Daniel Petzen:
I forgot to attach the PHD2 log; Please find it attached: PHD2_GuideLog_2025-01-27_205755.txt
I also did a stacking of NGC 1535 and Centaurus A. I only applied an unlinked STF throught the histogram transformation. No processing apart from the stacking calibration (yes, I know the vignetting is bad - I'm experiemnting a bit with back focus and calibration frames).
Here they are, with warts and all:
NGC1535, 2h 28m, drizzle x2:
Centaurus A, 3h, drizzle 2x:
There are only minor guiding artefacts, and it looks like the Centaurus A image has less guiding artefacts.
Don't get me wrong; I'm happy with both images. A few hours more integration time tonight and they'll hopefully turn out quite good, but I was hoping for a much lower RMS error, which, in my experience, just subtly does wonders to the end result when it comes to clarity and details. Out of curiously... What kind of RMS performance were you expecting from the mount?
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Reg Pratt:
Out of curiously... What kind of RMS performance were you expecting from the mount? I saw Dylan O'Donnell tracking at 0.25" total RMS with his EQ8-Rh with a C11 on top, so I was hoping for sub 0.3", or at least around 0.4". I knew I wouldn't be able to do that out-of-the-box, but I didn't expect 0.6" plus with large random spikes. The theoretical RA RMS error is 0.11", so I was a bit disappointed to see 0.4"-0.5" average on RA, but that may be PPEC and PHD2 interference and, quite possibly, inaccurate RMS reporting from PHD2 because of seeing conditions, focus, focal length etc.
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Daniel Petzen:
Reg Pratt:
I second this advice. 0.37" RMS is actually very good performance but it could be that PPEC and the RA encoder are interfering with one another so best to turn off PPEC. Your RA peak to peak 1.82 arc/sec so seems like the encoder is doing a very good job.
You will also want to either increase your exposure time in PHD2 or use the delayed exposure setting. 1.5s is too fast. Mounts with little to no PE don't need corrections very often and benefit from larger periods between exposures. Try increasing to 5-8s and see if that helps.
I didn't realise the PPEC was RA only. I can definitely see how that will make things worse. I'll disable that tonight.
1.5s was the sweetspot for my old mount. That will be completely different now. I'll put this on the list of things to test tonight. How would PPEC work in DEC? Permanent periodic error correction is designed to allow you to fine tune out the periodic error in the tracking gear... The mount only tracks in RA, so periodic error is only going to affect you in RA.
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Alex Nicholas:
How would PPEC work in DEC?
Permanent periodic error correction is designed to allow you to fine tune out the periodic error in the tracking gear... The mount only tracks in RA, so periodic error is only going to affect you in RA. No, you right, I ballsed that one up. I was rushing through too many things to stop and think. I'll approach things a bit more methodically tonight.
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Daniel Petzen:
Reg Pratt:
Out of curiously... What kind of RMS performance were you expecting from the mount?
I saw Dylan O'Donnell tracking at 0.25" total RMS with his EQ8-Rh with a C11 on top, so I was hoping for sub 0.3", or at least around 0.4". I knew I would be able to do that out-of-the-box, but I didn't expect 0.6" plus with large random spikes.
The theoretical RA RMS error is 0.11", so I was a bit disappointed to see 0.4"-0.5" average on RA, but that may be PPEC and PHD2 interference and, quite possibly, inaccurate RMS reporting from PHD2 because if seeing conditions, focus, focal length etc. We have to keep in mind though that guiding is heavily impacted by seeing, wind, high clouds, etc. We also have to consider that there will be variations in performance from unit to unit because of manufacturing tolerances. I doubt his mount is performing that well at all times. We are all guilty of showing our peak performance and not average performance at times. I've got plenty of screenshots of my own of 0.25" total RMS but the truth is that only happens on nights of excellent seeing and on average its usually in the 0.4" range. 0.37" is exceptional guiding especially with a payload that isn't exactly small. I bet you will see it drop much lower for periods of time when the seeing allows. Both your axes are guiding at pretty much the same level as well. Renishaw can make even outright shabby gears perform well. Also, it looks like you're using a guidescope. At that FL an OAG would be advisable.
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Worth mentioning, as I'm sure someone else has, but having PPEC enabled in the mount, and having PHD2 trying to correct for things, and having absolute encoders reading each of those movements as 'erronious' is a recipie for sub-par results.
Whilst Dylan O'Donnell has achieved 0.25" RMS with his EQ8-Rh, that doesn't mean that you can 'expect' that, either out of the box or after tuning… Its kind of like seeing someone build a race engine for a car, just because one man made 2500hp, doesn't mean anyone else will, and just because you once saw a supra do a 7s quater mile does not mean they are 7 second cars out of the box.
There will be MANY nights of fine tuning the mount (physically, obsessing over balance, worm gear meshing and polar alignment etc. and in software, training your pointing model and tweaking many guiding parameters) to achieve a ~0.2" to 0.3" RMS.
And the theoretical RMS is, well theoretical… You're never getting 0.11" accuracy, because you will never having seeing that good anyway, and atmospheric refraction will cause bigger errors than that, unless your mount is tracking at a variable rate that is compensating for atmospheric refraction, and then you're also in seeing conditions that are sub arc-second.
0.4" is quite good from what is, at the end of the day, a cheap mount… Yes, its expensive depending on the purchasers budget etc - but, if you look at all the mounts in that weight class with absolute encoders, the EQ8 is VERY cheap by comparison…
That's not to say that 0.4 is t he best you'll ever get - but its nothing to be upset about considering you've not really spent a lot of time with the mount yet and figured it all out… Give it 6 months of fine tuning and tweaking of settings…
Remember, Guiding assistant is a good help, but its not a silver bullet - if I run guiding assistant on my harmonic mount and take its suggestions verbatim, my guiding jumps to ~0.8" RMS, if I put it at the settings that I spent 4 nights fine tuning, testing and finally settling on, I guide at 0.35~0.45 RMS time and time again..
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Reg Pratt:
We have to keep in mind though that guiding is heavily impacted by seeing, wind, high clouds, etc. We also have to consider that there will be variations in performance from unit to unit because of manufacturing tolerances. I doubt his mount is performing that well at all times. We are all guilty of showing our peak performance and not average performance at times. I've got plenty of screenshots of my own of 0.25" total RMS but the truth is that only happens on nights of excellent seeing and on average its usually in the 0.4" range.
0.37" is exceptional guiding especially with a payload that isn't exactly small. I bet you will see it drop much lower for periods of time when the seeing allows. Both your axes are guiding at pretty much the same level as well so I feel like your DEC encoder is doing very good work. Renishaw can make outright shabby gears perform well.
Also, it looks like you're using a guidescope. At that FL an OAG would be advisable. Yes, I'm starting to tune my expectations based on what I'm seeing and what people are telling me. It'll be a journey of optimising, improving, calibrating, tinkering, testing, tuning and perhaps even a Dec zero backlash upgrade when my finances allows for it. Bottom line is that I need to have realistic expectations, especially with my current setup, For instance, the guide scope is a long and painful story. I just can't get my OAG to work; it's either blocking the main sensor or have too much distortion to track properly. I've lost many a night to that OAG, but I'm still working on it.
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Thats a bummer to hear about your OAG but tbh the guidescope is doing a great job. I really do think that you will see periods of even better performance when seeing is better just keep in mind guiding isn't a constant thing. Everyone's mount has a range based on conditions. On my bigger mount that range is 0.25 - 0.5" depending on the seeing. Add a little wind and/or high clouds and it'll rise into the 0.6" range. It all depends on conditions. Continue to use the mount and I am confident you too will see that the mount will perform better on some nights than others.
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Alex Nicholas:
Worth mentioning, as I'm sure someone else has, but having PPEC enabled in the mount, and having PHD2 trying to correct for things, and having absolute encoders reading each of those movements as 'erronious' is a recipie for sub-par results.
Whilst Dylan O'Donnell has achieved 0.25" RMS with his EQ8-Rh, that doesn't mean that you can 'expect' that, either out of the box or after tuning... Its kind of like seeing someone build a race engine for a car, just because one man made 2500hp, doesn't mean anyone else will, and just because you once saw a supra do a 7s quater mile does not mean they are 7 second cars out of the box.
There will be MANY nights of fine tuning the mount (physically, obsessing over balance, worm gear meshing and polar alignment etc. and in software, training your pointing model and tweaking many guiding parameters) to achieve a ~0.2" to 0.3" RMS.
And the theoretical RMS is, well theoretical... You're never getting 0.11" accuracy, because you will never having seeing that good anyway, and atmospheric refraction will cause bigger errors than that, unless your mount is tracking at a variable rate that is compensating for atmospheric refraction, and then you're also in seeing conditions that are sub arc-second.
0.4" is quite good from what is, at the end of the day, a cheap mount... Yes, its expensive depending on the purchasers budget etc - but, if you look at all the mounts in that weight class with absolute encoders, the EQ8 is VERY cheap by comparison...
That's not to say that 0.4 is t he best you'll ever get - but its nothing to be upset about considering you've not really spent a lot of time with the mount yet and figured it all out... Give it 6 months of fine tuning and tweaking of settings...
Remember, Guiding assistant is a good help, but its not a silver bullet - if I run guiding assistant on my harmonic mount and take its suggestions verbatim, my guiding jumps to ~0.8" RMS, if I put it at the settings that I spent 4 nights fine tuning, testing and finally settling on, I guide at 0.35~0.45 RMS time and time again.. In hindsight, I think I was trying to buy myself out of the constant tuning and tinkering with my EQ6 Pro. I now realise that this is just the same, but perhaps at a different level. I need to give it more time. I need to build a pier, sort out an OAG, find the perfect guiding approach and lots of other things. This will be a process, just like with my EQ6. That's OK. That's why I do backyard astronomy and not renting a remote telescope. I think the key thing is to set my expectations, becuase I was almost panicking last night. I really though I've made a huge misstake. I've used the guiding assistant as a bit of a silver bullet. That is a good point. i think I need to explore some of the settings for myself. It'll be easier when I have a pier so that it's a bit more consistent. Do you get 0.35-0.45" RMS on a harmonic mount? Nice!
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Daniel Petzen:
Do you get 0.35-0.45" RMS on a harmonic mount? Nice! After 6mths of tuning it and using all my cloudless but moon soaked nights trialing different settings and configurations, yep. It's certainly still at the whim of the seeing, and performs dramatically better pointed near the celestial pole compared to the celestial equator, but, its usually in that 0.35 to 0.45 range with an OAG on a 840mm focal length. Absolutely, it will still be a tinkering game - I've had to mechanically tinker with NEARLY every mount I've ever owned... Mounts from Skywatcher, Losmandy, Avalon Instruments, ZWO, Emcan Astro... The only one that performed with 0 issues at all was my Astro-Physics, despite it being 2nd hand, and quite old, I set it up, got it aligned and it did exactly what it said on the tin... I had a similar moment of panic when I first set up my Losmandy G11 Gemini II that I bought brand new, and to begin with, I couldn't even get it to point REMOTELY close to the selected coordinates... That mount was the one that needed the most love, but $ : performance ratio wise, once I'd fiddled with it, was the best mount I ever bought.
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@Daniel Petzen : I just wanted to echo what others have said before, and add some more comments. 1. your guiding graph looks good! I checked out the log and in the 3h5m session you achieved an average total RMS of 0.49" - this is a very good result! If there are bad frames due to hiccups then just sort them out... 2. your result quality is ok - but it could be better (don't get me wrong - I am a picky person here) This is not because of the guiding, but more related to the optics and conditions. I see heavy vignetting, elongated stars to the corners, background illlumination and potential straylight / lightleak issues. I personally would concentrate on this issues first (buy a dew cap to help with straylight, check the corrector distance, probably think about a faster optic that is more suitable for deep sky photography). Of course everything is fixable today with good flats, good processing and some BlurXTerminator magic... your gallery proves that you are able to produce superb results! 3. change your guiding setup! According to the PHD Logs your guidescope has a focal length of 190mm and the guidecam pixel size of 5.9um - your main optics is > 2000 mm - this does not fit! The pixel size is WAY too large for the guidescopes FL and the FL of the guidescope is too short for your main optics! Your average 0.49" is actually a variance of 0.08 pixels on the guidecam chip... it simply cannot get any better... If you want to stick to a guidescope use a sensitity guidecam with 2.9um pixels (i.e. IMX 462m based), or use the your IMX174 with an OAG, or switch to a guidescope with a higher FL.
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Regarding the tuning of the DEC backlash. TBH, I would not worry much about it for now - if your guiding setup fits, then PHD should be able to handle that... You only have to get active here, if your DEC guiding worse (by factors) than your RA. I did my tuning just for fun... I discussed this in the EQ8 Groupsio Mailing List: https://groups.io/g/SkywatcherEQ8/topic/98300937 |
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Nico Gärtner:
@Daniel Petzen :
I just wanted to echo what others have said before, and add some more comments.
1. your guiding graph looks good!
I checked out the log and in the 3h5m session you achieved an average total RMS of 0.49" - this is a very good result!
If there are bad frames due to hiccups then just sort them out...
2. your result quality is ok - but it could be better (don't get me wrong - I am a picky person here)
This is not because of the guiding, but more related to the optics and conditions.
I see heavy vignetting, elongated stars to the corners, background illlumination and potential straylight / lightleak issues.
I personally would concentrate on this issues first (buy a dew cap to help with straylight, check the corrector distance, probably think about a faster optic that is more suitable for deep sky photography).
Of course everything is fixable today with good flats, good processing and some BlurXTerminator magic... your gallery proves that you are able to produce superb results!
3. change your guiding setup!
According to the PHD Logs your guidescope has a focal length of 190mm and the guidecam pixel size of 5.9um - your main optics is > 2000 mm - this does not fit!
The pixel size is WAY too large for the guidescopes FL and the FL of the guidescope is too short for your main optics!
Your average 0.49" is actually a variance of 0.08 pixels on the guidecam chip... it simply cannot get any better...
If you want to stick to a guidescope use a sensitity guidecam with 2.9um pixels (i.e. IMX 462m based), or use the your IMX174 with an OAG, or switch to a guidescope with a higher FL. 1: Ah, yes, you're right. I misred it. Now when I've "calibrated" my expectation, I can see that is good guiding. 2: You're right. My image train is terrible. I'm using this video ( Celestron SCT Back Focus Study at Native and Reduced Focal Lengths) as a guide, but it's currently cobbled together with all sorts, including an OAG (that is probably where the light leak is. 3: Yes, this is embarrasing. The ASI174MM was picked for OAG guiding. It's completely wrong for short focal length with my C9.25. I know there is some debate abotu the focal length ration between the main scope and the guide scope, but what I'm using is just wrong. I know :-) I have the Sky-Watcher Startravel 80 with a focal length of 400mm. I just managed to get it to work (the backfocus wasn't achievable without a diagonal or additional bits not included), so at least I'll get 400mm. I have a 2x Barlow that I will test with next time. I'm also keen to get a new guide camera once I've recovered from buying the mount (thanks for the tip about IMX 462 - I'll look into that).
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Daniel Petzen:
AstroShed:
The EQ8 mounts are solid, and beefy, but have one big flaw, the backlash is absolutely awful, I see you did not run that test in your guiding assistant, you need to, that could well be your issue, it was with mine, it was so bad I scrapped my mount and bought a CEM70
I did run the backlash test third time:
DEC has a backlash, but I saw that it's not being compoensated for becuase the mount has absolute encoders (highlighted area).
I couldn't find a way to tell PHD that I only have absolute encoders on RA only. I'm starting to think that I should tell PHD that the mount don't have absolutel encoders. That is about the same amount of lash I had in DEC, I managed to adjust down to 2200ms, but no lower, the issue with these mounts is the ring gear not being perfectly round, and there are tight spots, so you can only adjust out the backlash at the tightest spot, or you get binding. If you run you mount with a power supply that shows the amps being used, you will see when it hits the tight spots as the power draw goes up quite a lot, this is then the where you need to adjust the lash. not sure how much help it would be, as your guide figures are very good. the spring loaded worm idea is the best option, and looks very simple to do.
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I tried to take a methodical and scientific approach to my attempts at guiding last night, but it did descend into chaos fairly quickly. I disabled PPEC, switched to a 400mm guide scope (from a 190mm), I verified polar alignment and double checked everything. I too an unguided image of NGC 1532 while I was running the guiding assistant and it looked great:  The PHD2 guiding assistant was very inconsistent and reported a backlash of 2210-3107 with very different graphs, but I did run it against NGC1532 that was sinking fast (60-35 degrees 23:00-01:30), so the position of the telescope was different. The second unguided sub was a bit worse:  I tried the following: 1. Unguided: worked well early in the evening (NGC 1532 lat > 40) 2: Bump guiding (RA MinMove 0.5-1, exposure time 8s): this worked very well to start with, but RA started to slip to the north when the altitude decreased (graph below) 3: High MinMove and aggression: I tried with 10s, 8s, 5s, 4s, and 2s exposures and MinMove 0.25-1, but the guiding was going past 1.0" RMS at this time, not matter what I did 4: Encoders switched off:: I only did "full" guiding liek this. PPEC didn't seem to have any impact. I saw both improvements as well as problems (spikes). 5: Switch back to my old 190mm guide scope, full guiding RA+Dec, 2s exposure, no encoders: this dropped the RMS error from 1.15" to 0.7", but I only got 10 minutes before the clouds rolled in. This is the PHD2 log from about an hour's worth of experimenting:  This is the log from when I ran EQ6 style, i.e. no encoders, 190mm guide scope and full PHD2 guiding at 2s exposure (with a lot of star lost because of clouds):  This is the full PHD2 log for the evening: PHD2_GuideLog_2025-01-28_194122.txtGetting high precision tracking last night was extremely frustrating (there may have been some swearing in the night), but 95% of the subs were stable enough to pass a blink test (a unusually kind Blink test, but still...), so I can get by with the guiding I have with my current equipment, but most of my future plans are on hold for now. It'll be until next week before I'll have clear skies again. Yet again, thanks to everyone who has helped me with this. I really appreciated your help and advice. I learnt, the hard way, that the best way to become a better astrophotographer is to listen to those who has more experience than yourself.
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Daniel Petzen:
I tried to take a methodical and scientific approach to my attempts at guiding last night, but it did descend into chaos fairly quickly.
I disabled PPEC, switched to a 400mm guide scope (from a 190mm), I verified polar alignment and double checked everything.
I too an unguided image of NGC 1532 while I was running the guiding assistant and it looked great:
The PHD2 guiding assistant was very inconsistent and reported a backlash of 2210-3107 with very different graphs, but I did run it against NGC1532 that was sinking fast (60-35 degrees 23:00-01:30), so the position of the telescope was different.
The second unguided sub was a bit worse:
I tried the following:
1. Unguided: worked well early in the evening (NGC 1532 lat > 40)
2: Bump guiding (RA MinMove 0.5-1, exposure time 8s): this worked very well to start with, but RA started to slip to the north when the altitude decreased (graph below)
3: High MinMove and aggression: I tried with 10s, 8s, 5s, 4s, and 2s exposures and MinMove 0.25-1, but the guiding was going past 1.0" RMS at this time, not matter what I did
4: Encoders switched off:: I only did "full" guiding liek this. PPEC didn't seem to have any impact. I saw both improvements as well as problems (spikes).
5: Switch back to my old 190mm guide scope, full guiding RA+Dec, 2s exposure, no encoders: this dropped the RMS error from 1.15" to 0.7", but I only got 10 minutes before the clouds rolled in.
This is the PHD2 log from about an hour's worth of experimenting:
This is the log from when I ran EQ6 style, i.e. no encoders, 190mm guide scope and full PHD2 guiding at 2s exposure (with a lot of star lost because of clouds):
This is the full PHD2 log for the evening: PHD2_GuideLog_2025-01-28_194122.txt
Getting high precision tracking last night was extremely frustrating (there may have been some swearing in the night), but 95% of the subs were stable enough to pass a blink test (a unusually kind Blink test, but still...), so I can get by with the guiding I have with my current equipment, but most of my future plans are on hold for now.
It'll be until next week before I'll have clear skies again.
Yet again, thanks to everyone who has helped me with this. I really appreciated your help and advice. I learnt, the hard way, that the best way to become a better astrophotographer is to listen to those who has more experience than yourself. I actually find the PHD2 guiding assistant very hit and miss too, the figures it reports on backlash and in PA are always different and sometimes a lot different, as in your case with your backlash readings. This does not help your cause either as you have no real idea how much lash you actually have, I wish they would sort it out, and make it more reliable and more accurate.
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AstroShed:
I actually find the PHD2 guiding assistant very hit and miss too, the figures it reports on backlash and in PA are always different and sometimes a lot different, as in your case with your backlash readings. This does not help your cause either as you have no real idea how much lash you actually have, I wish they would sort it out, and make it more reliable and more accurate. It would be good to have a benchmarking tool for the mount to get some absolute measurement. If it turns out that it's the Dec backlash that is causing the problems, then I'll upgrade with spring loaded Dec wormwheels. I'll just have to get the motorcycle sold :-) I just finished processing NGS 1532 ( NGC 1532, Haley's Coronet (Daniel Petzen) - AstroBin). I only had to drop 9 frames out of 60, and only three of those were dropped because of guiding errors. I would have liked longer integration time, but I'm pleased with the result given the circumstances.
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4 nights, close to 30 hours troubleshooting, tantrums, words, angry emails, and, of course a lot of help, and I could take this snapshot before I went to bed last night:  That didn't come easy. I've tried pretty much everything, including some mad stuff. I still need to analyse all the final settings and logs, but I think the key was to find the right adjustment frequency, so the most important part was to find it at 4-6 seconds exposure (guiding delay didn't work). Worth noticing in addition to the settings in the snapshot is: - Encoders on
- PPEC/adaptive PPEC off (jury is still out on that one, as I think it need more training)
- Hysteresis for RA, ResistSwitch for Dec
- Using an 800mm guide scope (I may get an Celestron OAG, my ZWO OAG just doesn't work), but the focus is still terrible
- Backlash compensation + guide direction auto (I had guide North or South to avoid backlash to start with).
- Dither in RA only - it did WONDERS for how dithering affected guiding. It went from chaos to unnoticeable.
This I'm considering - Camera suitable for guide scopes (IMX 462 based)
- Celestron OAG
- Finishing building my pier
- Dual Losmandy saddle. I want to run have both my telescopes mounted at the same time and can use my Askar 103APO as guide scope.
- Zero backlash upgrade
There is much more to do and I need consistency above all, so PHD2's comments about backlash and failing to pulse the mount are still worries. For instance PHD2 said this after my last calibration:  I then went on to do 2h19m at 0.4" RMS and then 1h 0.33", so I'm not sure how serious to take this.  The polar alignment was near perfect, but the backlash was reported at 3,212ms. I don't want to sound like a broken record, but thank you for all your help with this!
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You are just not going to get your guiding any better than those results with this mount, the mount is a mass produced Chinese mount with cheap parts and poor QC and large tolerances, if you want better, then bin the EQ8 and buy the 10 micron, it’s as simple s that, or you will drive yourself to distraction..
I went through all this with my EQ8, and the best thing I did was to bin my EQ8 and buy a CEM70 which is far better, but still not as good your results here, but I am happy at my imaging scale.
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AstroShed:
You are just not going to get your guiding any better than those results with this mount, the mount is a mass produced Chinese mount with cheap parts and poor QC and large tolerances, if you want better, then bin the EQ8 and buy the 10 micron, it’s as simple s that, or you will drive yourself to distraction..
I went through all this with my EQ8, and the best thing I did was to bin my EQ8 and buy a CEM70 which is far better, but still not as good your results here, but I am happy at my imaging scale. I'm sure you're trying to help, but being told to "bin" my new mount and get a new one that isn't "a mass produced Chinese mount with cheap parts and poor QC and large tolerances" is not exactly the kind of help I'm after, but rather getting what I have to work, as I don't have another $20k kicking around.
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Daniel Petzen:
AstroShed:
You are just not going to get your guiding any better than those results with this mount, the mount is a mass produced Chinese mount with cheap parts and poor QC and large tolerances, if you want better, then bin the EQ8 and buy the 10 micron, it’s as simple s that, or you will drive yourself to distraction..
I went through all this with my EQ8, and the best thing I did was to bin my EQ8 and buy a CEM70 which is far better, but still not as good your results here, but I am happy at my imaging scale.
I'm sure you're trying to help, but being told to "bin" my new mount and get a new one that isn't "a mass produced Chinese mount with cheap parts and poor QC and large tolerances" is not exactly the kind of help I'm after, but rather getting what I have to work, as I don't have another $20k kicking around. I have tried to help already in this thread along with many others, you are getting very good guiding with the mount, better than most will ever get, yet you are still not happy, I was just pointing out you will not get any better with a mass produced chinese mount.. so either just get out under the stars and enjoy what you have or bin it and spend another 20k…simple really…
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Your Daniel Petzen:
4 nights, close to 30 hours troubleshooting, tantrums, words, angry emails, and, of course a lot of help, and I could take this snapshot before I went to bed last night:
That didn't come easy. I've tried pretty much everything, including some mad stuff.
I still need to analyse all the final settings and logs, but I think the key was to find the right adjustment frequency, so the most important part was to find it at 4-6 seconds exposure (guiding delay didn't work).
Worth noticing in addition to the settings in the snapshot is:
- Encoders on
- PPEC/adaptive PPEC off (jury is still out on that one, as I think it need more training)
- Hysteresis for RA, ResistSwitch for Dec
- Using an 800mm guide scope (I may get an Celestron OAG, my ZWO OAG just doesn't work), but the focus is still terrible
- Backlash compensation + guide direction auto (I had guide North or South to avoid backlash to start with).
- Dither in RA only - it did WONDERS for how dithering affected guiding. It went from chaos to unnoticeable.
This I'm considering
- Camera suitable for guide scopes (IMX 462 based)
- Celestron OAG
- Finishing building my pier
- Dual Losmandy saddle. I want to run have both my telescopes mounted at the same time and can use my Askar 103APO as guide scope.
- Zero backlash upgrade
There is much more to do and I need consistency above all, so PHD2's comments about backlash and failing to pulse the mount are still worries.
For instance PHD2 said this after my last calibration:
I then went on to do 2h19m at 0.4" RMS and then 1h 0.33", so I'm not sure how serious to take this.
The polar alignment was near perfect, but the backlash was reported at 3,212ms.
I don't want to sound like a broken record, but thank you for all your help with this! I have to agree with AstroShed on the point that you're not going to get better performance than 0.25" without going to a premium mount. That being said the guiding you've been able to achieve is excellent. I'd pat myself on the back and call it a day.
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Daniel Petzen:
I then went on to do 2h19m at 0.4" RMS and then 1h 0.33", so I'm not sure how serious to take this. As the new owner of a Mach2GTO I can say you're getting comparable performance when using a "floppy" telescope (i.e. one that has moving optical parts that makes repeatability impossible). Something to consider that I've been having great luck with is to try the ZFilter algorithm with super fast guide exposures (I'm using 0.25 seconds) but then tune the filter to respond to only changes in the star positions that you'd notice if you took 4 second exposures (Xfac : 20) This way you get a chance to "bump" the mount every quarter second but it will only correct for movements that are apparent in 4+ second exposures. This works better because you get to issue corrections during the entire next 4 second exposure based on the results of the prior 4 second exposure. So you're bumping the mount gently but constantly instead of abruptly but occasionally. NOTE: I have to warn you that you will have to completely ignore the guide graph ... it will show all of the seeing effects; instead you should turn on trend-lines and look for completely flat lines... That shows you're tracking perfectly. Also, you can "eyeball" the bullseye and see of all the points look to be centered. I'm thinking of modifying PhD2 to add a "center of gravity" display to the bullseye so you can see any slow moving trends. (You'd want that "center of gravity" to be pinned exactly in the center of the bullseye.) You can test it to check the FWHM and Eccentricity of successively longer exposures. They should stay pinned at what you get with say a 1-3 second exposure. So far tonight I'm up to 21 minute exposures with no change.    |
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Reg Pratt:
I have to agree with AstroShed on the point that you're not going to get better performance than 0.25" without going to a premium mount. That being said the guiding you've been able to achieve is excellent. I'd pat myself on the back and call it a day. Thanks for responding in a friendly and helpful way, Reg. It's clear that I expressed myself a bit unclearly in my summary post; I am patting myself on the back and am very happy with where I'm at now. I'm absolutely thrilled to have touched on a total RMS error of 0.25". I had clear skies last night again and was guiding for 6 hours around 0.4", which is wonderful guiding and exactly the consistency I'm after. The set of actions I listed were all about how I can improve it even further, remove variables from the equation and reduce inconsistencies. This is important to me as I eventually want to move to even longer focal length. I managed to get my EQ6 Pro mount to guide at 0.33" for an entire night, so I'm sure I can improve the EQ8 a bit as well, but if I can get an average guiding of 0.4" with my EQ8, then I'd be happy as Larry.
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Also, I picked up a Starlight XPress Active-Optics unit second hand for 1/2 price and put it on my buddy's Askar 107PHQ (which not the sharpest scope in the world) and just look at the difference. Left image is 300 sec H with A/O guiding at 0.1s (10 hz) — HFR 1.79; Eccentricity 0.38; RMS 0.59 Right image is 300 sec H regular guiding at (maybe) 2s (0.5 hz) — HFR 1.79; Eccentricity 0.38; RMS 0.38 The right image was shot last fall when the seeing was arguably better (see the difference in guiding RMS).   The difference is remarkable: The left image has much better noise characteristics and there is much more detail visible. I think that keeping the star's light aligned on it's pixel really makes a difference; though the experts would say guiding at that high a rate is useless. To me the proof is in the image. Here's the video that gave me the idea to try it. He does the same comparison and gets the same results as above: https://www.youtube.com/watch?v=i8IPQrED8iY&ab_channel=NormanRevereOh, and what's even more remarkable is that I'm getting similar guiding results with this telescope and AO unit on an old EQ6-R which used to struggle keeping things at 0.5" rms (with lots of wild excursions) and which as periodic error in excess of 30 arc-seconds. Now that EQ6-R is turning out identical guiding performance to the Mach2GTO setup right next to it and imaging the same target at the same time. I'm up to 960 second exposures with no increase in HFR or Eccentricity. It's remarkable that a $700 used mount with a $700 used AO unit can perform similarly to an $12k mount; at least tonight with the equipment I own. Now I've got to find a way to squeeze it onto my C8-XLT w/ Starizona SCTCORR-4 reducer. :-)
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