191 replies to this topic

[vidrazor]

Went out and shot a couple hours of Bode's Galaxy. In Siril the run looks like this:
 

Those spikes show increased star size. The actual images at those spikes generally show increased trailing. They're about 8 minutes apart, which I understand is the same as the worm period - but it's not periodic error in the usual sense where the mount "wobbles" due to imperfections/out of round in the worm or ring. (I'm not sure why star size generally increases over time; maybe seeing or transparency was getting worse. There was no dew that night.)

So this raises the question: why spikes? Is there some kind of microscopic damage to the worm? Poor manufacturing QC? Or is it just binding at a high point? I don't have any good way to examine the worm for small defects. I'll try resetting the spring a little to give it more freedom of motion, but I suppose that will create extra play as well so I'm not sure I'll get the desired outcome...

Hmm, the worm period for the SGP is listed as 600 seconds, as you can see below, so 10 minute intervals. So, stiction? Perhaps loosening the spring may be helpful. I guess simply try it.
 

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[xploeris]

That 8 minute figure is taken from counting subs, not from checking timestamps - if there was an 8 second delay (for data transfer) between each sub, that would make the spikes about 10 minutes apart. I don't think the delay is quite that long, but I can check the times on the files. But there's definitely something periodic happening there (the spikes are pretty regular) and I think the worm is the primary suspect because what else has a period in that ballpark?

The problem with adjusting the worm spring is that the worm/motor assembly is in the way, so it's guesswork. But I found that if you take off the dec bracket holder and clutch, there's a narrow space that you can get a hex key into so you can adjust the spring while testing the assembly for proper springiness. (When you put the dec bracket holder back on, of course you have to take care that it's aligned with the polar scope. It would be a simple matter to drill a small "keyhole" in the clutch to allow adjustment without taking anything apart, and I might end up doing this.) I backed the spring out just to the point where I could feel some play in the assembly, and then about a quarter turn back in to ensure constant spring contact.

Having the spring backed out this far does mean that it's easy to generate RA play; just wiggle the dec blacket and the ring pressing against the worm causes the assembly to lift slightly. I suppose this will eliminate binding, but make proper balance absolutely crucial and make the rig more sensitive to wind and vibrations. I also suppose there's an optimal setting that applies exactly as much force to the worm as it can handle without binding, but finding that point will take a lot of testing. I guess it's a good thing we're heading into the part of the year where nights are tragically short but usually clear? Perfect for testing...

[vidrazor]

That 8 minute figure is taken from counting subs, not from checking timestamps - if there was an 8 second delay (for data transfer) between each sub, that would make the spikes about 10 minutes apart. I don't think the delay is quite that long, but I can check the times on the files. But there's definitely something periodic happening there (the spikes are pretty regular) and I think the worm is the primary suspect because what else has a period in that ballpark?

I can agree with this, because yes what else? See if you can get a more accurate figure, as the gearing is standard, so the worm period should be as stated.

I read somewhere that the new Star Adventurer GTi has a 180 tooth gear vs the 144 tooth gear in the SGP and single axis SA. If off-shelf gearing was used in the SGP, I wonder if it would be possible to get higher tooth off the shelf gears to use? Depends on cost of course, there's a point of diminishing returns in hacking up gear, especially a unit like this. I also wonder if the electronics would work normally with different gearing. I would imagine it would not be an issue.

 

Still, I have yet to get my SGP to give me more than 25 seconds unguided, and that just seems off to me. I feel closer to 60 should be possible, and like yourself I don't know where the problem(s) lie.

[xploeris]

Hrm. The motor in the SGP is designed to go at certain speeds to match Earth's rotation. You'd have to replace both worm and ring (so the teeth match). If you turn that worm at the same speed as the stock worm, will it turn the ring at the same speed as well, or does changing the gearing throw off the ratio? (My brain won't calculate gear ratios across a worm/ring interface.) This assumes that the 180 tooth gear is the same size. If It's bigger, then it won't fit anyway. I don't think swapping the worm and ring is the way to go, even if you could get the parts cheap.

Yeah, I don't know how much potential there is to hypertune or mod one of these trackers. It's already belt-driven. I guess you could polish the worm and swap out the bearings for ceramic, but would it even matter? If I can get it to reliably give me 60s photos unguided, that'll be great.

[vidrazor]

Hrm. The motor in the SGP is designed to go at certain speeds to match Earth's rotation. You'd have to replace both worm and ring (so the teeth match). If you turn that worm at the same speed as the stock worm, will it turn the ring at the same speed as well, or does changing the gearing throw off the ratio? (My brain won't calculate gear ratios across a worm/ring interface.) This assumes that the 180 tooth gear is the same size. If It's bigger, then it won't fit anyway. I don't think swapping the worm and ring is the way to go, even if you could get the parts cheap.

Well yes, obviously the worm would need to have a matching tooth size.That's why I think it would work. However it depends on whether off the shelf gearing was used, or if those gears are custom made by iOptron. If they're readily available for a reasonable price, it would certainly be worth the swap I think.

[xploeris]

Again, if the gear ratio is different, the mod won't work (unless you can tweak the electronics to change the rotation speed).

Or, if the worm doesn't fit in the stock assembly, then it won't work without machining a new assembly. Or, if the ring is a different diameter then again you need to machine a new assembly to make the worm mate with it. So we would need to find out those details before we could guess whether this was a feasible mod.

I don't even know if the QC is any better on the 180 tooth set - if not then you might not see substantially better performance. You'd be better off getting "stock" part designs machined to higher tolerances - but that's $$$ for a precision custom job, at which point why not just buy a better mount instead? Equatorial/GOTO and better machining to boot. (It's the same issue as with cars - for what it costs to mod a beater you could have just bought a better car.) But I'm obviously no expert, so anyone is welcome to prove otherwise...

Someone in one of these threads mentioned using a multimeter to read the voltage across the motor to see how hard it was working. Seems to me that might be a viable way to test whether you're getting binding or stiction without having to go outside, and it would be a lot more precise than squinting at test photos or trying to judge performance by eye or the pitch of the motor whine.
 

edit: Checking the file creation times confirms that 8 minutes worth of exposures took about 10 minutes to make. So that's cleared up.

Edited by xploeris, 28 May 2023 - 03:10 AM.

[xploeris]

Last night I went out and did some more test shots - the first batch since I loosened the spring. All of them had visible RA trails at 60s, though some are worse than others. Siril's plot seems to show their roundness "bouncing" up and down every couple of frames, suggesting it's speeding up and slowing down. This could be due to wind, I guess, but I set up behind a barrier and all I could feel was a very light breeze.

In the longer term, stars are drifting eastward in my FOV, which seems to suggest that my mount is running slightly too fast. This was also something I noticed before my last adjustment. I'm not sure what could be causing this - some people have reported the mount running too fast when overcharged but I've used it for a few nights without recharging so that shouldn't be the problem. I also ran the calibration routine on it so it shouldn't be that.

(Aside: I'm not happy with my PA. Getting a good alignment in the polar scope is a PITA as usual, no thanks to the SGP's janky wedge - and I'm not spending $200 for the WO wedge, thank you! - but the three point alignment routines in NINA and APT seem to put me wildly off target, rather than refining the PA I get visually. Sometimes I perform the requested adjustment, wait for an update, and see no significant change in the reported error! - which makes me distrust these tools. Curious to see how well Sharpcap's routine works, supposing I can get it to run with my DSLR. In any case, the trailing I'm seeing in RA is much worse than whatever I'm getting in dec or field rotation.)

I'm beginning to wonder (a) if there is actually something legitimately wrong with my mount that can't be adjusted out, since whatever its inherent inaccuracy might be it should still be tracking the sky over the long term, not outpacing it, and (b) if I'm chasing precision that the mount simply can't deliver.

If there is a problem with the mount, it's out of warranty by now, and I doubt whether it's worth sending it in for maintenance/repair and paying a significant fraction of the mount's cost to get it fixed, knowing that it would still be a cheap, imprecise tracker afterward. If I'm going to spend that money, I think I'd better spend it on autoguiding and brute force the problem, and then if I decide to upgrade the mount later I'll still have my guiding equipment.

Still, I'd like to do more testing. Tonight I'll try different balance settings - almost even, a little more east-heavy, a little more east-heavy than that, etc - and see if I can't find a happy place. Maybe the right amount of weight will smooth out the mount's movement without making it strain overmuch. I should have done all this last summer, but at the time I was still using a ball head that I couldn't balance completely because I didn't want to spend money on rails and clamps and pan heads, and... well, live and learn.

[vidrazor]

Still, I'd like to do more testing. Tonight I'll try different balance settings - almost even, a little more east-heavy, a little more east-heavy than that, etc - and see if I can't find a happy place. Maybe the right amount of weight will smooth out the mount's movement without making it strain overmuch. I should have done all this last summer, but at the time I was still using a ball head that I couldn't balance completely because I didn't want to spend money on rails and clamps and pan heads, and... well, live and learn.

Perhaps try tightening the worm past where it originally was and see if that has a beneficial effect.

 

Certainly better to invest in autoguiding gear, but of course it's best to get them mount running as mechanically good as possible. That will make autoguiding that much better.

 

I've been working with other mounts and gear lately, but I want to get back to the SGP and see what else I may be able to do to hopefully improve it's performance. I had shown iOptron a video of the worm in action, a still from it is below, and they mentioned I should clean and re-lube the gears, and to use Super Lube on them, which I thought was weird because that's obviously not what they use on the gear.
 

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Edited by vidrazor, 28 May 2023 - 06:56 PM.

[xploeris]

I suspect iOptron uses some bulk industrial lube (or their Chinese manufacturer does). Superlube may be the closest suitable product that's readily available to consumers. Or, more cynically, they may use an inferior lube that iOptron can't be bothered to clean off and replace before sale, but hey, if you're gonna do it, might as well sub in the good stuff...

[xploeris]

Bit of an update: I've been shooting M101 for the last couple of nights, trying to get that supernova. I've got about 200 60s subs so far. The majority of them, I would say, have acceptably round stars. The rig is sensitive to wind, as I predicted, but about 3/4ths are keepers, and I don't have the problem where I lose framing because the mount is running too fast.

What's my secret? More weight on the east side. Forget "slightly" east-heavy, make the motor work for it. Hold that ring firmly against the worm.

There's probably a better setting for that spring, but this has been very encouraging.

[vidrazor]

Bit of an update: I've been shooting M101 for the last couple of nights, trying to get that supernova. I've got about 200 60s subs so far. The majority of them, I would say, have acceptably round stars. The rig is sensitive to wind, as I predicted, but about 3/4ths are keepers, and I don't have the problem where I lose framing because the mount is running too fast.
What's my secret? More weight on the east side. Forget "slightly" east-heavy, make the motor work for it. Hold that ring firmly against the worm.
There's probably a better setting for that spring, but this has been very encouraging.

Well I always thought that was part and parcel about improving tracking, but how much of that bias has always been a bit of a mystery, at least to me.

 

Another variable is the tripod. Unless you have a really stable one, best to not extend the legs, or extend them very minimally. This typically requires crouching down to do a (at least rough) polar alignment, but the rig will be far more stable.

[xploeris]

All the advice I've read indicated a slight east bias to hold the gears firm, but it also said that it was critical to have these little mounts balanced, especially with larger loads. So I'd get it balanced in RA as best I could and then make it slightly east heavy, but not very much. Wish someone had told me to make it heavier a long time ago. Then again, the retaining ring on my mount WAS too tight, and then the worm WAS having periodic issues, so I feel like my tinkering legitimately helped in the end.

I've always used the same tripod, so that's not a variable for me - it's equally bad for every run! (I kid, I kid - though it's a relatively cheap tripod. I only extend the thickest leg sections, and I have a stone bag.)

[vidrazor]

All the advice I've read indicated a slight east bias to hold the gears firm, but it also said that it was critical to have these little mounts balanced, especially with larger loads. So I'd get it balanced in RA as best I could and then make it slightly east heavy, but not very much.

Do you balance in DEC as well?  Depends on how you mount your scope and camera, but I noticed that because the SGP bearings are so good, once you move your scope and camera to a target, you knock the DEC balance off (yes, the SGP is that sensitive, other mounts are not), so if you rotate the rig back to 12 o'clock in RA, it will be imbalanced again and fall one way or the other. Most people compensate by readjusting RA, but DEC is what really needs to be readjusted. So at least the way my rig is set up, I move it forward or backward as needed on the Vixen or Arca mounting bar the rig is mounted to on the DEC axis, as you can see below (this setup is for using DSLR optics, as opposed to a scope), and move it until it is balanced again. This will require re-aiming the rig to the target, which is straightforward for me since I use plate solving to manually align to a target.

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Edited by vidrazor, 30 May 2023 - 10:23 PM.

[xploeris]

I didn’t used to balance in dec when I just had a ball head attached to the dec bracket, but I found that having the center of gravity off the plane of the bracket did weird things to RA balance as it moved around. I ended up cobbling together something very similar to your rig, with the camera on a sliding rail mounted on a pan head.

Haven’t decided how I want to mount a guidescope yet… I had thought to put it under the vixen rail, but that might cause me problems if I’m aiming south, and I don’t have a ton of extra room at the end.

[vidrazor]

Haven’t decided how I want to mount a guidescope yet… I had thought to put it under the vixen rail, but that might cause me problems if I’m aiming south, and I don’t have a ton of extra room at the end.

If you're using photographic lenses with a tripod mount, I can't think of a better arrangement than this. The lens is mounted to the L-bracket, which is mounted on the Vixen bar (or Arca bar if an Arca setup is used). This allows lateral adjustment if necessary for balance.

 

On an actual scope I just mount it on the scope itself, like this.

[vietspace]

Hello all,
I'm looking for a tracker that I can put in my checked baggage when flying. I really like the iOptron SkyGuider Pro, but it has a built-in battery and can't be placed in checked baggage.
However, is it possible to remove its built-in battery and use only an external power source, such as a power bank?

[DanielJStein]

Hello all,
I'm looking for a tracker that I can put in my checked baggage when flying. I really like the iOptron SkyGuider Pro, but it has a built-in battery and can't be placed in checked baggage.
However, is it possible to remove its built-in battery and use only an external power source, such as a power bank?

You can’t really remove the battery. Besides, an item like that really should go in your carry on. The only time I have checked a star tracker was my ZWO AM5 and it was a hard Pelican case with the Trekpak dividers which are meant to be pretty robust. I would recommend a carry on sized Pelican that you can put all of your camera gear in, then check anything less valuable. Another solution is to just put the tracker body in the carry on, but you can check the alt-az base, tripod, etc.