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Dithering With PhD2 and Nina

ZWO ASI120MM Mini ZWO ASI294MC Pro Acquisition Dithering Deep Sky
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Benjamin DeHaven avatar
Benjamin DeHaven avatar
How long should a dither take? I currently have my settle timeout at 80 seconds and often get a settle timeout error and have to manually stop the frame and restart it after the settling takes effect. I never used to have this problem and am going to try a new st-4 cable next time I go out based on my research. I currently am having problems with a 120mm guide scope with a ASI 120mm mini guide camera. The main scope is a135mm Rokinon with an ASI 294mc pro. I have the dither set for 5.5 pixels and the settle tolerance set to 2 pixels. I have tried many combinations of numbers and the problem is not solved hence my plan to switch cables. The rig guides very well but dithering is causing me great pain lately. Any additional info you need to guide me (pun intended) to an answer let me know and I will provide. Thanks! 

Benjamin
Christian Großmann avatar
Christian Großmann avatar
Hi Benjamin,

this sounds strange. In my case, dithering usually takes some seconds including settling time. But from time to time, I see those timeout messages on one of my mounts which has more backlash in its mechanics. This you can even see in the PHD2 graph. My DEC axis has challanges to correct in this case. I am not sure, if the timeouts only happen if the scope is pointing to a specific position. It can be, because usually all works well. Maybe the balancing of my equipment is a bit off. Then, the mount will have problems with guiding corrections only if the axis are nearly horizontal.

If the problem occours only lately in your case, maybe it's time to readjust the mount. I found that the usual guiding has no problems, but dithering has. Here the correction steps are larger.

These are just some thoughts…

CS

Christian
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Marc V avatar
Marc V avatar
You did not mentionned which mount.

Before going st-4, I would test the USB cables. 

You also mentionned issues with you guide camera, do you use a usb hub? It is powered?
Benjamin DeHaven avatar
Benjamin DeHaven avatar
Marc V:
You did not mentionned which mount.

Before going st-4, I would test the USB cables. 

You also mentionned issues with you guide camera, do you use a usb hub? It is powered?

Gee, the mount. Can't see why you might need that info! The mount is a Skywatcher HEQ-5. I do have a powered USB hub. Connection to the guide camera is solid...
Benjamin DeHaven avatar
Benjamin DeHaven avatar
Ok, still  having issues. Downloaded PHD2 Log Viewer. It seems that my mount suddenly stops responding to the Dec pulse guide and then suddenly overshoots. Is that what I'm seeing? And if so, the cause is backlash maybe, or maybe time to regrease the gears? Why is it mostly only showing up during dithering? Am I on the right track here? I exposed the gears in my HEQ5 and I didn't notice any real difference in backlash between the RA and Dec gears. And everything seemed as I remember it when I first opened it up a year ago. Thanks in advance!



Same Time Period, Zoomed in on the Y axis:


Benjamin
Ali Alhawas avatar
Ali Alhawas avatar
A quick thoughts..
Its backlash issue..
or USB cable need to replaced..
You can test backlash in PHD2 I suppose or NINA by manually move Dec. in opposite direction and see how does it response ?
Goodluck !
Frédéric Ruciak avatar
Frédéric Ruciak avatar
Hi, according to the graphs I see, it seems that DEC is having a lot of backash. When you see a serie of pulses in the same direction and the DEC not responding and then overshooting it looks like backlash. Have you tuned PHD2 according to your backlash? The very first thing to do with PHD2 is to run the guiding assistant with backlash measurement ON, and PHD2 will finish with a report telling you what backlash you have and the recommended parameters. If there is a lot of DEC backlash you can decide to live with it and tune PHD2 to compensate it or first try to reduce it by tuning your mount if possible. I have a AM5 that came with too much DEC backlash, I put more tension on the DEC belt, I still have some but PHD2 is managing it knowing that I put the parameters accordingly for the DEC algorithm.
Hope this helps, Frédéric
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Christian Großmann avatar
Christian Großmann avatar
I have this kind of behavior with my EQ5 Pro mount. I tried to adjust the gears of the mount for months without really increase the performance on that. At least in EQMod there is a setting for the DEC backlash. I increased this number and suddenly, the mount worked perfectly well. It was a bit of a challenge to find the right setting, because PHD2 always told me different values. But at least it worked and still does.

In your guiding graphs, you see that DEC is always correcting in the same direction. When the problem occurs, the direction of the pulses changes. This is the best sign for backlash. Oh, this is what @Frédéric Ruciak had noticed...
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Benjamin DeHaven avatar
Benjamin DeHaven avatar
Frédéric Ruciak:
Hi, according to the graphs I see, it seems that DEC is having a lot of backash. When you see a serie of pulses in the same direction and the DEC not responding and then overshooting it looks like backlash. Have you tuned PHD2 according to your backlash? The very first thing to do with PHD2 is to run the guiding assistant with backlash measurement ON, and PHD2 will finish with a report telling you what backlash you have and the recommended parameters. If there is a lot of DEC backlash you can decide to live with it and tune PHD2 to compensate it or first try to reduce it by tuning your mount if possible. I have a AM5 that came with too much DEC backlash, I put more tension on the DEC belt, I still have some but PHD2 is managing it knowing that I put the parameters accordingly for the DEC algorithm.
Hope this helps, Frédéric

No, I was unaware of this feature nor was I sure of the problem. I will run this next clear night and report back if it doesn't mitigate the issues I'm having. Thank you!

Benjamin
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