[SOLVED] Ongoing mount problems

[lazjen]

A while back I posted some PHD graphs showing an issue I was having with my G11 with RA - it's still happening. It's spikes around like a large magnitude earthquake, while the DEC is a lot more refined and is no issue at all.

Admittedly the weather has stopped me from doing a lot of testing, however I've managed the following:

* Cleaned and regreased the RA and Dec gears/worms with Super Lube. This had no noticeable effect on the problem (it still needed to be done anyway).

* Tried different balance positions. I've tried slightly unbalanced in both directions and balanced. So far it seems that balanced appears to be the best, but the difference is not great enough to be truly conclusive.

* I've tried a few different orientations for drifting alignment (weights on west, east, dec at various points, most tests including the one below pointing straight up or close to it (maybe within 20 degrees?).

* I've tried adjusting the minimum move amount (lower seemed better), the hysteresis (no real effect), the aggressiveness (higher seemed better, but inconclusive at this point)

190905

The above I did last night. With a RA peak-peak of 26.8", it's just crazy. I think I can refine the PA further to get below 1', but I doubt that should have much impact on this problem.

I don't believe this is a seeing issue as I would expect DEC to be effected just as badly then.

I have a feeling it's some form of backlash that's happening, especially since the RA is always being adjusted as it tracks across the sky. Is there a way I can specifically measure the backlash? Would it be worthwhile swapping over the DEC and RA motor, gears, worm?

Also, I'm open to any other suggestions. I'm going to go through the Gemini settings tonight (again, but with extra care) to see if anything is wrong there, but I'm doubting it. If there's something specific you think I should look at, please let me know.

[billdan]

Gooday Chris,

I hope you are aware that with guiding assist turned on that guiding is actually turned off, guiding assist gives you an idea of your raw mounts performance.

So making PHD changes will not affect the results shown by guiding assist. (except for PA adjustments).

Also backlash shouldn't be an issue as RA only goes in one direction (never reverses), only slightly slower or faster.

Hope this helps
Bill
EDIT: Having said all that, 28 arcsecs Peak-Peak sounds a lot for a G11 should be about 7 or 8. Try turning the dart board on next time and watch the stars move and see if there are any sudden jumps in RA causing this. Could be a binding worm??

[lazjen]

Yes, I'm aware the guiding was off, but that was the point to show the difference between DEC and RA.

While guiding is on, the DEC line is near flat, the RA is the seismograph.

I did have the "dart board" on actually and the RA was jumping back and forward making a low lines of dots.

I hope it's not the binding worm since I cleaned and re-greased the worm, etc. However, it's the first time I've ever done that, so maybe I did not do a good enough job.

Note that this problem existed before I did the re-greasing too.

[billdan]

Chris,

The movement backwards and forwards is normal I think, I get that with my EQ6. What we don't want is for the star to suddenly jump off the 'dart board' and not return, this would suggest that something has changed, either a change in balance, cable snag or worm/gear binding.

When I did get a sudden jump with my EQ6, adjusting the weights cured the problem. At the time I thought I was East heavy but it wasn't heavy enough.

Regards
Bill

[lazjen]

Well, I've just switched the DEC and the RA worm and greased it up again. I'll see if that makes a difference tonight.

Yeah, I remember seeing it moving around a bit in the past, but always within the "dart board". This however has been jumping all over the place, well beyond the dart board.

Edit: I've checked the cables as well, but that's ok - no real change there.

[lazjen]

The change has definitely made things better. So it was either the worm change or the re-greasing or both. :)

I've got a bit to go yet to determine how much better. I think at this point it still isn't enough.

[lazjen]

Ok, it didn't help. The RA peak to peak is still around the 27" mark.

I'm noticing a periodic movement though, peak to peak of around 200+ sec.

Read the manual: "The cycle of the periodic error is equal to one rotation of the worm gear, which is four minutes for the G-11" That's probably it then for the cycle.

[lazjen]

Finished for the night, no better than last night. The mount is basically useless with such a variation in place (27+" peak to peak) - can't plate solve, can't guide well, etc.

I guess it's try some other approach tomorrow... :(

[billdan]

Hi Chris,

Sorry your worm swap didn't come off, at least you eliminate them as the cause.
Last night I tested my EQ6 with guiding assist and my P-P arcsecs in RA was 28 arcsecs (normal for EQ6) and PA was varying 1-3 arc minutes.

I then did a 2 hour run at NGC300 and the photo below shows the guiding I got after 2 hours. Even though only the last 800 secs are shown.

You should also be able to guide out your PE error if its a slow PE curve, if its high freq movement that would be more difficult.

Bill

[alistairsam]

hi Chris,

with your re-greasing and worm mesh, do you have any play in RA at all when parked. its more obvious with an OTA than without.
what type of grease have you used?

I've tuned EQ6's and what I noticed was that with anything other than superlube, although the mesh seemed tight, it was in fact spongy with oscillation in RA as you're seeing.

Other thing you could do to identify is capture video of either a planet or the moon and watch the movement. that'll give you a better idea of tracking issues than a guide monitoring.

Finally, the guiding is it with an OAG or a guide scope?
have you examined the RA ring gear closely?
you've eliminated issues with the worm by swapping but how about the gear?

Cheers
Alistair

[gregbradley]

First check a few basics:

1. Is your mount accurately polar aligned? How accurate and how did you align it?

2. Is your mount level?

3. If you can see the worm, is there any grit or brush hair from applying the grease in the gearing causing a spike?

4. Your autoguider is working, connected and commands to move the mount from the autoguider software are working.

If the above are all good then it sounds like a mechanical issue like grit or rubbish in the gears, poor gear mesh or some other mechanical fault etc.

Huge PE sounds like something is totally off, not just a little bit. So it should stand out when found.

Greg.

[lazjen]

I'll answer the posts soon, but I have some extra info. I took the cover off the RA worm so I could examine it more closely. I had read issues about the mount blocks for the worm - there are 2, one at each end and they're independent of each other.

I tried some adjustment of the blocks and I can see some improvement although I'm finding it hard to lock into place. I managed to get a peak to peak of 16 at one point, but as I write, I've locked it in place again and am back to 25-27 range. I find it a flawed design to be made in such a manner and can now understand why people replace it.

Anyway, with the adjustments though I can almost guide again - almost (RMS is a about 1.15 for RA). I haven't tried to plate solve again, but unless I can get a clear few second image without blurring, it'll be pointless.

So there's something significant here that I'll have to manipulate further. Unfortunately I can't think of a logical approach other than fiddle and test - with each test needing to take > 4 min for a worm revolution.

[Ryderscope]

Hi Chris,

Some more advice on balancing. May not help but if your busy trying things why not?

I find with my EQ8 that it is quite sensitive to balance and if I don't get it right I will see a reduced guiding performance. Not to the extent that you are seeing but worth looking at all the same. This may not be relevant to the G11 but worth checking.

As well as balancing in RA and DEC, I find with the EQ8 that it is important to also make sure that the weight is evenly distributed laterally across the top of the mount where the OTA sits. In other words if you are standing behind the mount looking at the OTA pointing towards the pole, the weight should be distributed evenly on each side of the OTA.

It may be easy to assume that this would always be the case but in many cases the finder scope will not be in the centre of the OTA but is off to one side. If this is the case and depending on the mount, the centre of gravity and balance point will shift as the mount moves in RA.

In my case with the EQ8, if I move the mount in RA and I haven't got this third balance point correct then it may seem to be balanced when checking in the 'classic' way with the counterweight bar horizontal. If I then move the mount manually in RA however I will find that there are points where the scope/counterweight will not stay in position and will drift to the centre of gravity point. This is further complicated with the EQ8 due to the position of the motors so may or may not be an issue in your setup.

I would recommend testing your set up by loosing the RA clutch and manually pushing the counterweight bar around in small increments. Make sure that each time you let go of the counterweight bar that the OTA/counterweight bar stops at the point you let it go. If it starts to move of its own accord when you let it go (even after you have checked the RA/DEC balance) then it might be worth checking the lateral balance as indicated above.

When I have had issues with this in my current set up I have simply rotated the OTA so that the lateral balance is correct. I note that you may be using a GSO RC10 so I am unsure as to how you may do this.

Sorry for long post but it might just be worth a shot.

[lazjen]

What's the scope on there? What focal length are you at? That's still pretty good - that RA RMS 0f 0.1 - I'm over 1.0 at this moment.

Btw, I've got my RC10 on, no reducer, so at 2000mm.

[lazjen]

I'm using superlube. What do you mean by play? It doesn't seem to do anything different to what I've seen in the past. As in, it seems to be about the same amount of effort to move it as before.

Video - I might try that sometime. I think I'd have to switch back to the DSLR for that as I haven't got anything else suitable. Would that be ok for it?

Guiding is normally with an OAG. When I first started this though, I was redoing my PA for the mount in the observatory (no pier yet) after a dark site visit. I've been using the main camera (Sbig ST-8XE) for the testing as it gave greater star choice (wider field of view) than the OAG.

Although I've cleaned/greased the gears, I haven't examined them more closely yet.

[lazjen]

1. I believe the PA is within about 1' now, although I haven't confirmed with any other tool/approach other than what's being reported in PHD2.

2. Mount is level, it's in my observatory with a laser level floor. I've had the same setup working in the past.

3. No. My changeover/regrease last night seems to have helped. I get the peak to peak of about 27 over 4 minutes, but it's a "sine wave" style now, with smaller spikes. I'm willing to say that a good proportion of those spikes off the main "wave" are due to seeing.

A problem with this wave though is that the rate of change throughout the cycle is not the same, with some rapid changes during the wave's "rise".

4. Yes, you're right. The guider is definitely operating.

And I also agree there's at least one mechanical fault here. As I mentioned in a previous post, it's looking like the worm block holders alignment/positioning is having an impact. I'll look into the gears more as well.

[lazjen]

I'll check this, but I think it won't apply to me. I don't bother attaching the finder scope most of the time, since I never really use it.

The set up is almost the same as what I've used in the past. I've taken out the focal reducer and put in a spacer before the focuser. Other than that, I can't think of any other configuration changes and those should have no real impact.

*********************

For those that have made suggestions:

Tomorrow night I'll try some adjustments again and test. I'll collect some measurements and post the details/pics. If there's any specific test and result pic you'd like to see, please reply and I'll try to get it.

I'm currently using PHD2 for testing.

[billdan]

Chris,
I'm imaging at 1000mm (8in F5) and guiding with an Orion Mini-Guider at 160mm (2in F3.2). I don't know how much those PHD RMS numbers translate to 1000mm.

I don't own an OAG but it would be interesting to see if the same RMS numbers were reported guiding at 1000mm vs 160mm.

Regards
Bill

P.S Good luck with testing, I hope you solve the problem.

[lazjen]

Thanks. I'll solve the problem one way or another, it's just how much time and/or expense is going to be involved in the final solution. :)

[tlgerdes]

Hi Chris,

Your are on the right track with your work block aligning, that and the distance the worm sits out. Common problem of people having the worm block too tight.

One solution to your block alignmernt is the newer one piece design available. But you should be able to get the 2 piece ones working well again. One practice Frogginator taught me was to use the with cover as an alignment square between the 2 blocks, then use a feeler guage set to around 0.5mm and gap the far end block from the mount and tighten.

Trev

[lazjen]

Cracked the problem(s).

I'm currently letting the PHD2 Guiding Assistant run as I post. It's gone through nearly 2 cycles and my peak to peak is now 11.7. (Conditions here are 20k winds outside obs; skippy sky thinks seeing is going to be a '5'; it's cloud free as far as I can see; I think the seeing is spiking around 3+")

So how did I fix it? Well, the regreasing, etc has certainly helped and I might be able to reduce that number further with some more work in that area.

However the key fix was improving the PA further. You might be thinking wtf, didn't I say it was within 1'? Yes, I did - I unwittingly was telling a Big Lie.

Here's the first lesson: don't believe the PA error reported by the guiding assistant. It's only loosely based on a true story.

When I started tonight, I wanted to capture a baseline before making any other changes, so I thought, go back to basics and check alt/az. I started with alt tonight as I was wanting to do final testing in sky where I would be adjusting the azimuth.

I set up the drift align tool for alt and then get a massive WTF?? From memory it was out by about 13'. As I went to make the adjustment I noticed the locking bolt was not as tight as it could be. I'm not sure why it's that way but I'm getting some suspicions that it's not locking as well as it should and slips over time.

I fix that up and then adjust the azimuth. It was pretty close, but I spent the effort to refine it further. As I was doing it, I noticed that the wilder fluctuations were disappearing.

Finally, I went back to the Guiding Assistant to find the peak to peak value and to see how the graph appeared. It was much shallower of course and didn't have too much rapid changes on the rise. The seeing spikes were more observable (to me at least I think).

I've switched off the assistant and am now guding. The "dart board" is getting better grouping. See the pic below (note the 'y' scale is +/-4"):

191017

So, stoked to be back in the game again. Thanks everyone for the suggestions.

[lazjen]

Ah yes, this probably helped a bit too. I used a metal ruler I had to assist in aligning them. :)

[alistairsam]

but if your PA was off wouldn't the drift be in Dec? ra shouldn't show pa misalignment

[lazjen]

Well, I don't know what else to attribute the changes to - I have done 0 hardware (or software) mods since last night, other than PA adjustment.

[billdan]

Good on ya Chris,

Glad to hear you are back in the game, now you have to wait for a new moon before you can get serious.
Who would have thought that PA error would change the RA Peak to Peak cycle by so much?
Just shows that in this game you have to cross the T's and dot the I's.

Cheers
Bill

[lazjen]

I've got a few other things I'd like to do before the new moon, but I might still take some images anyway. Probably Ha.

When I go out to tonight (hopefully), the first thing I'll be doing is a PA check on alt to ensure nothing has changed there.

[Ryderscope]

That PHD graph is looking much better Chris. I feel a sense of relief on your part as I know how frustrating it can be when things just aren't going well. Conversely it is a great feeling when you kick everything off for a nights imaging and sit back and watch it all happen. Keeping fingers crossed for the next report.

[lazjen]

As is usual in this game, the weather has intervened, delaying any further progress.

Time to work on other stuff for a while. :)