• ASI Mount

  • Getting the best performance from my AM5

Kevin_A I am not sure if the belts are set tight enough on my drives and may have some slop that is causing this… not sure.

Kevin, if the RA dithers are less than 7.5 arc seconds, there should be no backlash from the strain wave gears themselves, but it could come from other sources, like you said, a loose belt, or a cheap pulley (the ones that come with the ZWO EAF for example, are not what I'd consider good pulleys :-).

Now, if you limit each RA travel to be less than 0.75 arcsec (e.g., by setting max RA pulse to 100 ms), then a 3 arc second dither could take 4 exposure times to reach the destination, and longer, if it is not smart enough -- the ASIAIR, for example, does a slow exponential decay because it is lazily using PHD2 guiding to recover from a dither -- instead of appling a long enough pulse to directly drive the guide star to the new position and then restart guding.

Still, as you say, not a big deal when you can achieve better guiding.

Chen

      w7ay as soon as the skies clear i will do some testing. I will keep my dithers at 1-2 pixels and see how it recovers. I do not forsee a problem. I did open up and look at the Dec belt and it looks fairly snug with about 2-3mm of deflection when pressed so i am happy with that as it is normal.

        w7ay

        Thanks for the detailed explanation Chen and the pointers for future testing.

        As I suspect typically happens with new owners I started off with max dec and RA pulses of 1500ms and the aggression right up at 80% or so. Whilst the mount worked it was over-correcting and see-sawing back and forth but the average tracking was reasonably accurate.

        The weather has been awful so opportunities to tweak the settings have been very limited however the last outing showed improvement with the approach discussed in this thread i.e. smaller moves. Ended up at max moves of 350ms and the aggression down at 35% on dec and 40% on RA - seemed to get down to 0.5-0.6 over the 2 hour break in clouds.

        Next clear night (likely months away!) will drop the max moves from 350 with end game being 100ms, will drop in increments of 25ms and see what works.

        As has been mentioned on this thread, there seems to be an opportunity for ZWO to use the report to highlight the recommended guiding settings range for each mount. Wonder if its an unwise thing to do from a retailer perspective though in this day and age of internet / mail order and the associated ease of returns.. would we all be cherry picking mounts?

            ShinyredAM5 end game being 100ms, will drop in increments of 25ms and see what works.

            If you are going to slice it that thinly, give it a bit of leeway if your mount is not tracking at King Rate, and your target is near the horizon.

            https://canburytech.net/DriftAlign/DriftAlign_3.html

            Notice in the second of two charts about 1/3 of the page down that there is a 0.03 arcsec/sec rate added to the sidereal rate near the horizon, which accounts for something like 4 ms of the max rate when autoguiding at 0.5x sidereal rate.

            And, as I mentioned earlier, any mechanical flexure too.

            Make sure that you don't fine tune where the periodic error curve has the minimum slope. If you do that, the pulses will not be able to keep up at the higher slope regions.

            Chen

              @w7ay I did mine, absolute worst case is 0.228 arcsec/sec (violet line). What would be recommended settings for this case?

              • w7ay replied to this.

                raawe I did mine, absolute worst case is 0.228 arcsec/sec (violet line)

                As mentioned multiple times already on this thread, just set the max RA pulse duration to give this much movement for the gude rate that you use. Nobody knows your guide rate, so no one can give you any number.

                Chen

                  Looking at that PE trace made me think again. All the HD mount guide logs I have seen show short duration, large spikes in RA - even when guiding well. I wonder if these are actually at the point in the PE curve where the error reverses? In the case above this means the error rate goes from approx +0.23" per sec to -0.23" per sec virtually instantaneously - the peaks are sharp. Just a thought.

                  • w7ay replied to this.

                    Jhaunton In the case above this means the error rate goes from approx +0.23" per sec to -0.23" per sec virtually instantaneously - the peaks are sharp. Just a thought.

                    They are probably not instantaneous -- just appears so when the curve is shrunk on a finite graph.

                    The way to view it is to look at the periodic error curve as a Fourier Series. For good mounts, terms after the third harmonics are very small. For worse mounts, you may see large 5th, and even 7th harmonic terms.

                    In any case, each of these components of the Fourier Series is a continuous function -- and since they are sinusoidal, the sign change occurs when the magnitude of the first derivative of the curve is small. I.e., you don't go from +0.23" to -0.23", you go through +0.2...+0.10, ... +0.8,... +0.1, 0, -0.1, -0.2, ... etc. And the slopes near 0 are easy to guide away.

                    Mathematically, the second derivative (how fast the first derivative changes) is actually quite small. I.e., no sudden change in slope.

                    Chen

                      prastro It just works great. I took the data into Excel and calculated the first derivative. Then I generated an X-Y plot and obtained this:

                      The blue line is the PE, and the orange one is the first derivative. So, the maximum slope is close to 0,3 arc-sec/sec.

                      Thanks a lot for the tip.

                          biomedchad Are you saying that AM5 can't do an autoflip with NINA / ASCOM? If so, thats not correct.

                            fbitto Sure.

                            Mine is similar. I get a max first derivative of 0.3 as/sec every 430 sec (surprise!), which lasts for about 20 sec.

                            Interestingly, when I did it manually, I got 0.18-0.2 as/sec, but this is obviously much more accurate.

                            Can this also be worked into the sub length, if we know, say, that the guiding will be under stress every 430 sec for 20 sec?

                            Next step is to do it on the actual graph under load (PHD2 LogViewer) and not the unloaded ZWO chart.

                                fbitto

                                prastro

                                Could I ask a favour and grab your excel formula to calculate the first derivative? Have pulled data from my ZWO report, got it in excel and duplicated the graph with data points.

                                Whats the next step to get derivative values from existing 2 columns of data?

                                      @StarzLite forgot to ask you what kind of setup are you running? Main/guide scope mainly.

                                      ShinyredAM5 You have to divide two consecutive values of the error (y axis) by two consecutive values of the time (x axis).

                                      But the time axis is calculated from the displacement in degrees. To do that you'll have to multiply all the values in degrees by 239,3444 (430,82 sec per cycle divided by 1,8 degrees per cycle)

                                      In cell E3 the formula is =+(D3-D2)/(C3-C2), and so on.

                                      Then you can just look at the values in column E or use the formulas =MAX(E:E) and MIN(E:E) to obtain the maximun and minimum slope values

                                          Going to chime in and thank Chen and a lot of other people I've gleaned info from the past two weeks. First night out on the AM5 a couple of weeks ago I was averaging .87 RMS total and was completely amazed by that. Coming from a GEM28 I thought I would never regularly see >1"RMS. But after reading/applying some of the recommendations and tweaking the RA/Dec aggression (40/40, I know going lower would lower RMS even more so looking forward to that test), I was able to stay at .59" RMS total last night for over 3 hours. Equip is as follows: AT80EDT main scope, 60mm guidescope, 533MC main, 290mm guide, ASIAIR Plus and AM5. I set the Calibration step to 900ms (per PHD2 recommendation) and RA Dec duration to 200ms with a .5s capture.

                                          When it's a bit warmer I'll do some more tests but I'll take .59 any night of the week. Question on binning. I've seen bin2 recommended before on guiding with my GEM. Would it help or hinder in this case? Apologies in advance if that's been addressed above

                                          Finally some nice weather so I put those new numbers to the test. Was really conservative with the settings for the first run. Results are amazing. Max RA and Max DEC was set to 250ms and aggression to 35/45%, but I feel this can go even lower. Total RMS was dancing around 0.3" and was maintained for the duration of my imaging session (3h).

                                          I was using Askar FRA500 at native f/5.6. Off-axis guiding with ASI 290MM mini and main cam is ASI2600 MM Pro.

                                          A big thank you to everyone's suggestions.

                                          fbitto

                                          Thank you for the formula, just finished off the excel graph and its interesting stuff.

                                          Seems my mount spikes at 0.3 arc secs / sec worst case but mainly sits around the 0.2 level.

                                            fbitto In cell E3 the formula is =+(D3-D2)/(C3-C2), and so on.

                                            What about if you are on line D15 or D33? Is there a generic formula that can be used to cover all D values? I am an Excel dummy.