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Hi Folks, I just opened up my guide log from my last session and found something I can't explain:  The DEC axis has near-digital jumps over the course of the session. The funny thing is: Guiding was just enabled for RA and not for DEC... So do you have an idea what could have happened there? Field drift due to polar alignment should be continuous and always one directional, right? The night was also very still, no noreable wind was going on. The DEC-axis was slightly front heavy and the target was close to the horizon, so the minimal play in the worm gear should be taken out of the game by these measures. I am looking forward to your ideas! ps: yes, I know... The mount is super cheap and not a good choice for astrophotography, but I am trying to max out the equipment I have. Regards, Volker |
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Hi Volker, I suggest you use PECPrep to analyze if there are any periodic movements. I am however not sure if the log shows the movement in DEC if there is no guiding in DEC but only in RA. If you can do this analysis you will possibly find the main frequency and this gives you an indication which mechanical element is the culprit, if there is any. If you can do the analysis please post the result here, I may be able to provide some insight, since I owned this mount some time ago. There is also an active group for this mount in groups.io, so you might get some good support from the folks there. CS Gernot |
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I can't give you a frequency analysis of the DEC axis, because it is not periodic. I have the analysis of the RA available:  The RA gives the expected periodic error with a period of 10 minutes. The plot of the DEC in the opening post shows the actual data, even when the DEC guiding is not active. There are just no correction pulses send. |
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PECPrep does read the PHD guidelog and does a fast Fourier analysis, this gives you a frequency spectrum of the main frequencies in your data. FFT is a standard analysis aproach to analyze signals. If there are any main contributors to the movement you will be able to identify them and this may give you a hint at the faulty mechanical element if there is such. The worm period for this mount is 600 sec, so the main frequency that you see in RA is the frequency of the worm, indicating that there might be a problem with it. There could be other factors responsible for such issues, e.g. insufficient guide scope pixel scale, error in polar alignment, flexure of the guide scope etc You may need to double check on all of these to eliminate the potential basic issues. CS Gernot |
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You may have some looseness in the DEC section. Periodic error in the RA worm gear may push on the head causing movement. Check to see if there is any correlation to your mount's periodic error. Jeff |
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Thank you for your reply. I certainly have some backlash in my DEC axis. Seeing the worm period in the plot should be normal, the peak-peak error of the worm is 15" as analyzed by the PHD2 Log Viewer. This is about normal, actually quite good for such a cheap mount. The events in DEC do not correlate with the periodic error in RA. I have an idea, what might have caused the digital jumps in DEC: I just replaced the doublet in my guidescope, because it was faulty (tilted). This doublet caused my starshape to be extremely coma-like over the whole FOV. The brightness distribution of a star was not shaped like a single spike, but more like a double peak. This could have led to a jumping star center between the two peaks. Yesterday I had a short session with a new doublet in the guidescope. There was no single jumo in over 2h of guiding. I have tried dithering (not successful), so the plot is kind of messy. Sadly, I will not be at home for the next two weeks. I am looking forward to do further testing with the mount/guiding combiation. I will give you an update if I can figure out something useful. Could be interesting, even for other mounts. Regards, Volker |
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Good morning, despite the bad forecast, I managed to setup a decent guiding session. It seems that it worked a lot better now. Maybe the doublet in the guidescope was the responsible component in this topic.  Backlash / Dithering is still a thing. Not really suitable with this cheap mount. |
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Hi Voker, 0.8" RMS is not bad at all with this mount. This is what I was also able to achieve with the mount. CS Gernot |
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Hi Folks, I want to give you an update. Last night I was able to use the clear sky and the new moon to capture the Veil Nebula in an uninterrupted session. This is the guide log from beginning to meridian. No strange jumps in Dec. A decent performance of this simple mount with a total of 8.1kg payload... Sure, good seeing at an altitude of 850m helps to achieve the 0,71" total RMS, but still the mount performed well.  The result of this session is the best picture I have taken yet. https://www.astrobin.com/gqrk37/ Even dithering worked fine. As long as the amplitude is small. I have dithered by max. 7 pixels. |
