If so any suggestions on what setting to change to get round the timeout would be much appreciated.

[Bruce Waddington]

You need to upload both your guide and debug log files using the built-in function in PHD2 following these instructions:

https://openphdguiding.org/getting-help/

Setting aside all this other stuff, can you summarize your issue?  If you are dithering, you will obviously see shifts in the frames from your main imaging camera.  Are you saying these are larger than they should be?  What are the imaging scales (arc-sec/px) on both your imaging and guiding systems?  

Bruce

[Paul Goelz]

On 10/25/2025 3:26 PM, David Wilden wrote:
> Even though I've polar aligned to about 1 arc min overall error (using
> Sharpcap's tool) and each sub has circular (not elongated) stars , I'm
> still seeing a sizable systematic drift superimposed on the random
> dithering applied by PHD2.

I image with an AT115 refractor and an AT60 guide scope. Over the years
I have had times where there was noticeable drift from sub to sub. And
very occasionally, I had slightly elongated stars. The drift was
diagonal and would (usually) reduce in amplitude over the course of the
imaging session.

In troubleshooting, I always started with the assumption that the drift
was NOT due to PHD2 so it MUST be flexure.... relative movement between
the guide scope / camera and the main scope camera. My imaging train is
screwed together so that part is not flexing. But each time when I
tightened things like the guide scope rings and the main scope focuser
bushings (its a rack and pinion), the drift either vanished or became so
minimal that it was no longer an issue. Do not discount the effect of
all that weight hanging off the main scope focuser as the scope tracks ;)

Paul

--
Paul Goelz
Rochester Hills, MI
pgoe...@gmail.com
www.pgoelz.com

[David Wilden]

I thought I had given a full explanation of the problem in  the original posting and also a summary at the end under the heading "Conclusion". But to repeat, the problem is that I am seeing large systematic drifts from sub to sub which happen only when dithers are being actioned, and not between dithers. 

I will attach the log files as requested in a seprate update.

dave

[David Wilden]

Is it possible to delete or close this thread, as I realise that its an unintential  duplicate of the posting I made just before it and doesn't have the two analysis files attached which the original ones does have.

To explain, I'm new to the Forum and this is my first post. When I submitted it last night the title appeared in the list of messages, but when I clicked on it, it said it had been deleted, so I thought I had done something wrong and tried to send it again but forgot to reattach the analysis images.  I now understand it had not been approved at that time, rather than having been deleted which is what the message actually said.

Sorry

Dave

[David Wilden]

I've attached the  guide logs for the two runs and here is the link to the debug logs.

https://openphdguiding.org/logs/dl/PHD2_logs_DEtS.zip

Regards

Dave

[David Wilden]

Also now attached the two images of the graphs I referred to in the original post.

[Bruce Waddington]

Hi Dave.  I think you're probably barking up the wrong tree here with your focus on dithering.  I think the problem you have is probably differential flexure.  Your long guiding session on 10/25 provides some good insight once we exclude the long section at the end where you lost the guide stars, probably because you were pointing down in the weeds.  So a useful time period looked like this, showing only RA in red:

You can see nearly all of the RA corrections are "up", meaning they are guide corrections to the west.  This isn't normal at all for RA which typically moves back and forth between east and west corrections.  It's possible that the sidereal tracking of the mount is off but I think this is very unlikely.  It's more likely that the guiding assembly is loose and is moving around independently from the main scope.  PHD2 sees this behavior as a large rate of drift in RA:

PHD2 is staying very busy correcting for this at a rate of 1.47 px/min or about 90 px/hour on the guider image scale. I think it's likely that your main scope isn't seeing this drift and consequently the main camera images suffer from all these (unnecessary) RA corrections.

This behavior also explains the frequent failed settling periods.  You can see that the RA trace in the graph stays below the x-axis for extended times - it is generally under-correcting due to the combination of the high drift rate and the 80% aggressiveness value.  This under-correction after a drift is big enough to fail your settling requirement of 1.5 pixels for 10 seconds.  This is further compounded by far too many lost-star events.  You should disable 'Star Mass Detection' on the guiding tab of the Advanced Settings dialog.

There are other mechanical problems with your setup but perhaps it would be better for you to fix this RA drift problem before getting into all that.

Regards,

Bruce

[David Wilden]

Hi Bruce,

Many thanks for looking at the logs in such detail.

I agree it looks like the mount is constantly trying to push the mount west but I dont understand how this could be due to relative mechanical flexure between the guide system and the imaging system. I can see how any "sloppiness" in the guide scope mounting would increase the random errors but what sort of mechanical flexure could cause a long term systematic drift?

I've looked at my setup and can't see any obvious loosness. Both the imaging scope rings and the guide scope adjustment ring screws are tightened down, as is the screw which holds the guide scope mounting bracket  to one of the main scope rings. So I cant see what I can do to improve this, anyway

This weekend I did a little experiment where I captured about 15  subs, each of 2 min exposure on M33, first with guiding off, then with guiding on without  dither, then with guiding on with dither after every sub.  I've attached a plot of the alignment errors from Deep Sky Stacker for each run as well as the guide logs and debug log link.

https://openphdguiding.org/logs/dl/PHD2_logs_sWck.zip

The results (starting at 21:03 in the log) show a very regular drift of  approx -3 image pixels in RA per 2 min update when guiding without dithering is on  This equates to  about -2.7 arc secs per min.

This drift is not seen when guiding is off - there is still some movement between exposures, but it's much smaller and is primarily a drift in Dec with random variations in direction in RA. I think this is normal result of  imperfect polar alignment and possibly inherent mount periodic errors. I think this rules out the systematic drift being due to a systematic error in the mount's sidereal rate or an imbalance in the payload of the mount as this would affect the unguided performance as well.

Is it possible that rather than being due to flexure, the systematic error when guiding is due to field rotation caused by the axis of my guide camera not being well aligned with the main scope. I believe its about 1.5 or more degrees out, but not sure in which direction. The reason its so far out is that the Orion Guide scope has two adjustment screws and one spring-loaded pin. When well-centred, there is a real chance of the flexure as  it relies on the spring pressure keeping the scope fixed.  To prevent that I had to tighten the other screws down which pushed the scope out of alignment.

If you think its feasible that this is the cause, I can probably devise some sort of shims to allow me to lock down the guide scope in a better alignment.

Regards

Dave

[Bruce Waddington]

Perhaps you could send us a photo of your whole setup with a wide enough view to show the main scope, the guiding assembly, and the cable routing scheme.  I can't say whether the guide scope mis-alignment could cause this much RA drift but I doubt it.  Keep in mind that a shift in the guide camera sensor of 3 microns will create an apparent guiding error of 3.5 arc-sec.  A human hair is typically about 50 microns thick.  From your description, I suspect your guiding arrangement isn't very robust.

Bruce

[David Wilden]

Hi again,

I will take a photo of my setup tomorrow when it's light and post it.

In the meantime, I have been looking back through all my saved guide logs amd noticed something interesting.  All the runs where the target was west of the meridian don't show a systematic RA offset and all the ones to the east do, typically causing a drift rate of  ~-20 " per min. For targets to the west of the meridian the RA values are randomly distributed around zero as they should be.  

Then I  located two runs where the target crossed the  meridian during the guiding, and I can see the change from tracking below zero in RA to tracking around zero very clearly in the tracking data just at the meridian crossing.  I have   posted several screen shots showing this.  I have also attached the tracking log for 2025-10-25 which holds the data for two of the examples in the screen shots.

I think its interesting that the tracking drift/offset problem seems to disappear completely as it  crosses the meridian, rather than the offset  in RA remaining but changing sign.

Regards

Dave

[Bruce Waddington]

This sounds like a good analysis.  You will need to do a very careful check of the guiding assembly on both sides of the meridian but if something was simply loose, you wouldn't expect the problem to disappear on one side vs. the other.  Of course, that assumes the range of hour angles are roughly the same for the data sets you are looking at.  But a more obvious, simpler explanation would be a cable pulling or cable sagging problem that appears on the east side of the meridian.

Regards,

Bruce

[David Wilden]

I've attached some photos of my setup as requested.  I agree the length of hanging cables could be causing issues with tracking. The USB cables are quite light and flexible  fabric covered ones, not the heavy rigid ones that came with the camera, but the cooler power one to the  is more rigid. They were tie-wrapped to the one leg of the tripod but there is clearly a decent length of cables hanging down off the cameras, so not ideal.

I've just now changed it as per the last photo. The cables are now anchored to the dovetail  near the mount axes and also to the mounts pillar, so hopefully that will minimise any cable drag.

In the future I will probably get a miniPC to mount on the tube assembly so that will be better again

I checked yesterday and the guide scope was misaligned by 3 degrees relative to the main scope, not 1.5 degs as I thought. I've now aligned them anyway.

I've attached a close up of the way the guide camera attaches to the main telescope rings. the bolt and washers  arrangement may be a source of flexure, so I may put the mounting bracket back onto the telescope tube where it used to be which would be more rigid. I moved it because it was off  axis so harder to balance the system at all positions.

I'll see how this works next time I have a clear night.  Any other suggestions greatfully received.

Regards

Dave

[Bobby]

You have to find a better way to mount your guide scope. Check out:

https://www.cloudynights.com/forums/topic/767238-show-your-guide-scope-mounts/

for some ideas. It's way too high with that 'stalk', which contributes to in-balance and flex, and you only have single screws securing things. At the very least, replace the "thumb" screws with ones that allow you to use a tool to tighten.

You need to 'dress' those cables as well. See:

https://cosgrovescosmos.com/astrogear/wiring-and-cable-managing-your-telescope-platform

You may not think the weight of a USB cable can be important, but it is. Having them drag along the ground is not only bad practice, but could put your scope in danger if there is a snag.

Hope this helps.

Bobby

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[steve]

Hi
No. No chance with that arrangement.
1. Bolt the guide telescope in rings on a plate like this:

2. Now bolt that across the top of you main telescope rings.

3. I don't think the sw Crayford will hold the camera at that distance without flex. Alas, I don't think there are decent replacement focusers for the 72ed. One think which helps is to remove the rubber washers in the focuser and replace without using a paper/thin-card gasket instead.

4. Get those cables tidied! Make absolutely certain that none will drag on stationary surfaces as the telescope tracks. Check at ALL possible telescope angles.

HTH and CS,
Steve

[Bobby]

On 10/30/2025 7:29 AM, steve wrote:

3. I don't think the sw Crayford will hold the camera at that distance without flex. Alas, I don't think there are decent replacement focusers for the 72ed. One think which helps is to remove the rubber washers in the focuser and replace without using a paper/thin-card gasket instead.

He is not using an electronic focuser, so tightening the focuser down should suffice in eliminating any flex. I have several Skywatchers with similar focusers and they can be 'tuned' to work fairly reliably. (There are many references online.) He is screwed into the scope, which is very good.
He may want to consider getting a longer Vixen bar, or moving the one he has. From the pictures, it looks like getting a good balance is difficult. 
Skywatcher "ED's" are very prone to edge distortion, especially with larger sensors. This may mislead someone into thinking that the guiding is poor.
But all that being said, I see no reason why good images cannot be made with what is available. Remember: "a micron here and a micron there, and pretty soon your talking real movement".

Bobby

[David Wilden]

Hi,

I've made some more changes to make the guide camera mounting much more solid (see attached photo) and also routed the cables a lot better.  I've not had a clear night to test the impact of the changes yet. If necessary I will invest in a pair of tube rings to make the guide camera mounting even more rigid but I'm hoping what I've done is enough given that my camera pixels are 1.8 arc secs.

I would like to understand, though, the mechanism by which differential flexure causes a systematic drift in RA.  I can understand how it would increase the random error, but how does flexure cause the mount to continuously lag in RA? 

Regards

Dave

[steve]

Good modifications. But...
One of the main movement inducing items is the barrel into which the guide telescope slides. Weak spring loaded screws with a flexible rubber ring. If the drift remains, that's where I'd look next: lose the stalk.
HTH and CS,
Steve