Guiding Issue: Page 2 - ZWO User Forum

Guiding Issue

w7ay

RLamb Attached is a LOG and screenshots of guide settings from last night.

Notice that the frame rates of your 0.5 second exposures had dropped to less than 2 FPS. Looking at your guide pulses, it appears to be over correcting.

The ASIAIR is spending more time sending correction pulses to correct itself.

I would suggest first reducing the Max RA and Max Declination durations to less than 400 ms. At a autoguiding frame rate of 2 FPS, I use 150 ms with my RainbowAstro mount, albeit, the RST-135 has a smaller first derivative of the periodic error curve than your mount (contrary to the ZWO boasts, the first derivative is the important number when autoguiding, not the peak-peak error of the curve, which is important only for unguided tracking).

So, you might try starting with something in the region of 100ms or 400 ms first. And then tweak from there. A 2000 ms pulse (the setting that you are using) allows the mount to be moved by 30 arc seconds (!!!) per pulse; only cheap trackers require that amount of correction.

This will prevent any error in correction to throw the errors off that badly (I typically guide at 0.35" to 0.4" total RMS with the very small corrections that I use).

I also use very small loop gain (aggressiveness) with my mount (around 35% and 40% in ASIAIR). You might want to start at the minimum (10%, I think) and only increase the loop gain (by no more than 5% each time, and then waiting 30 seconds to see the response to each change) when the correction pulses cannot keep up with the periodic error.

Chen

RLamb

w7ay

Chen,

Thank you so much for your detailed response, analysis and sharing your experience with the RST-135. I have had a rare stretch of clear nights and was able to drop the Declination durations down to 100 last night. Unfortunately that didn't seem to have any meaningful effect when guiding east of the meridian as my RA is still randomly jumping to around the 4-5.0 RMS for several minutes at a time before settling down. Guiding west is still really terrific in both axis and I'm having no issues after the meridian flip. As this behavior is reliably consistent and only on the E side of the MF, I am beginning to consider this issue may not be due to user error but may be a mechanical issue with this particular unit -

phd2-guidelog-2022-09-20-205648txt.zip

833kB

w7ay

RLamb Unfortunately that didn't seem to have any meaningful effect when guiding east of the meridian as my RA is still randomly jumping to around the 4-5.0 RMS for several minutes at a time before settling down.

I concur with you that a big difference in East and West pier side points to some problem with the mechanics of the mount.

Have you tried roughly balancing the mount (RA axis) with a counterweight to see if the discrepancy is still there? It is hard to balance a strain wave gear mount. One way I have found to do it is to look at the current draw of the RST-135 while the mount is tracking. When the mount is more balanced, it draws less current (the mount has less work to do). Do not balance it completely, since that will un-mesh the gears. With my RST-135, I have found perfect balancing to be detrimental to guiding.

For that matter, you might check (can do this even in the daytime even without dark skies) the current draw of your mount while tracking pier East and tracking pier West. With the OTA on the west of the pier (target on east of Meridian) the RA motor will be trying to actually brake when it is tracking. When the target is on the West of the Meridian, the motor has to work against gravity to pull the OTA mass up.

If there is more strain (and thus more current draw) on the motors, you can try using a larger power supply and heavy gauge and short power cables (at least 18 AWG -- and not the fake Amazon "18 AWG wires") between the supply and the mount. Under no circumstance power the mount through the ASIAIR's power outputs (I do not use any of the ASIAIR's power outlets).

The most strain [sic] on the motors happens when the target is within 30º of the Meridian. So, if you see different behaviors with tracking near the Meridian vs tracking within 30º of the horizon, that is another sign there is insufficient torque in the RA axis (or there is some additional unwanted friction that the motor has to overcome). (My impression is that ZWO had used stepper motors in their mount. My RST-135 uses real servo motors. But I could be wrong, since I have not actually see it published.)

At this point, there is not much you can do even if it is the mount's fault, except to return the mount to the dealer, so you might as well see if there is something you can do to mitigate the problem. At least you can try to reduce the strain (semi-balancing the mount) and making sure the motors have enough capability (heavy power cables and large power supply).

Finally (I do this often when I am testing autoguiding algorithms), see if you can very lightly load the mount by mounting only the guide scope on the mount, and not include the OTA. In my case, I use a dual saddle plate with my mount, and it is easy to simply not mount the OTA. If you have only a single declination saddle, you may need to fashion something to mount just only the guide scope on the saddle.

Chen

RLamb

w7ay Thanks, Chen. I'll look at the draw and balance when I get another chance but it won't be several days with weather and personal travel lined up. I am powering the AM5 using the same power source Tim Connolly recommended from Amazon. The AAP is powered through the mount, which in turn powers the camera, dew heater, etc. The scope is well balanced to my eye, but even if it is off slightly that shouldn't matter with this mount as it's such a light scope (redcat with OAG, FW, EAF, APP).

Walf@ZWO

Show me the photos about your telescope pointing east and west if it's convenient，thank you！

Jan75

Hi Chen (@w7ay),
I want to thank you wholeheartedly. I didn't realize I was so off when it comes to max RA and DEC duration, now that I use the AM5 mount instead of my previous iOptron CEM25. With 300ms duration and low aggressiveness I'm getting far better results. One question I'd have is how long your Calibration Steps are? I picked 1000ms but feel that the choice was somehow random. Maybe you'd have a better piece of advice here. Again, thanks!

w7ay

Jan75 One question I'd have is how long your Calibration Steps are?

Congrats on the better guiding.

When you are calibrating, PHD2 will attempt to move it by 25 pixels in each direction.

The Calibration step size is simply how long (in milliseconds) PHD2 moves the mount by at the guide rate. For example, if the guide rate is 0.5 x sidereal (equivalent to 7.5" per second of time) and you had set the step size to 1000 milliseconds, then each movement will be 7.5 arc seconds. And depending on your guide camera's plate scale, it will correspondingly move a guide star by the amount of pixels that is determined by the plate scale.

Instead of actually calculating the plate scale, etc, the easiest (and laziest :-) way is to just do a test calibration run and count how many movements it takes to finsh the 25 pixels (the ASIAIR displays that while calibrating).

If you have a well-machined mount (like a 10Micron), just 5 of these movements should be sufficient to create an accurate calibration. Poorer machined mounts will need more measurements (shorter steps for each movement) so that PHD2 can form an accurate measurement from the average of those many measurements. With your current mount, I would recommend 12 to 20 movements (i.e., about 1.5 to 2 pixels per movement).

Chen

Jan75

w7ay Thank you again for your explanations. I will follow that empirical rule of 12-20 movements during calibration.

Maybe it's the proverbial bad luck effect of posting good news, haha, but right after I hit the Send button my guiding started acting up. It's a smooth uneventful curve with the total average RMS below 0.50" for several minutes until, boom, a high spike on RA, or on DEC, or on both, up to the +/-4" watermark and things go left without plausible explanation. Sometimes those spikes would repeat in almost a periodic manner but most often they are isolated.

I realized guiding on this mount was very sensitive to seeing and at least one occurrence of the problem was caused by a high altitude cloud moving through the guiding camera's field of view. But in other cases the sky was entirely clear and there was no wind, no dragging cables or any other issues with immediate explanation. I also tried a 2s exposure instead of 1s (that I saw as being recommended for harmonic drives), but that didn't improve things at all, quite the opposite.

I'd like to think that the AM5 being brand new would not have internal mechanical defects that would create those bumps and I haven't observed them over a long enough period so as to ascribe them to a dent in the harmonic drive (I'm assuming that if there was a dent in the drive that effect would pop up every X minutes/hours where X is the period of the drive rotation). So to me the first likely explanation is external to the drive itself. Here's an example of the Asiair (PHD) curve but I'm at a loss for new ideas here... So if you happen to have one I'd be quite interested.

Many thanks again,
Jan

Jan75

P.S.: I'm also starting to wonder about possible power supply issues. Investigating...

Jan75

Another example - this RA spike is also mirrored by a smaller DEC spike. Shortly after this, the mount started beeping loudly which means voltage was getting low. That could explain why the gear would have these sudden micro-stops that translate into a spike on the RA guiding curve. This does not explain the DEC shift though.

I replaced the first power supply (a battery) with another one (a 10A 12V power source that is connected to the wall socket). Guiding seems now to be smoother (see pic below) with still some spikes occurring but with a lower amplitude. The highest recorded ones are just above 2" so about half the size of the highest spikes when the mount is battery-powered. They are also more rare. Total RMS hovers around 0.7" - still a bit higher than what I was expecting from this mount though.

Partial conclusion: ZWO AM5 is more sensitive to power supply regularity than iOptron CEM25.

w7ay

Jan75 this RA spike is also mirrored by a smaller DEC spike.

This "coupling" is puzzling.

The next time you have a chance, and if it is possible, rotate the guide camera angle until one of the axis of the frame is parallel to the declination plate. I.e., turn the camera angle so that the red and blue vectors in the Calibration popup are pointing to the x and y axis, instead of rotated by some angle.

This will decouple the correction vector (i.e., the rotation matrix will have close to zero off-diagonal terms). This reduces one possible error, and at the same time make the PHD2 log easier to analyze by eye.

It is easiest to do by pointing the USB cable to the camera at one of the four cardinal points, like the example below, vertically up relative to its dovetail plate, for one of my guide scopes:

If you still see x and y graphs coupling after that, then the error is not (at least not likely) caused by the strain wave gears (i.e., no chance the gears on the RA motor and declination motor have flaws right at the same time). I.e., kind of good news.

So the places to look for are:

. wind
. slippage (dovetail clamps not locked down) or guide tube band not tight, mount not tight with tripod,
. loose spacer rings
. tripod not on stable ground
... stuff like that

All that being said, don't get too concerned about short term glitches on the guide graph. If it last for just one second out of a 180 second exposure, you will never see any ill effect on the final frame. Too many people take guide graphs as gospel, without thinking about what it means practically.

Rumen (one of INDIGO's principals) and I were just discussing it not many weeks ago, and we both think that Root Mean Square is a very poor estimate of guide errors because it over-emphasizes short large pulses (RMS takes the square root of the average of squares, and square of a large number is really, really large), while even if you move a star 1 arc minute away for a short time, it will do no harm since there is not enough integration time for enough photoelectrons to register on the plate.

The magnitude of error is not a problem - how long it stays away from perfect guiding is the problem.

My recommendation to you is that you can ignore the short (time) deviations, even if the magnitude (arc seconds) is large.

Chen

Jan75

Hello Chen @w7ay - thank you for taking the time to think about my problem and for suggesting some extra avenues to further investigation. I turned the guiding camera so that the USB port points directly downward (perpendicularly to the declination plate and to the telescope). Since my last post I've had two clear nights and here are a few observations:

The mount CAN drive smoothly; during the first night I managed to have eventless guiding almost all night, for multiple hours, with a low RMS:

Even though the spikes were still there from time to time, just shorter and lower amplitude, some symmetrical (DEC/RA), some just in RA:
With this mount, more than with my previous CEM25P, guiding seems to be quite sensitive to seeing, air turbulence etc. I noticed visible degradation of the total RMS when I went out to check on the mount; I shoot from my balcony and when I go out I leave the door half open. Likely this creates more turbulence as warmer inside air flows out of the door.
There was no wind at all but on the second night there was some high altitude nebulosity and the mount went all crazy. The thicker the clouds the more random the guiding was - seems logical although again the AM5 has higher sensitivity to clouds than the CEM25P where likely the counterweight inertia would attenuate fine erratic movements.
I also noted that on my first night the guide scope was slightly out of focus which didn't seem to be much of an issue with regard to guiding. I had worse results on the second night even though I had refocused the guide scope back.

Note the angle between RA and Dec vectors after calibration that is slightly greater than 90° even after I redid the calibration - I think the high altitude clouds might have simply made it impossible to calibrate properly but I'm not sure about the explanation.

All in all, while I can get very good guiding with this mount, it remains somehow unpredictable and seems to be sensitive to weather conditions. I still like the mount for its portability and quick setup, but its precision is not way above the CEM25P, at most just a little better.

I will keep testing and will report if I see anything interesting.

Thanks again for your kind attention and comments.

Jan

Jan75

Maybe as a quick follow-up on this discussion I'd like to briefly report on yesterday's shoot session:

The target was the Soul Nebula that is visible at a high altitude right now where I am (France). I did the guiding calibration in the same region which in retrospect wasn't optimal and I should have tried closer to the equator but that didn't seem to have much visible impact on guiding.
There was almost no wind and I made sure there are no turbulences created by me (closing windows, doors etc.). Since the day had been warm there was likely turbulence off of warmer roofs but again I didn't see much of that in guiding.
Overall the total RMS hovered between 0.3 and 0.4, sometimes venturing down to 0.25 and some peaks up to 0.65. No weird behavior in the beginning.
More irregular guiding observed in the second half of the shooting session due to two factors: partial obstruction of guide camera field by roof/wall (even a 30 % field obstruction led to more chaotic guiding) which is OK because that means the target is about to go off-limits anyways; second, battery. The latter is showing to be an important factor to the AM5 mount that appears to be sensitive to voltage fluctuations. Anomalies start appearing when the battery is about 2/3 and by about 1/3 the mount starts beeping loudly and guiding (tracking) goes crazy.
Conclusion so far: I'll need to buy a battery pack with regulated DC 12V/10A output.

kaiyung

From your description, I don't believe it's caused by the battery. Instead, imaging with the guide scope near the rooftop line will definitely cause wild swing for the am5 because of the Eddie current at the interface between warm surface and colder air. AM5 is very sensitive to bad seeing and you are seeing the effect of it chasing it.

Jan75

kaiyung Thank you! You are confirming my suspicion about the seeing. The harmonic drive has no backlash (so I understand) which makes it very reactive to guiding impulses; since the recommended sampling time is 1 second that clearly is good ground for "false positives" in guiding that are caused by atmosphere rather than tracking irregularity. I might try 2 second guiding intervals for targets that are close to the roof but I hear 1s is recommended so let's see... Being downtown Paris I won't be able to get rid of bad seeing at all so I just need to find some coping mechanisms.

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