Large swing in guiding - Please Help!

[PhotoGav]

I was rejoicing in the dark sky last night and gathering some long subs, however I noticed that my guiding was all over the place... It was inconsistent too, the Dec graph ranged from beautifully flat to Himalayan and that seemed to change between subs, so one sub had good Dec guiding, then after the dither, the next sub would be all over the place. The RA guiding was swinging back and forth from side to side all the time. Attached are various screenshots of the graphs.

I am wondering if the mount needs some grease on the gears and bits inside? Though I am terrified of opening up the mount and killing it! Please let me know what you reckon and if there are any other tricks I should try too. Balance is good, in both Dec and RA. The subs are OK, but the ones where the DEC settles and the RA is up and down are worse, showing slightly elongated stars. Last night was not the first time I have noticed this, it has been like it for the last few sessions, having been better in the past. I can't think of anything that has changed dramatically recently, except perhaps for better polar alignment these days.

 

Here's the graph with both RA and Dec all over the place:

 

 

And then the Dither...

 

 

After the dither DEC settles down and RA carries on all over the place:

 

 

Please help!

[Oddsocks]

It could just be normal backlash in the RA and DEC gears, when you set up do not try for perfect balance, leave something in the range of 0.5kg to 1.0kg unbalanced in both RA and DEC, this will create a bias in one direction between the teeth of the gears and ensure that the drive is not bouncing back and forth.

This method works really well until the mount/OTA gets close to crossing the meridian with the RA weights pointing towards the ground or the OTA pointing straight up, in this position the bias caused by the unbalance disappears and oscillation will return. When imaging close to the meridian a cheap solution that works well is instead of unbalancing the counterweights is to tie one end of a 2m length of 6mm diameter elastic shock cord around the OTA and the other end around something heavy on the ground or the tripod leg then shorten the elastic cord so that a force equivalent to 0.5kg to 1.0kg is pulling on the OTA in the problem axis, or at a tangent to both axis. This method will not be compromised during meridian crossing but you do have to watch out not to trip over the cord in the dark and remember to untie it before slewing off to a new target.

The longer you can make the length of cord the less its pull will change as it stretches or contracts as the mount tracks the object over the course of a few hours imaging. 

Elastic shock cord is sold by the metre in yacht chandlers with a large choice of diameters and in DIY stores but usually in only one or two sizes.

OTA clip-on weights do the same thing and allow you to unbalance in RA and DEC but at a rather higher cost.

 

[Davey-T]

Can't be much help but just to let you know you are not alone Gav, m.y setup was working fine but in my wisdom I bought an Altair 60mm guide scope and since then it's all gone to pot, graph looks like yours.

I've taken the opportunity to strip it down and regrease stuff and made a PTFE sheet to fit between the tripod and the mount to help with PA adjustments, was hoping to try it out before SGLX1 but not looking like it ATM, clouds here last night.

There is a later version of PHD2 now so you might as well give that a try, nothing ventured etc.

Dave

[PhotoGav]

Thank you Oddsocks & Dave. Interesting approach Oddsocks. I will check imbalance carefully next time. Not sure I'm brave enough to lay out trip wires quite yet, but that might be a solution!

If it ain't broke, Dave, but you don't need to hear that now!!! How easy was the mount strip down? Enjoy SGLXI, I wish I was coming this year, but work is in the way and I can't get out of it. Very annoying.

[Davey-T]

Could've, should've left well alone, I was swapping the guiding from the WO110FLT to the WO Star71 and imaging on the other but thought a guidescope between them on the dual mounting plate would be a good thing.
Went from a nice flat graph to sawtooth, only added a small amount of weight so no idea really, perhaps there'll be some helpful folks as SGLX1

Mount is Ioptron iEQ45 instructions  on their site so not to hard to strip down, mind I haven't tried it since so

Shame you can't come, Alans had to pull out as well so be missing you.

Dave

[SonnyE]

I agree with Oddsocks, a bit of imbalance keeps the gearing just slightly biased. (loaded)

What brought me to post is your RA and DEC settings, those are bit far out from what I typically run. Mine are usually 35 and 35, and my Hysteresis is typically 20. Have you ever used the Guiding Assistant in the tools menu? Sometimes that can help. But when I've had my RA or DEC settings high like yours, the mount over reacts to guiding commands, and the graph swings wildly like yours is doing. So what I've found is the lesser settings of 35 and 35 gives mine smaller steps and it doesn't swing out wildly either way.

Maybe these two PHD2 Basics video's can help you find what is off a little and causing the Himalayas.

PHD2 Basics 1

PHD2 Basics 2

But the RA and DEC aggression can make my graph run out like yours is showing. Worst case scenario where the guiding runs off, I've found that if I shut down PHD and restart it in the area I want to operate in, then let it set up where my target is, it usually fixes any wild hairs in my guiding. I like to refer to it as a hard boot of PHD. Sometimes I will hard boot PHD if my object does a meridian flip and my mount goes bonkers.

My mount and scope typically is tail heavy due to the mass of imaging and guiding equipment. I don't fuss too much about the imbalance because I'm only running ~13 pounds of gear, on a 30 pound rated mount. So my AVX is loafing along.

[PhotoGav]

Thank you for the advice, Sonny. The forecast is clear tonight so I will have a chance to try out your suggestions. So far guiding has worked flawlessly (or at least, equally out in all directions to give me round stars!) so I haven't had to start fiddling with the weird numbers underneath the guide graph. I will do some reading up to try and understand how PHD2 is working and hopefully will be able to alter those numbers with some educated guesses. I will definitely try out the Guiding Assistant tonight too, something I have done once or twice before, but not very seriously!

Fingers crossed.

If there is little or no difference, I am thinking it might be time to apply the Rowan belt mod to my so far trusty HEQ5 in the hope that that gives it a new lease of life.

 

[Starlight 1]

This was a good post on Phd and something not to be over look . hope this is right link.

I Think I Finally Understand My PHD2 Graph! - Imaging - Discussion - Stargazers Lounge

[PhotoGav]

Thanks Starlight. Generally I agree that the graph doesn't matter if the resultant image is good. However, I am having to worry as the resultant images are starting to suffer and have been better in the past, so something must be up... This is the joy of amateur astrophotography with cheap kit - all the tinkering required to get half decent results!

[Thalestris24]

Hi

I wish my guiding error was only +/- 1.5" !! I usually have my y-scale set to 16.... Your graph would look pretty good at that scale :). I think atmospheric effects can cause that sort of error - you can tell by just monitoring the guide star. Also, your RA osc is too small - you need it to be 0.5 to get round stars. You may have some RA backlash. You can tell a lot by running the Guiding Assistant to see how things behave without guiding corrections.

Louise

[PhotoGav]

Thanks Loiuse. Generally the mount behaves pretty well, it's just started to be a bit naughty recently, not sure why. Anyway, my tinkering this evening has been interesting. This morning I opened up the motor area and smeared the grease that was present all around the gears, it is certainly quieter this evening and the centering routine with plate solving is working much better. I also made sure that the RA was heavy in the east. I have been fiddling with the guiding and, despite the pretty suspect seeing here, it seems to be doing Ok, certainly my stars are round tonight and that is what really matters. Anyway, I have ordered the Rowan Belt Mod kit and look forward to seeing if that helps calm things down a bit. I am keen to get the mount humming for the next project which will involve the C8 and will require some accurate guiding...

[Thalestris24]

7 minutes ago, PhotoGav said:

Thanks Loiuse. Generally the mount behaves pretty well, it's just started to be a bit naughty recently, not sure why. Anyway, my tinkering this evening has been interesting. This morning I opened up the motor area and smeared the grease that was present all around the gears, it is certainly quieter this evening and the centering routine with plate solving is working much better. I also made sure that the RA was heavy in the east. I have been fiddling with the guiding and, despite the pretty suspect seeing here, it seems to be doing Ok, certainly my stars are round tonight and that is what really matters. Anyway, I have ordered the Rowan Belt Mod kit and look forward to seeing if that helps calm things down a bit. I am keen to get the mount humming for the next project which will involve the C8 and will require some accurate guiding...

My heq5 improved with the belt mod - need to balance it well, not east heavy. Someone else had some problems fitting theirs (search on here)

Guiding problems can be very frustrating - my avx has been misbehaving recently. It can be difficult to diagnose the root cause of a guiding problem...

If it's not one thing it's another! Sigh

Louise

[PhotoGav]

6 hours ago, Thalestris24 said:

My heq5 improved with the belt mod - need to balance it well, not east heavy. Someone else had some problems fitting theirs (search on here)

Guiding problems can be very frustrating - my avx has been misbehaving recently. It can be difficult to diagnose the root cause of a guiding problem...

You have just answered my next question! I was searching whether belt driven mounts should be east heavy and not really coming up with many answers, so perfect timing with your post, thank you! I had hoped and guessed that would be the case as the whole point of the belt system is to eliminate backlash, which is what the east heavy trick is trying to do too. In fact, part of my reasoning to go ahead with the belt mod is so that balance doesn't have to be out as I am in the process of getting to grips with SGP and am keen to use the auto meridian flip feature, but kept on wondering how to deal with the problem of shifting the weights after the flip to keep it east heavy, when I am in bed asleep enjoying the benefits of the auto flip!

Good luck with your naughty mount. I wish I could help!

[PhotoGav]

Thought I would post an update on this little problem.

I have fitted the Rowan Belt mod to the mount and initially had problems with the plate solve go to not managing to get within 20 pixels of the target and the guiding was not much better than pre-belt mod. Anyway, I opened up the motor area again yesterday afternoon and secured the motors to make the belts tighter in both RA and DEC. I was able to try out things last night and hey presto, the first itteration of plate solve go to was only 19 pixels away - result! Then the guiding, with my EdgeHD 800 and reducer, so focal length of 1422mm, performed superbly. Here is a screen grab of PHD2:

 

Sweet, I've never seen my set-up guide so well!

I'm now waiting for the next bit of required tinkering as one thing is for sure, nothing ever works perfectly, or at least certainly never for long!