What you need to know about guiding SWG mounts.: Page 2

What you need to know about guiding SWG mounts.

enviso

@Kevin_A thank you so much for writing this, and suggesting the settings to be used as a starting point.

I just purchased a new iOptron HAE69C mount and it didn't really perform out of the box as expected. My first session the guiding was 0.60" on average, but was oscillating a lot between 0.35 and 2.20" for the duration of the 4hr session. I was using my FRA400 with the WO UniGuide 30mm guidescope, which is not a big/heavy load. Apparently, my settings were completely wrong. I set the calibration step to 8000 following advice from that blog page, and left the Max Ra and Dec Duration at 2000. I will try using the settings you listed and see how it goes.

Thank you again for taking the time to share all this good information.

Kevin_A

enviso that blog page it totally incorrect regarding calibration size.
8000ms or 8 seconds is just plain bizarre! I wish that site would correct it as many people use that and it is just ridiculous! The more calibration steps the more accurate, and you want it to be done in around 12 steps per 25px…. So 8000ms would be around 3 steps so your guiding will be all over the map! The generic setting in asiair is 2000ms at 12 steps to go 25pixels, and Phd2 calculates a 120mm/120mm mini at around that too… just do your calibration near the meridian to the side you are imaging and once complete click on the toggle in guiding to use that calibration for the whole session no matter where your target is. It will be great. Try reducing the durations down to 700ms to start and aggressions to 50% as that puts everything in a not too high, not too low scenario using 1s exposures…. And bump up the gain on your guide camera to compensate for the faster 1s guide exposures.

enviso

Kevin_A thank you. I knew something was wrong but i didn't really know where to start or what kind of values i should aim for. Thanks for clearing that up. Will try this Saturday evening and come back with results. Nothing else on the agenda but to tinker with guiding accuracy. The moon is still up high, which i guess will affect my results, but i would like to try. I also went and purchased the 174mm mono and a 50mm guide scope (200mm FL), to replace the small 30mm scope and the 120 mini. Hopefully this will help move things in the right direction. It's a $4600 mount, and i intend to get every bit of performance out of it. Thank you again!

Kevin_A

enviso the only issue might be the pixel size of the 174mm. While the 174 is a much better guide camera and sensor, it is mainly used with off axis guiding due to the bigger sensor and bigger pixels. It has a huge size sensor compared to the 120 but it also has huge 5.7uM pixels which reduces the sampling rate when used on guiding focal lengths under 600mm. The bigger 200mm scope you got will help for sure and if guiding is only subpar, then use the 120mm mini for a test as its pixels are much smaller and may give better results. Big pixels make blocky stars for guiding. Good luck!

enviso

Kevin_A thank you. I will try the 174 first, using the 50mm guide scope. The plate scale is 6.04" per pixel, and i want 2 pixels, which means i want to move 12 arc-seconds per pulse. I use 0.5x sidereal rate as the guide rate, so this 12 arc-seconds corresponds to 12/(15*0.5) = 1.6 seconds. So, the Calibration Pulse should be set to 1600 milliseconds. For other settings i will use the numbers from your post above. Eager to see how it behaves. Thanks again!

Kevin_A

[unknown] yes, 1600ms will be fine too. My guide scope resolution using a 230mm guide scope and my 290mm mini is 2.6 and using my 220mm mini is 4.0. My imaging resolution 1.18.

enviso

Kevin_A How do you find the 220mini? I hear good things about it.
Also, some people suggested to try and put a UV/IR cut filter on the guide cam? Not sure how much effect that would have?

Kevin_A

enviso the 220mm mini is a good camera. I wish the would have made the pixels a bit smaller. I have found no difference with or without a uvir cut filter on a mono camera. I decided that my askar FRA was too heavy as a guide scope and the Askar FMA180 was too small and the Goldilocks ended up being the Sharpstar 50edph with reducer/ flattener at 230mm and much lighter at only 1.2kg.

enviso

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?

Kevin_A

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.

enviso

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

enviso

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.

eyecon

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!

Kevin_A

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.

eyecon

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!

Kevin_A

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

eyecon what mount HAE or HEM?

eyecon

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

eyecon

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”?

Kevin_A

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.

Kevin_A

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!

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