KC_Astro_Mutt Good guiding, then bad guiding.

That's because there are spots in your periodic error curve that has low first order derivatives (slope of curve) while at other spots, the slope becomes larger.

If the large errors are momentary (a glitch that lasts ony a second or two in time) and don't occur often (say, only two or three times in a long exposure), you can ignore the problem (if you are not OCD), since there are not enough photons in those jumps.

However, if you are consistenly lagging, the slope of the PE curve is too high for your parameters. Try increasing the "aggressiveness" first. But remember, back off when the feedback loop oscillates.

I think I had set the max DEC duration to 1500 and RA duration to around 500.

If the declination gears have the same quality as the RA gears, the declination pulse duration should not need to be anywhere close to the RA pulse duration (since you typically don't need to correct the declination axis at a rate anywhere close to the RA's siderral rate).

It is definitely possible that ZWO had sorted the gears and used the less good gears for declination. But unless they document the Declination curves, we would not know, and you will just need to measure it yourself -- you can try to simulate the drift alignment (for different decination angles) to see if there are bumps in the drift curve.

Chen

      tempus Would this alignment be of significance when guiding the AM5?

      No, unless you are debugging. The coordinate tranformation matrix (CTM, for you graphics/Postscript nerds) in PHD2 is is pretty reliable.

      But, when you are debugging guiding errors, it is useful to have the guide camera angle set so that the RA aligned closely to either the x or the y axis of the guide camera. You can actually get this from the camera angle when you do a plate solve from your guide camera.

      For me, it is just a small, one time adjustment. So it is worth it for me.

      Chen

        w7ay However, if you are consistenly lagging, the slope of the PE curve is too high for your parameters. Try increasing the "aggressiveness" first. But remember, back off when the feedback loop oscillates.

        Increasing the aggressiveness beyond 50% in either axis made things much worse, that's why I began working on the Max Duration settings.

        w7ay you can try to simulate the drift alignment (for different decination angles) to see if there are bumps in the drift curve.

        To be honest Chen, I'm getting weary of troubleshooting. I'm going to be reaching out to ZWO Customer Support to get this sorted out. It either needs to be repaired or replaced. I've applied the recommended settings, made adjustments, and I still have to throw away 40% of my subs. I suppose someone that enjoys tinkering would be happy to figure this out, but I'm a hobbyist, not an engineer. I just want a mount that works as advertised like I said earlier.

        Thanks for you help! You add great value to this forum!

        Cheers,
        Jeff

            KC_Astro_Mutt These settings just gave me the best performance to date.

            Still not super great, but quite usable.

            The errors consistently make excursions past 1 arc second. So these will definitely register in your images.

            The saving grace is that they are random, with no perceptable RA-declination correlation. So the error should distribute over all radial angles, and instead of creating a non-round star, the errors simply create stars that are perhaps 1" to 2" larger (but still mostly round).

            If your "seeing" is worse than 2" seconds to start with, the star diameter will not bloat that much. If your "seeing" is 3", for example, the bloat is perhaps just 10% to 20%.

            Chen

                w7ay Yes, when my guiding was say, 0.5"rms for a couple of minutes, the FWHM on my images were consistently around 1.5. Then the rms error would rise, and FWHM on those images rose to 1.9 to 2.0.

                • w7ay replied to this.

                    KC_Astro_Mutt Then the rms error would rise, and FWHM on those images rose to 1.9 to 2.0.

                    Yep, from the randomness of the error, I would bet the stars remained "round," though

                    I LOL every time I read, "my guiding must be good since I get round stars." The fact is that stars will remain round as long as the error is distributed over all radial angles.

                    Roundness also improves after stacking (since you are further randomizing). If you are interested, it has to do with Jointly Gaussian distribution which has a perfect circular symmetry, if the x and y are independent, and have equal variances:

                    https://en.wikipedia.org/wiki/Joint_probability_distribution

                    Note that this means that star at celestial equator will remain round, but the ones closer to the poles may not :-)

                    Here are some round stars from a strain wave geared mount (not too far from Equator :-) :-) (ASI2600 mono camera, with a Chroma Luminence filter, FSQ85+flattener):

                    http://www.w7ay.net/site/Images/Lagoon.png

                    Chen

                        w7ay I LOL every time I read, "my guiding must be good since I get round stars." The fact is that stars will remain round as long as the error is distributed over all radial angles.

                        I use the subframe selector in Pixinsight. I use 2 categories to choose the best images. Number of stars, and FWHM. I've gotten my best image processing using just those 2 parameters.

                          w7ay This is why it is imperative for strain wave geared mount to use very rapid correction rates. As I mentioned at the end of my previous post, it is not the camera exposure rate (FPS) that is important, it is the correction rate.

                          Exposures (i.e., sampling the centroid) can actually be done at a rate even slower than 0.5 FPS (2 second exposures). But the correction rates need to be fast.

                          It is just that with today's PHD2 (and ASIAIR) guiding, we can only issue one guide pulse per exposure. And because of that we have no choice today but make very short exposures (I need 0.5 second exposures with an RST-135 to guide it at 0.4 arc secnd total RMS).

                          You should definitely start a conversation with the coders looking after Phd2. Strain wave mounts are here to stay and will likely become the vast majority of the mounts for amateurs. Or maybe start "PhHD" (Push Here Harmonic Drive) guiding app ;-)

                              tempus You should definitely start a conversation with the coders looking after Phd2.

                              Currently, I am still modeling the behavior by using a simulator (which can run thousands of time faster than real time, so simulation runs takes less than half a second instead of an hour). I plan to take a while more to fully understand the problem; but already, the guide rate is yielding results exactly as predicted. My plan is not to create software for others, but trying to understand better the behavior of mounts with large harmonic distortion.

                              I will be eventually be implementing with real stars using Indigo (I have been programming computers since 1966 on an RPC-4000 and Flexowriter as I/O, and have control theory knowledge (up to Kalman filtering) from my EE background.

                              None of it should be very foreign to me, but I do have to start with first principles along the way, such as detecting stars in a guide frame estimating centroids -- even that topic needs work since multi-centroid algorithms like the one in ASIAIR does not take care of long term field rotation; I need to stop and restart ASIAIR guiding once every hour or two, for example, or the guiding will eventually become worse.

                              Chen

                                  w7ay i really like your idea on multiple pulses per exposure discussed earlier. That sounds very intriguing and may help greatly in theory. It may help out more on conventional mounts running 2-3s exposures. The only issue I see is how bad seeing plays into multiple shorter pulses. If only our skies were always lacking turbulence!

                                  • w7ay replied to this.

                                      Kevin_A The only issue I see is how bad seeing plays into multiple shorter pulses.

                                      Very likely it is no more than bad seeing plays with using a single long pulse. It is just a matter of distributing that same correction term over more pulses (to keep the error from building up before being corrected).

                                      Atmospheric turbulence is today pretty much solved by using the centroid of multiple stars. If you are not already using it, I would highly recommend taking advantage of the ergodic process of the twinkle, twinkle litte stars :-) Notice that when you go out on a clear dark night, the stars do not twinkle in unison. Different air masses are affecting each star individually. The centroids of N independently twinkling stars averages out to be the same as the centroid from an exposure of a single star that exposed for N times longer.

                                      Fortunately, the slope does not change much from one 2-second exposure to the next (i.e., the second derivative of the curve multiplied by 2 seconds). This small change of slope should allow us to issue corrections at 1 second intervals, but using the centroids from the 2 second exposures. And even that can be corrected, if we also include slopes in our centroid detection.

                                      Chen

                                          w7ay i do like the idea of correction more frequently so that the buildup does not get out of control. Tiny rapid corrections make much more sence than the big slow hammer aproach!
                                          I use multi-guiding and that helps a lot in the twinkling stars for sure..

                                            I wonder what the limit of the correction rate will be? Would the the rate at which the comms protocal in use (and underlying usb/wifi) can transmit/receive or the mount firmware's ability accept and process the corrections? Can the AM5 achive 5 correction commands per second? 10? Would this be for RA only (assume yes).

                                            Have you taken a look at the performance of the EC equipped HD monuts - in theory the encoders should do the same thing..lots of small/fast corrections and thus reduce the RA PE to such an extent that no guiding should be required. But this does not appear to be the case - even for the Renshaw equipped RST-135E, the residual PE is still +/- 5" or so. Much better than +/- 20" but this may be a limit of the mechanics I would really appreciate you views on this. I tried the iOptron HEM27EC thinking it would be an ideal mix of price, accuracy and portability but found the encoders did not deliver.

                                            • w7ay replied to this.

                                              Jhaunton Have you taken a look at the performance of the EC equipped HD monuts

                                              The two are actually quite similar. The "EC" mounts corrections are based on comparing the current geared shaft position relative to a precise optical encoder. With auto-guiding, we are correcting based on (what really is more precise and with infinite resolution), the stars.

                                              One big drawback with autoguiding is that we have to look through atmospheric turbulence. By using centroids of multiple stars that measurement much more accurate (error variances half when you double the number of equal SNR stars). At least accurate enough that the rest of the autoguiding mechanism becomes the tall tent pole.

                                              The other drawback, a much greater one, is that you can sample an optical encoder in a very short duration, thus, freezing the reading (close to an ideal Dirac delta sampling). With autoguiding, the stars are moving as you expose them for the duration of the guide exposure. So, you will always get a star trail, of which you need to find the centroid from, not the most ideal thing.

                                              But this is also where many multiple corrections also come in, you never hit the mount with a large enough pulse to cause the guide star to instantaneously move much, and then you don't allow the star trail for long before you hit the mount with yet another small correction. Instead of a large sawtooth waveform (where the leading edge of the sawtooth is the pulse correction, and traiing edge is the mount's periodic error), you are breaking the large amplitude waveform into two or more shorter and smaller amplitude sawtooths.

                                              This last phenomenon is greatly reduced by using that "guide using guide rate" paradigm that I mention earlier. I.e., you send a single pulse, like today per guide camera exposure -- but you don't change the pulse duration. You use a pulse duration that is as long as the exposure time. Instead of controlling the mount movement by the pulse length, you control it by puse amplitude (i.e., pulse sidereal rate). Essentially, you are sending a correction that is equal to the current slope of the mount's periodic error curve. The slope of the curve itself (i.e., the second drivative of the curve) does not vary by much every 2 or even 5 seconds.

                                              But this does not appear to be the case - even for the Renshaw equipped RST-135E, the residual PE is still +/- 5" or so.

                                              Yep, the "optical encoder in the sky" has much higher resolution.

                                              However, if executed properly (allowing both correction from the optical encoder, and from mount commands), the mounts with high resolution encoders can help a lot in allowing longer guide exposures when autoguiding. Unlike trying to simply reduce the peak-to-peak error from the gears themselves, a high end encoder reduce the error without including the higher harmonics. This is why in an earlier posting, I had replied to ZWO that it is more important to reduce the harmonics, instead of just reducing the amplitude of the strain wave gear.

                                              One other advantage of optical encoders is of course that they are functional in the daytime. For that, I have added a Hutech Hinode [sunrise, in Japanese, and also a NASA mission to the Sun] solar guider that I have been using for many years now. Not cheap (and needs to mount to support ST-4 :-), but worth its weight in gold if you do any daytime work .


                                              https://astrohutech.store/product/hinode-solar-guider/

                                              Chen

                                                2 months later

                                                Hi.
                                                I have observed something that may explain why the AM5 mount exhibits significant backlash in the DEC axis, at least in some units.

                                                pxl-20230417-193218463activets-2.mp4
                                                13MB

                                                In this video, there is a slight slippage between the gear and belt, which is very small but could be enough to account for the observed backlash of around 500ms (varying between 200ms and 800ms) in my mount. This slippage appears to be caused by the gear teeth being slightly narrower than the belt teeth.

                                                In defense of the AM5, I must say that my mount generally guides well, with RMS errors ranging from 0.4" to 0.6" on average. I usually do not experience issues with DEC guiding if the polar alignment is good and there is no wind. However, if DEC drifts in the opposite direction for any reason (such as bad seeing or dither recovery), it can sometimes take up to 10 or 20 pulses (or even more) to recover, despite having backlash compensation enabled in PHD2. When this happens, it can be a bit frustrating, especially considering that a gear with slightly wider teeth could potentially improve the situation.

                                                  isaakPr
                                                  Hi isaaPr,
                                                  Thank you for your video and experience sharing.
                                                  This slippage may be caused by the belt isn't tightened properly or the shape of the belt teeth does not match the one on the motor side.
                                                  For tightening problem, you can loose the motor and re-tighten the belt.
                                                  For the backlash problems, we are adding the backlash compensation into the Firmware.

                                                      a month later

                                                      isaakPr In this video, there is a slight slippage between the gear and belt,

                                                      I have a bit the same issue with DEC lagging. How did you get to the gear and belt? Through the bottom plate? How did you remove it? I took the 4 hex bolt out but there seem to be something holding the cover in place.

                                                          ASIMount@ZWO For tightening problem, you can loose the motor and re-tighten the belt.

                                                          ZWO,

                                                          I am more than willing to check the belt tightness.
                                                          Please provide instructions on how to verify the belt tightness, how to tighten it and starting by how to open the mount.

                                                            ANY REPLY from ZWO....?? Anyone left in support? Do you have some procedures available for opening the mount, checking the belt tightness and tightening if needed?
                                                            A reply would be appreciated as I don't seem to be the only one with lots of DEC backlash.

                                                            11 days later

                                                            tempus It is possible to access the internal gears by removing the front plate, where the connectors are located. Although I think that tightening the belt will not help, it is already tight enough. It's a problem with the shape of the belt teeth, I think.
                                                            For me, the best way I have found to minimize this problem is to achieve very good polar alignment (I have used drift alignment in phd).