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Another CG5-GT "spikes during guiding"-thread...

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#1 DHB001

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Posted 15 April 2010 - 06:23 AM

Last night, I was trying out my radial guider on M51, hoping that my flexure would be a thing of the past. Well, I set up my equipment at around nine pm, went to bed around 4.30 am, and all this time in between only yielded 2 hours worth of usable data. That’s absurdly inefficient, and was caused by 2 things, 1) sudden jumps during guiding (see accompanying screenshot), and 2) difficulty finding a suitable guide star through my OAG (lost quite some time realigning after the meridian flip).

I want to find a solution for the first issue, because it has been bugging me ever since I got my current equipment about a year ago. I’ve already read a lot of threads about these jumps, and I think I’ve done as much as I could possibly do to minimize these jumps. Besides having relubed my mount, and in the process eliminating as much backlash/slack as possible, I have taken the following precautions:

1) Polar alignment procedure via my mount. This is accurate enough not to cause significant field rotation, but is sufficiently inaccurate to ensure a little drift in the DEC, so I can guide the DEC north only or south only, which keeps constant traction on the gears.

2) Make the RA axis a little east heavy, and the DEC axis back heavy (i.e., just a bit more weight on the camera side than on the corrector plate side). Again, this should ensure constant traction between the gears on both axes.

Now, I’m only using a C6, a Rebel XS / EOS 1000D DSLR and an Orion SSAG autoguider, the combined weight of which should be well within the operational limits of the CG5 mount. Yet, I still manage to get these jumps during guiding. What’s causing this? Does the fact that M51, when passing through the meridian, for me is almost right above me in the zenith, have anything to do with these spikes? Have I inadvertently made the imbalance on my axes too big? I only move the scope and the counterweight about 1cm (or just under half an inch) away from what I perceive to be the point of balance. Those of you who use this mount as well, do you preload your axes, and if so, by how much?

For those who want to see my guide logs, please see the links below. The first run was to determine whether to guide north or south on the DEC, and the last 3 to 5 runs I experimented a little with some settings, like dec algorithm, and aggressiveness and hysteresis combinations. But everything in between is ordinary guiding with the same settings used throughout (except after the meridian flip, when of course I switched between north only and south only dec guiding).

I really hope someone can help me combating this issue. Thanks to anyone who can offer any advice.

Guide log 1: http://www.mediafire.com/?jjeujy22mmy

Guide log 2: http://www.mediafire.com/?yjny3yii2wz

Bear in mind that guiding was done at 950-1000mm, which, with my setup, equates to a resolution of approximately 1.0 arcsec/pixel for the SSAG.

Attached Files



#2 rmollise

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Posted 15 April 2010 - 06:33 AM

If the spikes were not periodic in nature, the obvious culprit could be wind or other vibration.

#3 DHB001

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Posted 15 April 2010 - 07:32 AM

Hi Rod,

Thanks for your response. The spikes were not periodic. According to the guide log, the first spike only happened after a little under a thousand corrections, but later during the night, they began to happen more frequently. Oddly enough, while most spikes were from the DEC axis, a couple of spikes were from the RA axis...

It was a clear night with no wind, so although otherwise a plausible cause, in this case it certainly could not have been wind. I cannot think of anything that could have caused vibrations. My setup stands in the middle of my backyard, on a terrace of street tiles. These tiles don't sag or wobble, and are really solid. Once I'm done focusing and framing, I use USB extension cables for my guide and imaging cameras, so that I don't have to get anywhere near my scope once it's busy imaging.

#4 seeker372011

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Posted 16 April 2010 - 02:27 AM

Wierd. I too would have said wind in fact i am Impressed it even brought it back after such huge spikes

#5 DHB001

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Posted 16 April 2010 - 04:09 AM

And what's even worse, I know my mount is able to pull off precise guiding for extended periods of time. The link below is to a guide log of last September, when I was imaging the Elephant Trunk nebula for 5 hours straight. I was also using a separate guide scope in side-by-side configuration at the time, so a more heavy and balance wise complex configuration than a single tube plus OAG. Can't think of anything I did that night that was different from other nights I have been imaging. It was the only one time everything went 100% according to plan. So I'm really puzzled by the recurrence of this spike issue...

Log of everything going perfectly with sbs config: http://www.mediafire.com/?ong2zzimduy

#6 bardo

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Posted 16 April 2010 - 04:42 AM

mirror flop? play in the OAG somewhere? backlash compensation on?

#7 DHB001

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Posted 16 April 2010 - 07:31 AM

Mirror flop? --> I was already having these spikes when I was using a guide scope (a refractor), so that seems unlikely.

Play in the OAG? --> extremely unlikely. The SSAG sits all the way down in the guider tube (no play possible), screwed tight with two screws. The screw which allows the guide tube to rotate I also screw really tight once I'm done rotating. Believe me when I say everything on the OAG is screwed down tightly. :)

Backlash comp? --> never ever used. Always on zero, both Dec and RA.

At first I thought perhaps my polar alignment was still too "correct" to induce sufficient drift in the Dec guiding, so that stiction might have caused the spikes, but on a few occasions, it was the RA that began to spike, so those spikes were definitely not caused by stiction... :question:

#8 Eddgie

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Posted 16 April 2010 - 09:12 AM

I am going to venture a guess. It is ONLY a guess. I do not image, but I have had this problem with visual observing.

As the load moment on the mount shifts over the course of a session, what happens is that the loading of the worm gear against the face of the worm wheel can change. The result is that the worm wheel can "Bounce" off of the face of the worm gear and then "Catch up."

The culprit is usually too much space between the worm and the worm wheel faces.

The easiest experient to see if this is the case is to always positivly load the faces. When imaging on the eastern hemisphere, slide the counterweight down a bit so that the worm is always pushing "Hard" against the wheel.

When working on the west, raise the counterweigt so that the weight of the telescope is pullling back against the movment of the mount.

I have seen these mounts bounce many, many times even visually. As the loading moment shifts due to tracking, the pressure (especially if the scope is CAREFULLLY balanced) can change causing slight jumps.

Just a thought, but I know it happens visually. One of my mounts (my LXD-750) required a lot of fussy adjustment to get the worm tight enough but not so tight is would stall. The telescope has a VERY long weight and balance moment, and changing from a light eyepiece to something like a 31mm Nagler would cause it to start bounching if the balance changed in some attitudes.

This can also happen in both axises of course.

These mounts tend to be sloppy mechanically. I don't mean that in a negative way saing that they don't have good table manners. I mean that they are not assembled to particularly close tolerances.

Try loading both axises more positivly and see if the condiition improves.

Regards.

#9 NewAstronomer

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Posted 16 April 2010 - 09:34 AM

Power supply? Or weight shift. I dragged out my SVP recently for quick planetary views with my new refractor and I'm using 4 D batteries , except 1 of the 4 is really a rechargable AA in a shell. Things seem to be fine for awhile them blamo, the mount shifts rsdically on the RA, then a little latter tracking and DEC movement stops working but the green light stays on...I throw in another AA and it seems fine...but keeps happening.

I'm going to buy 4 new D's before next session to see if that is in fact the problem. What do you use for power?

#10 DHB001

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Posted 16 April 2010 - 11:52 AM

Eddgie,

Thanks for your suggestion and explanation. I know exactly what you're referring to. I did preload both axes last week, and I thought I had imbalanced them enough, but now I'm not entirely sure anymore.

By the way, how much preload is enough, and when is it too much? What will happen then? Will it not constantly overshoot if the mount is too much out of balance? :confused:

At this point, I too believe a balance issue is the most likely cause of all this.

NewAstronomer,

I use a celestron powertank. I bought all my current equipment about a year ago, and although scope and mount are secondhand, the powertank is brand new. I'm pretty sure that it's working fine. I haven't had any trouble in the power department to date. Slewing, tracking etc. all goes just fine, never a glitch or anything, so although theoretically it is possible that it's not working 100%, I have no real suspicions at the moment that that is in fact the case.

#11 DaemonGPF

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Posted 16 April 2010 - 12:34 PM

1+ for load shift/balance. My CG5 was very picky about balance.

#12 Eddgie

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Posted 16 April 2010 - 02:22 PM

I would think just enough to ensure that the worm faces stay in contact.

The fact is that the gearing in these mounts provides a HUGE amount of torque magnification. These mounts will drive in even severely out of balance conditions. Now, I don't recommend using more pre-load than you need, but just having the mount SLIGHLY out of balance is sometimes not enough.

As the telescope's angle changes, and this is ESPECIALLY true with refractors, them load moment often changes. This is because the load is often very assymetric.

Many people balance with the OTA level to the ground and the polar shaft level to the ground. What you MIGHT find is that the mount is ONLY fully balanced in this ONE position.

As you change orientations, the moments of leverage get all twisted around.

The suggestion I would make is to balance in the orientation similar to where the mount will be when it is actually in use.

How much? I would expect that for imaging, a little more pre-load is a bit better than a not. Again, the motor torque is hugely magnified and you are NOT going to hurt anything.

This may cause the current drain on your power supply to go up a hundred milliamps or so, but you aren't going to hurt anything.

I could be wrong, but if it is irregular, then I think that this is one of the most likely issues.

Of course you may have some other slop somewhere. The spur gears between the motor gearbox shaft and the RA shaft should be checked for mesh along with the worm, but again, if you pre-load, it should not matter.

Try preloading with enough load that the mount is CLEARLY out of balance and let us know if it solves the problem.

If it does, and later you decide you want to attack the issue of the slop, let me know and I will walk you though how to check and what to do.

Regards.

#13 Eddgie

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Posted 16 April 2010 - 02:25 PM

Oh, and of course it would be unfair to not to mention this. You COULD have binding becuase of too much pre-load already, but this is unlikely unless your gears are dry. If the worm gears DO get dry, the teeth can chatter against one another. Unlikely, but not out of the question. If the mount is very old and has never been greased, it could be an issue.

#14 DHB001

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Posted 16 April 2010 - 05:38 PM

Thanks DaemonGPF and especially Eddgie for your input.

Eddgie,

As far as gear slop and potentially dry worm gear teeth are concerned, one of the first things I did after buying my equipment last year, was re-lubing the mount (with Super Lube, which I've read so many good things about here on CN) and ensuring that backlash/slack between the gears (both worm and spur) was minimized. I did this by following the instructions of Tim Doucette (see here), which is based on the well known instructions of Astronomy Boy, but for the newer CG5-GT.

By the way, what exactly do you mean by binding, Eddgie? I know what stiction (static friction) is, but I'm not sure I understand what you mean by binding. Don't think it's the same, or is it?

I will definitely test pre-loading my mount "excessively" asap, and as soon as I've got results, I will let you all know. The weather forecast is very favorable for the weekend, so unless this Icelandic volcanic ash is seriously blocking the sky (which I think it won't), I should be able to report back very soon.

Thanks again to all you guys who came up with ideas the last couple of days. I've got a good feeling we'll soon have this issue sorted out.

#15 Eddgie

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Posted 16 April 2010 - 07:22 PM

Well, the "Binding"would simply be tiny momentary chattering of the worm wheel/worm gear teeth or if the mesh is not correct on the spur gears, you can have a tiny amount of hitching of the gears as the successive teeth come into contact.

Anyway, look forward to the results.

#16 DHB001

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Posted 17 April 2010 - 07:51 PM

Well, it's now 2.50 am here, and I just finished what turned out to be a very disappointing test session. It seems the spikes at best were just as bad as earlier this week, but it was mostly worse. Tomorrow I'll post the guide logs of last night, plus the notes I made of the things I tried and at what time I tried them, so that you can see the effects (or lack thereof) they had on the guide graph.

#17 Eddgie

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Posted 17 April 2010 - 08:39 PM

Well, I am sorry that my suggestions were not helpful.

Don't know if it is elelctrical or mechanical now. Wish I had another idea.

Hopefully someone else will have some ideas.

#18 DHB001

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Posted 18 April 2010 - 05:22 AM

I've processed the guide log in Excel, and also provided it with comments about what I did, for those interested.

The guide log in Excel, with comments: link

I think one or more of the issues below could be causing my problems.

1) the DEC is too stiff, causing stiction.

I already regreased my mount once before, before I got hold of Super Lube, and it doesn't matter what lubricant I use, the Dec axis won't spin as easy as the RA does. I think it has a lot to do with the fact that the Dec has no bearings whatsoever, unlike the RA.

2) as a consequence of 1), the net result of scope preload and deliberate polar misalignment to induce permanent drift in the dec was insufficient to prevent stiction to occur, creating spikes during guiding.

3) preloading works, but not when imaging in zenith.

M51 is very high above the horizon for me, so I'm always imaging this object very near zenith. With the scope pointing nearly straight upward, there's almost no lever arm for the force, created by the preload, to exert torque on the dec gears.

4) preload in dec and deliberate drift from slight polar misalignment are in opposite direction.

This is something I just thought of.

Could it be that if the drift caused by polar misalignment is south in the dec, but the scope preload on the dec axis is to the north, that guiding the dec becomes hard to do for PHD (or any guiding software for that matter), because corrections tend to overshoot?

Maybe I should do only one of two things: either balance the scope in Dec and do a rough polar alignment so that there's a slight unidirectional drift in Dec, or do an accurate polar alignment and preload the Dec axis....

#19 bardo

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Posted 18 April 2010 - 07:21 AM

just put the motors on a vixen and be done with it ;)

#20 Richard Scott

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Posted 18 April 2010 - 08:00 AM

I worked for years with a CG-5 and I got a few spikes but nothing like you are experiencing.

Two thoughts,
#1 Try lapping the gears by dry running both the RA and DEC without any OTA the 10 or 20 full 360-degree rotations. If there is a bit of crud or two on one or more of the gear faces, that may rub it off.

#2 Try a AC/DC power converter. I found that my CG-5 ran a lot happier at 14/15 volts than it did at 11/12 volts. The extension cord is something of a pain, but after I went the converter route I never went back.

Good luck
Richard

#21 groz

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Posted 18 April 2010 - 10:17 AM

When you are set up for imaging, where are all the cables located. Every time I've seen jumps start happening, then go out and look at the scope, a cable from the camera was caught up somwhere and dragging. As the scope moves around, the cables can really end up in some odd locations if they are not all tied in place.

After learning that lesson, cables now always end up tied off in locations such that they wont have this problem. All the cables from the cameras and filter wheel go to a common point which is attached to the mount head, then a loop of slack before the bundle attaches again to the base, and from there off to where they need to go.

#22 vdb

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Posted 18 April 2010 - 11:35 AM

The last couple of days I've been imaging M51 as well with the same mount, I have a 70mm refractor as guider, parallel mounted to a Mak-Newt 150mm ...
Guiding is reasonable well, but when approaching zenith it goes from "ok" to bad ... I don't have the spikes as you do though, it just drifts away. I also try to preload, but that doesn't seem to work. My polar alignment is rough, so guiding stays in one direction for dec ...

#23 DHB001

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Posted 18 April 2010 - 01:53 PM

I've checked everything on the dec axis again today. Gear mesh, backlash, the works. Couldn't find a thing wrong here. Moving the axis manually via the spur gear goes very smoothly for the entire 360 degrees. I did however notice that the worm gear cover of the dec axis, where you attach the scope or side by side plate to the mount, was making contact with the rest of the mount, i.e., there was no gap between it and the dec axis housing. So I dug up two small washers, so that the cover now has about 1 millimeter of clearance from the dec axis housing. I have no idea if the drag from the direct contact between the two metal parts could possibly have caused jumpy shocks when guiding, but I figured the washers could only make the mount perform better.

I also followed your advice, Richard, and I've turned the dec axis via the handcontroller around for 10 or so full rotations, so that any crud should likely have been washed away by now.

Groz, thanks for thinking along. I did in fact at one point suspect cables being stuck as a possible cause, but unfortunately the cables had ample room for manoeuvre at the time I was having these jumps, so they're probably not the cause of this problem.

I still think this is a mechanical problem, most likely related to balance. My next step will be to try balancing my scope the best as I can, and let the dec drift only be caused by a slightly inaccurate polar alignment. At least then I'll be sure that I have only one source of dec drift, not two, which should make further troubleshooting easier.

I'll keep you guys posted.

#24 DHB001

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Posted 18 April 2010 - 05:38 PM

Okay, so I just did some more testing, this time with a carefully balanced dec axis, so that the only source of dec drift was going to be a slight inaccurate polar alignment, and the results are the same as yesterday and earlier this week. Still frequent spikes. I did try to change some parameters in PHD, like aggressiveness and hysteresis, and although you can see some minor changes in the guide graph, changing these parameters does nothing to eradicate the spikes.

I had to stop testing because it was becoming cloudy, and while disassembling my rig, a new and very interesting symptom presented itself. I wanted to set the mount to its two index positions, so that removing the scope etc. would be a bit easier. So I loosened the RA and Dec knobs to move the axes, and I swear to God, when I wanted to move the Dec axis back to the index position, it was unusually stiff.

After moving the dec axis from left to right a couple of times, moving it was as "smooth" as usual again (i.e. still much stiffer than the RA). But that first turn, it definitely was much stiffer than normal.

What does this symptom indicate? I have greased my mount with Super Lube, which is supposed to be such a good lubricant. Does it mean I have used to much, or perhaps to little grease? Does it indicate another problem? It's only like 10 degrees C, or around 50 degrees F, here outside. That's well within the operating limits of Super Lube, so it should have been lubricating just fine.

Thanks in advance for those who can help me with deciphering this new clue.

#25 Gregk

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Posted 18 April 2010 - 09:34 PM

My Cg-5 was acting up a while ago and to cure it Dec drifting not so much spiking I had to use a 1 sec guide camera exposure in PHD time If I use a bright star I reduce the gain of the DSI. You could have a build up of grease but you would have seen that

Greg






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