Manual Guide not moving mount

[Jay]

I have something strange going on. I'm trying to fix dec backlash, and tried to do a star cross test. Running the test through PHD2, I would get inconsistent results. Sometimes I would get a single line, sometimes the stars look like they did not move at all. I tried to do a star cross with manual guide, but when doing so I found that manual guide would not make the mount move at all. I increased the guide size to 5000ms, and tried each direction multiple times but no movement happened. Interestingly enough though, when trying a manual guide direction the camera would seem to stop taking pictures, and the screen would update after the 5000ms expired. Process Lasso would show restraint from ProBalance on PHD2, so I stopped that from happening and optimized windows with no luck.

NINA can slew in all directions, calibration works in all directions, and guiding "works" in all directions.

I'm not sure what could be causing this.

[Bruce Waddington]

Just a reminder - when you joined the forum in January, the 'welcome' message you got included this:

When you need help, be sure to follow the instructions for uploading your log files to our server (the “getting-help” link).  If you don’t do this, you are unlikely to get much help because discussions here are fairly technical, and we try to provide advice on the basis of objective data rather than personal anecdotes.

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

Regards,

Bruce

[Jay]

Apologies. 

https://openphdguiding.org/logs/dl/PHD2_logs_Gab2.zip

[bw_m...@earthlink.net]

Well, there are many problems here, so it's hard to cover them all.  If we start with your initial concern, Dec backlash, we can see a typical result of the GA backlash test:

 

 

You can that the Dec axis never started moving in response to a continuous stream of 1000ms south guide pulses.  This is a fairly extreme example but there is clearly a mechanical problem with the axis drive system, and bi-directional guiding in Dec is not going to work well until the problem is fixed.  When making adjustments to the drive system, you have to walk a fine line between "too loose" and "too tight".  "Too loose" creates simple backlash, "too tight" creates stiction (static resistance) and it can be hard to distinguish the difference simply by looking at guiding data.  It's equally important that the scope is well-balanced in Dec.  Seeing that the axis moves ok during slewing tells you nothing - slewing typically applies at least 50X of the mechanical energy that's used for pulse-guiding.   This problem is most serious when the Dec axis needs to be reversed in order to move by a substantial distance.  These situations occur when you require large dithers in Dec which is what you were doing - often as large as 24+ arc-seconds.  Are you sure you need to be dithering by such large amounts?  The other reason the problem occurs is because you appear to have unwanted mechanical movement of the guiding assembly.  One typical example looks like this:

 

 

This is a huge amount of movement in the context of guiding.  It isn't caused by guiding, it's the result of unwanted movement in the guiding assembly or, less likely, in the entire telescope.  The unwanted movement can be caused by loose fasteners or by cable snags/drags, but whatever the source, you will need to eliminate it in order to make any progress.  These unwanted movements are also a reason for the many lost-star events you see.  These can be further reduced by disabling the 'star mass detection' option on the Guiding tab of Advanced Settings.

 

You also made some operational errors that further complicate the picture.  You didn't bother to create a dark library which is a basic requirement.  You should also stop trying to use your session manager app until you've developed a clear view of what's going on with the equipment and have resolved the basic problems.  You can't automate a system that isn't working reasonably well on its own.

 

Regards,

Bruce

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[Jay]

Thanks for your input thus far. There certainly is a lot going on here. Unfortunately this is my first mount, and I’m working hard to figure out exactly what the issue is, aside from the obvious significant backlash. I’ll leave my questions listed:

1. Are you suggesting I’m dithering too far in the Dec axis? I don’t remember changing this setting, and only changed dithering to spiral as I’ve heard it’s better for backlash.

2. I don’t think it’s the guiding assembly, but rather the telescope itself. The Dec axis seems to have no play when there is no telescope mounted, but significant play when the telescope is mounted. Further, as far as backlash and the Dec axis are concerned, for whatever reason, there is a difference in backlash when the scope is pointed E/W along the Dec axis; when pointed west there can be backlash even though there is no backlash when pointed east. I feel as if balance could be at play here, but it seems like there’s no combination of tightening/loosening the worm gear adjustment that resolves this without making one side too tight/too lose. Does this sound like this is a problem of the assembly itself?

[Michael Waring]

"you require large dithers in Dec ......... 24+ arc-seconds."

If you are Dithering the imaging camera the usual 12 pixels, that suggests your imaging pixel scale is 24arcsec/12 = 2 arcsec/pixel.

Correct ?

If it's only 1arcsec/pixel then you only need to Dither 12arcsecs max.

Michael

Wiltshire UK

[steve]

I think you have the telescope balanced in RA and DEC, but not the third axis. With the clutch free, the telescope must stay put no matter where it is pointing. Add weights east/west to make this happen. Don't know which telescope you have but this should give you an idea...
HTH

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[Jay]

Interesting point. I did a quick trial, and it seems like the scope stays wherever I point it, and I don’t see any drifting in any direction.

I’m using an EQM-35 pro with a belt drive instead of the original gear drive. I can definitely say that I can physically feel the play in the dec axis, and see it along the length of the OTA. It would be difficult to say whether or not this has an affect on imaging, as I’ve had good and bad results. Determining what caused the bad results exactly might be challenging. 

If it’s binding in one direction, does that point to gear misalignment?

[Jens Scheidtmann]

Dear Michael,

Michael Waring <treasurerb...@gmail.com> schrieb am Sa. 11. Apr. 2026 um 10:40:

"you require large dithers in Dec ......... 24+ arc-seconds."

If you are Dithering the imaging camera the usual 12 pixels, that suggests your imaging pixel scale is 24arcsec/12 = 2 arcsec/pixel.

Correct ?

If it's only 1arcsec/pixel then you only need to Dither 12arcsecs max.

Michael

Wiltshire UK

What’s the reasoning behind having at max 12 imaging pixels dither?

And why would you want to dither such a large amount?

I believe there are 5 main reasons for dithering:

A) banding in the imaging camera

B) Fixed-pattern noise in the stacked image

C) Dealing with hot and cold pixels  

D) You want to have different sub-pixels (different color filters) sample the color of a star, i.e. getting better colors in your OSC camera by averaging different positions of the star on the sub-pixels (Usually there‘s a 2x2 matrix of these sub-pixels, so sub-pixel height and width are 1/2 of the pixel pitch).

E) You want to drizzle, i.e. increase the spatial resolution of your image, by leveraging the statistics of having the pixel boundaries at different physical positions.

(A) needs really large dithers (10th of imaging pixels), if dithering fixes this at all. 

In case (B) I would expect that a couple of imaging pixels (3-4) are enough. As the total distance goes with avg(step size)*sqrt(N), N being the number of sub-frames you take, which will dominate for large N. (sqrt(N) being the RMS distance from the origin after N unit steps of a random walk, see

https://math.stackexchange.com/questions/103142/expected-value-of-random-walk)

In (C) I would expect hot pixels to be dealt with by Darks and cold pixels by an amount of 3-4 pixels as in case (B). 

In (D)+(E) random movements by 0.5 imaging pixels max provide enough movement. 

So in most cases, dithering 3-4 imaging pixels should be enough. 

I appreciate any comments, pointers or corrections.

Best regards,

Jens

[steve]

The axes should be free to rotate. A small tap with your little finger should be enough to move either axis. It should then come to a stop and, more importantly remain there.

HTH

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[wes McDonald]

Hi Bruce 

Is that Backlash Analyzer a program you could post?  I have never been able to see the backlash graph except during the guiding assistant test which shows it but is ephemeral.  All these years I have been just taking pictures of the thing.   I didn’t think the PhD log viewer shows it? 

Thanks

Wes. 

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[Bruce Waddington]

The balance of the scope on the mount has nothing to do with the worm gear adjustment, it has to do with the balance of the payload on both mount axes.  Dec balance involves positioning the scope/camera payload at the neutral point in the dovetail plate or the mounting rings, whichever you have; and RA balance involves moving the counterweights on the Dec axis up and down to achieve balance in RA.  You should be able to loosen both clutches and move the scope manually and see that it doesn't move once you release it.  You're not trying to convince yourself that everything is "probably ok" at this point, you're trying to find a problem. When you've moved the scope to a particular position, you need to gently poke it in both Dec and RA to see if it tends to drift in one direction more than the other.  Unless you're using a Newtonian type scope where the camera payload is positioned well away from the optical axis, you probably don't need to worry about 3D balance.  If you do all this and you still see a big difference in backlash behavior on opposite sides of the pier, it probably is a mechanical problem - e.g. something may be slightly out of round.  When you switch pier-sides, you're rotating the Dec worm wheel by as much as 180 degrees so the mechanical interface to the worm wheel is changing.

Bruce

[Bruce Waddington]

Hi Wes.  The GA's backlash test results aren't as "ephemeral" as you might think. :-)  When you run the GA, look at the bottom-center button - it will say 'Review previous'.  That lets you look at a history of the last <n> results - I don't remember, I think 'n' might be 5.  But when old results are aged out, the last previous backlash test run is kept in the list.

I'll post a link to the backlash test analyzer app when I get the chance.

Bruce

[Jay]

Well, I've made some headway after following some advice here... as much as I hate calling this an improvement.

Clearly it is not a good star cross test, but at least I can see all directions in a single frame. Though I'm not quite sure why the "up" direction is straight and the "down" direction has a noticeable curve (backlash still?). Interestingly enough, I ran a second one immediately after:

I assume this is due to significant backlash worsening with each iteration as the mount gets stuck between N/S. Despite this, calibration assistant showed my south moves at approximately 10px, at South 1 which is a vast improvement considering I often would see the south moves create no appreciable change in px distance. 

Sorry if my units of measurement are incorrect, long day at work and back in tomorrow.

[Jay]

I took the advice here: revisited mount mechanics, dark libraries, balance, dither settings. I seem to have had a decent enough night last night, though I think what I'm seeing is still a good amount of backlash increasing total RMS which I will probably accept as this mount will be used for wide-field when traveling. If anyone is willing to evaluate in case they see something different, it would be appreciated.

https://openphdguiding.org/logs/dl/PHD2_logs_i6yt.zip