• ASI Mount

  • What you need to know about guiding SWG mounts.

Yeah, smaller pixels would have been nice. They should release something like that again definitely. Meanwhile a used 290mini is hard to come by.

With regards to testing my mount guiding with the new settings, i think you are right, i will keep the current setup with my SkyWatcher EvoGuide 50ED (242mm FL) and the 120mini. The 120mini has smaller pixels (3.75µm) while the 174mm has 5.86µm size pixels. But is this guide scope focal length overkill for the FRA400+reducer (280mm f/3.9)? Guiding a 280mm main scope with a 242mm guide scope?

    enviso guide scopes do not ever care what your imaging scope is. Overkill for that focal length… definitely. My Rokinon 135mm f2 setup uses a 162mm guide scope. Think of it this way… it never hurts an image to have better guiding than the image resolution requires. The limiting factor is still always going to be the sky, but most serious imagers try to get guiding rms at 1/2 to 1/3 of the imaging resolution which is based on the Nyquist Theorem. So, if your main scope is 3.0” then your guiding should be around 1” to 2”rms. The lower your guiding resolution is the small and finer the adjustments that can be made in phd to keep the rms lower. Some mounts do not need overkill guide scopes but others do and are a pain to guide well. The flatter and more in control your guiding is the less bloat your stars will have from moving all around during guiding.
    FRA300 with 240mm guide scope.

        Kevin_A thank you. I guess i will not be changing my configuration then. The 533MC Pro resolution is 2.77"x2.77" per pixel on the FRA400 with reducer (280mm). 1.94"x1.94" per pixel on native focal length of 400mm, I will have to test all configs to learn how it behaves.


        FRA400+0.7x reducer with 200mm guidescope

          7 days later

          Kevin_A

          So i went out last weekend with my iOptron SWG mount and used the settings you suggested, and it worked.
          The seeing was average but the guiding was stable with no erratic spikes or oscillations, around 0.40" for the duration of the 2hr session. I experimented a bit with the settings and ended up using 600ms for Ra & Dec Max Duration. I tried everything from 150 to 750 but it seems to have liked 600ms best. I also played with Aggression, and ended up at 55% for Ra and 35% for Dec. Moving Ra from 55% to 100% didn't seem to have much impact, and the same was for Dec. I will keep these settings as is and adjust accordingly in the next session. Hopefully will be able to dial it in perfectly. Thank you so much help and for sharing your knowledge.

            a month later

            Kevin_A just found this thread after posting my question earlier today. I tried similar settings after going through your experience and the great analysis and explanation from @w7ay but I'm finding that shortening the max pulse width is not making much of a difference. I just noticed that my minmo settings were a bit high because I think they came from the PHD2 guiding assistant recommendations so no sure if that could be it. The one thing I can confirm for my setup is that more frequent guide exposures definitely helps but that lowering aggression makes things worse in my case. Not sure what this actually means though. Would appreciate your thoughts!

                eyecon every AM5 mount reacts differently and the only settings that will be similar will be the fast 0.5-1.0s guide exposures along with smaller Minmo settings of 0.1px.
                Other things that help is doing PA twice to verify accuracy, a rigid tripod, a big aperture quality guide scope that has no flexture and using a higher gain setting to ensure multiple star guiding on good centroids due to lower SNR star signal from the short and fast guide exposures. Sky quality and turbulence is a big factor too but that you cannot change. I usually set my aggressions to 50% and durations to 450ms these days based on my specific mount and get good results. Good results are at around 0.7rms and below. Do not expect nor care if you do not get 0.3rms as that is not the normal for this mount… it is just when all sky, PA and calibration settings align perfectly. Some nights my results are 0.8, sometimes 0.7 and some nights 0.45rms. So that tells me the sky is dictating my results. But, my stars are round all nights and that is what matters.

                    Kevin_A yes makes total sense. As mentioned in my other thread, I’m not necessarily obsessing about rms error but rather trying to a) determine if there’s anything wrong with my mount and b) trying to optimize the settings to get the best possible results given the seeing conditions. I have a fairly decent guide scope with a good image scale and Kevin_A I’m already running the Asi178mm at relatively high gains to ensure a high SNR. I’ll try to lower the minmo settings to better understand how my mount is behaving. Again, I was just wondering if the fact that I had different results with shorter guide pulses and lower aggression meant that there’s something potentially wrong with my mount. I appreciate your replay!

                          eyecon can you share a image of your log? The 178 guide camera has big pixels that are not ideal on smaller guide scopes but lets look at a guide log pic first before jumping to conclusions. Here is one of mine from awhile back for reference. Just show me an area of concern and that shows a few cycles. I need to see the data from the log as I have shown.

                            Kevin_A the mount is an AM5 and I have a separate thread with the logs and details here: https://bbs.zwoastro.com/d/17900-yet-another-am5-guiding-thread

                            Happy to share an image of the guide logs when I’m at my computer but my concern was not not any specific section of guiding performance but rather the fact that my AM5 did not respond to guide parameter changes in the same way yours has. I was simply trying to improve guide performance in general and RA guide performance in particular.

                            For your reference, the 178 is paired with a 50 mm F4.8 so a bit under sampled

                              Kevin_A after carefully looking at your log screenshot it appears your guiding is occurring at a much larger scale than mine since we both use a similar FL guide scope(240mm) but your image scale asi120mm is 60% larger; I believe your are even more under sampled than my setup. I wonder if that’s a reason your guide performance on average could reach better numbers? Is it possible that with this degree of under sampling, you are not able to detect as much error resulting in seemingly better guide performance?

                              Please don’t take this the wrong way, I’m not saying there’s anything wrong with your setup. I’m just curious to understand if under sampling is effectively acting as a filter making the guiding results appear better (since a more under sampled setup is theoretically less capable of resolving star movement details) . Do you notice any significant increase in your main images’ FWHM when you get .8” rms total error vs say .45”?

                                  eyecon my pixel scale is not too large…. It is fairly normal as your pixels are small on the 178. Mine is 3.22. I would not say that is undersampled. I usually use a 290mm mini as I do not use the 120mm mini much anymore and only use it on ultrawide 15mm setup.

                                    eyecon I am not sure if super small pixel size makes that much of a difference personally as the 240mm scope is real crappy and stars were bloated on the 120mm mini and I got good results and perfectly round stars in my images. Now I have a FMA180 Pro and the stars are pinpoints dots on my 290mm mini but the sampling rate is exactly the same as before (3.2) so maybe it is splitting hairs if my stars are still good. I think 3 is normal, 2 is oversampled and 6 is whack. But it still depends on your imaging scopes resolution required to get round stars. I think my new guiding scope and camera are overkill as some people get good results with a crappy 30mm guide scope and 120mm mini. It all comes down to how good the mount is and if it can be tamed!

                                        Kevin_A thanks for sharing this additional information. Agreed on the FHWM values, ultimately sampling comes down to the diffraction limit and seeing but 3-6 pixels seems to be a good range. I can’t remember info from the logs but I think with my setup the average was 4 so not that far off. But I do still wonder if the lower the image scale, the higher the apparent RMS error for a given mount since technically the centroid algorithm has more pixels to work with and would detect smaller variations(?) both your fra180/290 mini and your 240mmFL scope/120 mini have a 3.2-3.3 arcsec/pixel scale where as my Evoguide 50/178mm combo has 2.02 arcsec/pixel scale. So won’t my setup be able to “notice” 1 more arcsec of error than your setup resulting in a higher rms error (assuming our mounts behave about the same in general) ? I guess I can probably test guiding with my imx571 camera on the same scope to see if get better results …just to test my theory

                                            eyecon my biggest issue may be Zwo’s implementation of phd2. It is not as good as the standalone phd2 and it really relies on great PA wheras it should not be so reliant on exact PA.
                                            Yes, techically on paper your pixel scale should do better but I don’t think these mounts are accurate enough nor responsive enough combined with poor seeing to work precisely at such small differences between 2-3 arcsec pixel scale. At least with these AM5 mounts. They are fairly crude and motor resolution is not fabulous. They are amateur mounts afterall and just intended for great portability. I think you are right about some pixel resolution masking of the real results and even though some insist they are guiding at 0.3rms… their stars may not be perfect. Who knows!

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