I've been given a belt modded EQ5 Pro mount and am trying to get up and running with a second setup. The attached screenshot shows the guide log from last night. Guiding was mostly good but I was getting these regular massive spikes throughout the night (Some are on dither points but others aren't). As a result, I had to delete a lot of subs due to double stars, as though the mount had jerked half way through a sub. There was no wind last night and the mount was very stable and well polar aligned. Had anyone experienced this before? I'm not sure where to start looking. TIA.  |
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The EQ5 has never been known for its tracking performance…
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Cable snags or slippage come to mind. Additionally, damage to a gear is possible. If you transport if with the clutches locked you can damage the gears. Even putting the scope on roughly can make marks on the gears. I'm not sure which spikes are dithering, but if it's periodic, then it might be gear-related. Maybe others a lot more knowledgeable will chime in.
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andrea tasselli:
The EQ5 has never been known for its tracking performance... I got 0.5" out of the box but maybe I just got lucky?
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Hi Craig,
I remember that a while ago someone wrote in a different post about his experiences with the belt mod on this mount. It turned out, that the shape of the teeth of the belt was problematic in combination with the gears. If I remember correctly, the edges of the belts teeth slipped above the edges of the gears teeth every time the gears need to take another tooth. The result was a similar guiding graph. The solution was to use a belt with a different tooth profile (with rounded edges instead of sharp ones). But I am not sure, if this is the case with your mount. Because the spikes are periodic, this might be the case.
I own an EQ5 but without any mod. I did some work on it with the original gears and had to set some higher values for the minimum correction pulse while guiding. Those tweaks worked nearly perfect. But this Information won't help you at all…
I'm not sure if I'm right, but maybe it's worth to keep an eye on that..
CS
Christian
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You said regular – do some math, see if it really is regular, like a worm period multiple. Also compare to a log of imaging, does it follow any event like a filter change (especially if using an OAG and using filter offsets without a stop/start guide). Other possibilities include wildlife - a bird (e.g. owl) stopping by. I've literally had a tree frog jump onto, and then off the OTA and you get a blip. Cable snags and such have been mentioned, I had one of those "armored" USB cables (plastic rippled wrapper) and found it left a little vibration every time one of those ripples was pulled over a spot on the mount.
Those are pretty big and always in DEC (right?). DEC sudden excursions won't be caused by tracking errors, it generally requires something intervene and move the axis. I guess it could be the mount responding badly to a small DEC move and making a giant one, but I'd look first for external causes.
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On a second thought it looks like cable snag, tension builds in for a while (with quasi periodicity) and then it shifts and then it repeats again. Happened to me a long while ago. Now I always check that the cables are free to move wherever the scope is pointing.
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I had my rig do similar at the beginning of the last session in windy conditions.
After scratching my head while it did that few subs, I shut everything down and restarted and it recurred.
Then I doubled the guider exposure tine, restarted and all went normally the rest of the night.
I had a suspicion the cause is an oddity of some sort in the ASIAir guiding algorithm in windy conditions where it just goes berserk and slews in Dec, loses the guide star (I got the STAR LOST warning each time), then decides to go back to where it was and recovers the guide star.
What’s interesting is that it’s a deliberate slew because the movement is larger and faster than is possible if it was using the guide pulses - it’s set to 0.25X sidereal and the pulse duration 200 milliseconds.
The ASIAir has current firmware. Mount is a CQ350.
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That's the declination trace, it won't be anything related to the worm period simply because the dec motor doesn't have one – it isn't always running. Cable snags seem the most likely culprit.
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Francesco Meschia:
That's the declination trace, it won't be anything related to the worm period simply because the dec motor doesn't have one – it isn't always running. Cable snags seem the most likely culprit. Yeah, I think I said that before really paying attention to the color. Unless it's a MyT, with it's cross talk.  (Glad mine is gone). I do find it interesting there isn't much RA impact at the same time. That's kind of a wimpy mount for a cable tug to have that much DEC impact without affecting RA at all. It makes me wonder if something is literally sliding/moving as the OTA's angle changes. Notice that's over 7 hours of time. If it's something with freedom to move (very slightly) only in DEC. Was the DEC axis very well balanced? A related possibility is if as the angle changed the effective tension on the DEC axis overall changed and there's a tiny bit of give somewhere that occasionally slips? or related, if there's a big temperature change over that time, dissimilar metals in (say) a dovetail plate and long OTA might result in periodic slipping in rings as one grows faster than the other (ok, that's grasping a bit at straws, but when dealing in arc seconds everything is made of rubber -- that's the saying right).
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Linwood Ferguson:
I do find it interesting there isn't much RA impact at the same time. That's kind of a wimpy mount for a cable tug to have that much DEC impact without affecting RA at all. It makes me wonder if something is literally sliding/moving as the OTA's angle changes. Notice that's over 7 hours of time. If it's something with freedom to move (very slightly) only in DEC. This is a very good point. It's worth exploring further. For the original poster: you mentioned dithering, but do the spikes have any correlation with anything at all happening in your sequence, like switching filters or moving the focuser?
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I would definitively check the belt on Dec.
Had a similar effect on RA and it turned out that the belt was too tight, deformed and slipped over the edge of the gears. Each time a tooth of the belt slipped over the edge, it kicked the axis. That explains also that some of these events coincide with dithering events, because specially at that time the Dec axis has to work.
As Christian mentions, if the teeth do not fit the gear very well and the belt is tightened too much, this can also produce this effect. Reduce the tension of the belt in that case. The teeth of the belt might slide more softly over the teeth of the gear. Or get a belt which fits better.
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Thanks for everyone’s replies. I’ve been working through this with all of those comments in mind. To Summarise:
I don’t think this is a cable snag issue. I’ve routed all the cables to the back of the camera so that there is only one trail to the PC on the ground (My intention is to get an ASIAir and mount it on the scope to tidy this up but as it’s my second setup, I’m building slowly).
At first glance, the massive spikes seem periodic but when I look closely, they are fairly random and don’t co-include with a filter change, dither, etc.
I did have problems with the belt initially stretching and then jumping over the teeth but I’ve since replaced the belt and the teeth on the belt fit well on the gear. The next time I get it out, I’ll sit with it for a while and closely monitor the dec axis to see if there is anything I can physically see. I have modified the layout of the belt system so I’m hoping that will help.
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This is a close up look of what a typical spike looks like. The dec axis quickly moves off, the mount tries to connect but can't. I'm wondering if it's backlash  |
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It looks like a textbook case of static friction (stiction) to me. PHD2 is commanding a series of North corrections that don't seem to have effect, then all of a sudden the cumulative result takes effect, and PHD2 takes some time to bring it back under control with a series of South corrections. This series doesn't result in an overcorrection because static friction is already broken, so PHD2 only needs to take up the slack due to backlash.
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Backlash is usually shown by repeated pulses being unable to correct an excursion, so I get what you are saying, but the problem with that theory is that the excursion itself seemed so sudden and large. I don't see how backlash per se could be the cause, though having backlash comp disabled may be the reason it took so long to recover (for a belt drive without encoders I expect you have substantial backlash innate in the system).
It's certainly worth turning on compensation (unless it's off for a reason?) and running guide assistant to give it a starting point.
But I still think it likely there's something loose in the direction of the Y axis that periodically slips (or something akin to that). Keep in mind 10" is probably so small if you were watching carefully when it happened you couldn't see it by eye. It's large as an error, but small as a physical motion. The fact that the RA axis keeps on tracking well (if that's true in all these cases) seems to exclude some external impulse (wind, something bumping the mount, etc.). I would still think it's something in the drive itself (where that last pulse instead of a slight motion did a very large one), or something loose where when you hit a certain balance or weight distribution it just slips or flexes a bit.
I just read @Franchesco's note. I'd agree (e.g. where I said "instead of a slight motion did a very large one"), except that it wasn't really trying hard to move in DEC before hand. It may be worth looking carefully at each one, see what the last few intervals of DEC looked like before the spike.
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It may be worth looking at what happened before the beginning of the portion of log shown, if Craig can. Have many North guiding command accumulated, without apparent effect? That would've built up the elastic energy that then released suddenly.
Dithering in the presence of backlash also shows in this way, but Craig said there's no dithering here.
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Definitely the belt. I have a belt modded eq5 and get 0.5 to 0.7 no spikes.
Easy fix. Take of the side plate and touch the belt with your finger. If it's too loose it will flex more than a pencil width. If too tight it won't budge by more than 2 to 4 mm.
Adjust accordingly
Happy guiding
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Here's something for thought, from the theoretical view. I have an SCT, so don't have experience with the short FL setup you're using.
I see that your guide scope and camera give you 6.45 arcsec per pixel resolution. Also, I believe that the ASI120MM is a FRONT lit sensor, meaning you have dead zones around each pixel. It's entirely likely (in my mind) that your guide camera is picking up your guide star with merely a single pixel. The dead zone around the pixel doesn't see the slight drift in tracking until suddenly you're getting a lot of photons on the neighboring pixel. Suddenly, your mount gets a command for a big correction, on the order of your pixel size of 6.45 arcsec.
Maybe a solution is to actually slightly DEFOCUS your guide scope, so that you have neighboring pixels picking up photons? The centroid model in PHD2 will definitely appreciate the additional pixels' data.
Yeah, this is definitely in a different direction, and I can't back it up because of lack of ability to check it. But, look at the spikes - they seem to be in 'quanta' of that 6'ish arcsec realm.
Good luck with finding the cause, and hope you have clear skies!
Mark
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…. except that you would think you have errors in both RA and DEC. It was just a thought. It could be that the belts, etc in above discussion are also suspect and it shows more on DEC due to more issues on that axis.
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If it has had the belt mod that means someone opened it up.
I would suspect one of the RA bearings has grit in it, and every time it rolls over that it pushes the axis up/down which will appear as a fast spike in dec.
I've mentioned this before - at the typical dimensions of the parts in these mounts an arc second corresponds to about 1 micron. YOU CANNOT SEE particles that small. Even if a part looks clean naked eye I would guarantee it isn't when examined under a loupe or a microscope.
If the mount has been opened by someone in surroundings that are less than scrupulously clean it is virtually certainly the worms and bearings are contaminated.
You could attempt to strip it down, degrease and clean, but by the time you reassemble the chances are it will be worse not better - unless you have a cleanroom environment similar to a semiconductor lab.
This is a major reason I will never buy a mount secondhand. Especially one that has been futzed with.
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MaksPower:
If it has had the belt mod that means someone opened it up.
I would suspect one of the RA bearings has grit in it, and every time it rolls over that it pushes the axis up/down which will appear as a fast spike in dec.
I've mentioned this before - at the typical dimensions of the parts in these mounts an arc second corresponds to about 1 micron. YOU CANNOT SEE particles that small. Even if a part looks clean naked eye I would guarantee it isn't when examined under a loupe or a microscope.
If the mount has been opened by someone in surroundings that are less than scrupulously clean it is virtually certainly the worms and bearings are contaminated.
You could attempt to strip it down, degrease and clean, but by the time you reassemble the chances are it will be worse not better - unless you have a cleanroom environment similar to a semiconductor lab.
This is a major reason I will never buy a mount secondhand. Especially one that has been futzed with. Actually, the latest version of the EQ5-Pro comes with belts as standard. My EQ6R-pro did as well. I'm not sure how long that has been the case though.
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I've come to the conclusion that his mount just isn't suitable for imaging. The person who gave me it said that it was tracking and guiding well but that's just not what I'm finding. As it is second or even third hand, I don't know the history of the mount but something is mechanically very wrong. I've decided to wait and see what Black Friday deals ZWO run this year and get an AM3 if they are on offer. I think last year the deal was a free tripod when you bought the mount head.
Thanks for all the suggestions. I've just decided that it's not worth loosing all the clear nights messing around with the EQ5 Pro.
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I wouldn't discard it before attempting to fix it. As you say it's been used and it clearly needs tuning, which eventually they all do. When I started out in this hobby I did the same thing, bought an old HEQ5 PRO which I modded and tuned myself. It seemed like a lot to take on at the time, but it's not rocket science (thankfully). Adjusting backlash can be a bit cumbersome, but otherwise I found it a neat/teaching experience.
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Well, I guess you have nothing to lose by "having a go" at stripping, cleaning, regreasing and reassembly…. if successful it's cheaper than a new mount.
Just remember cleanliness is key, and if possible I'd find someone with an ultrasonic bath cleaner large enough to take the worm and worm wheels to be certain they really are free of any loose particles. Failing that, thorough scrubbing of the teeth with an electric toothbrush and solvent.
And dont drop any of them on a hard surface.
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