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Old 16-06-2015, 07:17 PM
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Weird Oscillations

This is something I cannot seem to nut out for myself, so I thought I would check with people to see if anyone has any ideas.

The oscillation in my RA gets worse with longer guide exposures. See the two images I have included.

My balance is very good and the oscillation from the graph appearance is not coming from guide settings.

Though my Min=0, Max=0.2

I have similar settings for the Dec and as you can see the results are markedly different.

This is one of the reasons why I bought an AOX.

Using harder aggression settings or different min or max settings produces worse results. So what could be causing this oscillation?
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Old 16-06-2015, 08:50 PM
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I have seen autoguiding oscillations on my PMX before. What sorted it was making sure the balance was shifted to the east a bit so the gears were engaged with a slight amount of imbalance. That stopped it immediately.

Min 0 Max .2 does not make sense. That would mean any correction greater than .2 would be ignored. Perhaps your setup is near perfect but I have never seen guiding all night that was below .2. So I would try min .1 and max 3.0. Which is the usual setting. If it makes it worse that would be unusual and I would suspect backlash. Perhaps your cam has popped up (the pin in the 3 way switch mechanism) or your drive belt has slack in it.

But try the making it slightly imbalanced first and see if that sorts it. You don't actually want the mount perfectly balanced north to south. I have watched it go back and forth from a minus number to a plus over and over much the same every time when it was.

That cam pin easily pops up. All it takes is forgetting to turn the 3 way switch to lock and take the heavy imaging train off.

If you PA and PEC are near perfect you will probably get best results from aggressiveness setting down low like 3. Also longer guide exposures like 3-4 seconds.

Is this the PMX or PME you are having trouble with?

Greg.
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Old 16-06-2015, 09:55 PM
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Thanks for the response Greg.

The funny thing is that guide exposures of 0.5 flatten that oscillation right down.

I'll give the balance a look at and try the balance to the east.

I had not considered the cam, but I will take a look at that too.

I have used similar Min and Max settings as you suggest and the results go haywire. It's definitely something mechanically to do with the RA. Longer guide exposures make the problem worse. Though the stars are still round.
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Old 16-06-2015, 10:15 PM
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Thanks for the response Greg.

The funny thing is that guide exposures of 0.5 flatten that oscillation right down.

I'll give the balance a look at and try the balance to the east.

I had not considered the cam, but I will take a look at that too.

I have used similar Min and Max settings as you suggest and the results go haywire. It's definitely something mechanically to do with the RA. Longer guide exposures make the problem worse. Though the stars are still round.

Sounds weird. But hopefully the loading up the gears slightly helps. If not then possibly the cam on the PMX.
What program are you using to guide with? Maxim? Sky X? CCDsoft?

Greg.
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Old 16-06-2015, 10:21 PM
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I am using Maxim DL 6.0. I doubt it is guiding as I tried with SkyX and got similar results. So hence the thought of mechanical.
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Old 17-06-2015, 08:35 AM
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OK, for what this is worth since I'm not nearly as experienced as you and Greg. First, I agree with Greg that the max move of .2 seems way too small for such long exposures. Consider that the longer the exposure the more the mount could have drifted off, thus I have observed that longer exposures either need a larger max move or higher aggression (or both) compared to shorter exposures. So, when you decrease the exposure you are obviously correcting more often and the smaller max move is now able to keep up with the drift. With the longer exposure it seems to be taking a whole lot of smaller moves to bring the mount back to proper tracking so you get that wavy sort of guiding. Anyway, That's my first reaction. I hope you get it sorted!

Peter
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Old 17-06-2015, 08:38 AM
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I am using Maxim DL 6.0. I doubt it is guiding as I tried with SkyX and got similar results. So hence the thought of mechanical.
In my experience it was fast to fix. I even adjusted it whilst it was guiding and it retained the guide star. Just a quick slide down of one the counterweights 25mm is probably enough. At first it was very disconcerting and I could see myself having to pull it apart and playing with that cam pin again. That cam pin screw is tiny and easy to damage the hex slot in the top so its hard to get really tight. I think I'll buy a dozen of them and have them in stock.

It would be great if someone sold a pack of commonly used allen key stainless steel screws. Imperial and metric.

Greg.
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Old 17-06-2015, 08:40 AM
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I am using Maxim DL 6.0. I doubt it is guiding as I tried with SkyX and got similar results. So hence the thought of mechanical.
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Originally Posted by PRejto View Post
OK, for what this is worth since I'm not nearly as experienced as you and Greg. First, I agree with Greg that the max move of .2 seems way too small for such long exposures. Consider that the longer the exposure the more the mount could have drifted off, thus I have observed that longer exposures either need a larger max move or higher aggression (or both) compared to shorter exposures. So, when you decrease the exposure you are obviously correcting more often and the smaller max move is now able to keep up with the drift. With the longer exposure it seems to be taking a whole lot of smaller moves to bring the mount back to proper tracking so you get that wavy sort of guiding. Anyway, That's my first reaction. I hope you get it sorted!

Peter
Protrack is good tool to correct for longer exposure drift. On my PME I found a 00 point T-point model Protrack with tracking corrections activated made the difference to getting rounder stars with the CDK17.

But using it recently on the PMX results are not so clear.

Greg.
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Old 17-06-2015, 10:18 AM
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Thanks for the response Greg.

The funny thing is that guide exposures of 0.5 flatten that oscillation right down.

Though the stars are still round.
Possibly you are getting some feedthrough of PE, due simply to the fact that guiding tries to correct the average error that existed in the past, when the guide exposure was taken, not the error that exists when the mount moves. Why not continue to use 0.5 second exposures, or just ignore it if the stars are still round?

Last edited by Shiraz; 17-06-2015 at 11:03 AM.
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Old 17-06-2015, 11:45 AM
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Originally Posted by PRejto View Post
OK, for what this is worth since I'm not nearly as experienced as you and Greg. First, I agree with Greg that the max move of .2 seems way too small for such long exposures. Consider that the longer the exposure the more the mount could have drifted off, thus I have observed that longer exposures either need a larger max move or higher aggression (or both) compared to shorter exposures. So, when you decrease the exposure you are obviously correcting more often and the smaller max move is now able to keep up with the drift. With the longer exposure it seems to be taking a whole lot of smaller moves to bring the mount back to proper tracking so you get that wavy sort of guiding. Anyway, That's my first reaction. I hope you get it sorted!

Peter
Hmmm, that makes sense. I am not sure if I have tried a larger max move with longer guide exposures. I think I have, but that was quite a few months ago. I have been trying to nut this problem out for some time with the RC and PMX. It's worth a try for using longer guide exposures. I just found this all to be an intriguing problem and something that I could not seem to sort easily with trial and error. I have spent a fair bit of time reading up and have tried CCDware's online calculator and nothing really worked well except for the really short exposures with small max and min numbers. It might be worth trying a stronger aggression too. Thanks Peter for the suggestions.

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Possibly you are getting some feedthrough of PE, due simply to the fact that guiding tries to correct the average error that existed in the past, when the guide exposure was taken, not the error that exists when the mount moves. Why not continue to use 0.5 second exposures, or just ignore it if the stars are still round?
That makes sense Ray that it could be feed through of PE. In any event I will be using shorter exposures when I install the AOX. Or I will need to use the shorter exposures to use the AOX effectively. The only real problem is guide star selection on many galaxies can be a problem. If I cannot use the AOX on dimmer stars then I would need to be able to use longer exposures to guide on and here in lies the problem I suppose. 1 second is about the longest I can go before I start to see the oscillation take effect and that sometimes is just on the board line of star fade and guiding with a barely visible star. Anything over 1 second and the stars start to elongate in RA. Since the DEC is about as flat as you can get it, the elongation is quite obvious. Thanks for taking the time to respond.

Quote:
Originally Posted by gregbradley View Post
Protrack is good tool to correct for longer exposure drift. On my PME I found a 00 point T-point model Protrack with tracking corrections activated made the difference to getting rounder stars with the CDK17.

But using it recently on the PMX results are not so clear.

Greg.
I have protack enabled on the PMX. I found it does help to manage the guiding and I did a 250 point model. With PEC the guiding with short exposures produces quite round stars and tight guiding. The good seeing allows short exposures but as I explained to Ray, having the option of being able to use shorter exposures would really help in areas with dim guide stars. I'll try the counter weight shift and see what effect that has.
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Old 17-06-2015, 12:22 PM
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I am now curious as to the outcome. Let us know.

Greg.
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Old 17-06-2015, 07:27 PM
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Paul,

This is really intriguing. I only had a TEC140 on my PMX so you are a lot heavier. However, I never saw the oscillating issue both you and Greg have seen. Hopefully Greg's unbalanced idea will work though it does go counter to SB's advice with these mounts.

Have you tried unguided + Protrack as a diagnostic? It would seem that you ought to be able to go at least 5 min or so if you have a good model and accurate PA, though I guess this also depends on your FL...But, surely a few minutes. If not then perhaps something else is wrong, like PE not sufficiently corrected. Have you measured it recently? What does a guiding log look like if not making any guiding corrections? Is it wavy? Ray's idea makes sense because I doubt that a quickly changing PE could be corrected with .2 max moves + 10 sec guide exposures. But, .5 sec exposures or so probably would keep up with PE.

Anyway, just a few random thoughts.

Peter
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Old 18-06-2015, 07:18 AM
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What may be happening here Peter is the too balanced and not loaded to the east if there is a bit of looseness in the gears causes the autoguider to correct one way then correct back the other way. Like the guider is correcting the corrections.

That would make sense why constricting the guiding down with min/max works. Only some of the corrections are getting through with a max move of .2. So if the mount wanted to do a 1.0 correction its only letting a .2 correction or perhaps none at all (not sure if it lets up .2 through or rejects anything over .2 completely). So the oscillations then are more muted.

A loose popped up cam pin I think can increase gear looseness.

Greg.
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Old 18-06-2015, 08:14 AM
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Paul,

This is really intriguing. I only had a TEC140 on my PMX so you are a lot heavier. However, I never saw the oscillating issue both you and Greg have seen. Hopefully Greg's unbalanced idea will work though it does go counter to SB's advice with these mounts.

Have you tried unguided + Protrack as a diagnostic? It would seem that you ought to be able to go at least 5 min or so if you have a good model and accurate PA, though I guess this also depends on your FL...But, surely a few minutes. If not then perhaps something else is wrong, like PE not sufficiently corrected. Have you measured it recently? What does a guiding log look like if not making any guiding corrections? Is it wavy? Ray's idea makes sense because I doubt that a quickly changing PE could be corrected with .2 max moves + 10 sec guide exposures. But, .5 sec exposures or so probably would keep up with PE.

Anyway, just a few random thoughts.

Peter
Peter, I have not tried unguided seriously. There have been a couple of times the clouds have got in the way and upon reacquisition the star has been close to the line if the last corrections was to bring it back to the centre line.

PE only done a couple of months ago and this problem was around long before that. It proved helpful to reducing the problem. See my comments below about Greg's suggestion.

Quote:
Originally Posted by gregbradley View Post
What may be happening here Peter is the too balanced and not loaded to the east if there is a bit of looseness in the gears causes the autoguider to correct one way then correct back the other way. Like the guider is correcting the corrections.

That would make sense why constricting the guiding down with min/max works. Only some of the corrections are getting through with a max move of .2. So if the mount wanted to do a 1.0 correction its only letting a .2 correction or perhaps none at all (not sure if it lets up .2 through or rejects anything over .2 completely). So the oscillations then are more muted.

A loose popped up cam pin I think can increase gear looseness.

Greg.
There might be a possibility that the worm gear is not seated properly. When the gear is engaged it seems well locked off but there seems to be the tiniest of movements. However, that might just be the depth of the worm in gear not being quite right. When I changed the worm gear I thought I had everything seated well but it might just be a very small fraction out of place. I will check this just to make sure. It's a remote possibility but worth checking anyway.

It could also be the cam pin and I will check this at the same time.

The belt I think is the correct tension but it will be checked at the same time.

There is one other possibility and that might be the PMX cannot support the load and it might be time to change over the PME to the dome and put the PMX into the roll off roof so that is carrying the FSQ only. I would not have thought this would be a problem but it might part of the issue. It is a drastic step to undertake.

In any event I will need to eliminate all possible contingencies and whatever remains will be the likely cause. Another bug hunt.
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Old 18-06-2015, 08:48 AM
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Paul,

If you are going to try to reseat the worm be aware that the major adjustment is not just the cam pin! There is a video by Chris Venter that is quite popular that skips a most important step! That step is the adjustment of the 1/4-20 bolts and is discussed in the SB pdf on replacing the worm. If the 1/4-20s are wrong it won't matter what you do with the cam stop. BTW, in Chris's defence, when he made the video, the 1/4-20 adjustment was not part of the procedure in the Bisque pdf. Anyway, if you feel any slop in the worm/ring gear meshing it is likely to be the 1/4-20.

Peter

PS You may ask why I know about this. It's because whilst I had my MX I must have removed and replaced the worm 15-20 times trying to solve an unsolvable issue that was only resolved by me sending the mount back and trading up to the MEII. Mine had some sort of defect in the PA bearing (which is speculation on their part). I'm sure that is uncommon but took ages to diagnose. SB was very good about making things right, but it took me more than a year to prove the case. That part was frustration beyond belief.
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Old 18-06-2015, 09:08 AM
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Paul,

If you are going to try to reseat the worm be aware that the major adjustment is not just the cam pin! There is a video by Chris Venter that is quite popular that skips a most important step! That step is the adjustment of the 1/4-20 bolts and is discussed in the SB pdf on replacing the worm. If the 1/4-20s are wrong it won't matter what you do with the cam stop. BTW, in Chris's defence, when he made the video, the 1/4-20 adjustment was not part of the procedure in the Bisque pdf. Anyway, if you feel any slop in the worm/ring gear meshing it is likely to be the 1/4-20.

Peter

PS You may ask why I know about this. It's because whilst I had my MX I must have removed and replaced the worm 15-20 times trying to solve an unsolvable issue that was only resolved by me sending the mount back and trading up to the MEII. Mine had some sort of defect in the PA bearing (which is speculation on their part). I'm sure that is uncommon but took ages to diagnose. SB was very good about making things right, but it took me more than a year to prove the case. That part was frustration beyond belief.
Thanks Peter,

I knew about the video and the omission out of the video. I have the worm gear replacement PDF here somewhere and I was just looking for it on the drive. I might have to download it again. I used it last time when removed the old worm gear that was out of spec.

I think it is worth just going through and checking everything. I am pretty particular when working on mounts but I just want to check just in case this is the issue.
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Old 18-06-2015, 11:36 AM
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There might be a possibility that the worm gear is not seated properly. When the gear is engaged it seems well locked off but there seems to be the tiniest of movements. However, that might just be the depth of the worm in gear not being quite right. When I changed the worm gear I thought I had everything seated well but it might just be a very small fraction out of place. I will check this just to make sure. It's a remote possibility but worth checking anyway.

Its possible but my PMX has a slight amount of rock in it too and it doesn't seem to come into play.

It could also be the cam pin and I will check this at the same time.

Thinking about it I think all the cam pin does is set the amount of push of the worm onto the gear wheel so it will slip under load rather than wreck the gears.

The top screws are the ones that adjust the tension of the worm onto the gear wheel and will prevent it from slipping if too tight but will stall the motor or if too loose then the gears will slip too easily. 2.25 to 2.5 turns up from fully screwed in is what SB says they should be at.

The belt I think is the correct tension but it will be checked at the same time.

There is one other possibility and that might be the PMX cannot support the load and it might be time to change over the PME to the dome and put the PMX into the roll off roof so that is carrying the FSQ only. I would not have thought this would be a problem but it might part of the issue. It is a drastic step to undertake.

In any event I will need to eliminate all possible contingencies and whatever remains will be the likely cause. Another bug hunt. [/QUOTE]

Not being able to handle the load is possible also although I would have though that would show up more as elongated stars rather than an oscillation. I am finding the PMX is not handling the RHA very easily and that weighs around 33kgs plus the 30kgs of counter weights. It can but everything has to be spot on.
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Old 18-06-2015, 12:27 PM
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Quote:
Originally Posted by gregbradley View Post
There might be a possibility that the worm gear is not seated properly. When the gear is engaged it seems well locked off but there seems to be the tiniest of movements. However, that might just be the depth of the worm in gear not being quite right. When I changed the worm gear I thought I had everything seated well but it might just be a very small fraction out of place. I will check this just to make sure. It's a remote possibility but worth checking anyway.

Its possible but my PMX has a slight amount of rock in it too and it doesn't seem to come into play.

It could also be the cam pin and I will check this at the same time.

Thinking about it I think all the cam pin does is set the amount of push of the worm onto the gear wheel so it will slip under load rather than wreck the gears.

The top screws are the ones that adjust the tension of the worm onto the gear wheel and will prevent it from slipping if too tight but will stall the motor or if too loose then the gears will slip too easily. 2.25 to 2.5 turns up from fully screwed in is what SB says they should be at.

The belt I think is the correct tension but it will be checked at the same time.

There is one other possibility and that might be the PMX cannot support the load and it might be time to change over the PME to the dome and put the PMX into the roll off roof so that is carrying the FSQ only. I would not have thought this would be a problem but it might part of the issue. It is a drastic step to undertake.

In any event I will need to eliminate all possible contingencies and whatever remains will be the likely cause. Another bug hunt.
Not being able to handle the load is possible also although I would have though that would show up more as elongated stars rather than an oscillation. I am finding the PMX is not handling the RHA very easily and that weighs around 33kgs plus the 30kgs of counter weights. It can but everything has to be spot on.[/QUOTE]

So it is unlikely this is the cause of the problem but I will check it just for the sake of completeness.
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Old 18-06-2015, 04:41 PM
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Gday Paul
Have you collected some guided vs unguided data independently ( ie with something like PEMPro or PHD2 just logging in a passive mode).
If you do, you can then do an FFT analysis on the data and see if there is a specific freq showing up in both plots, which would indicate the guiding is reducing but not eliminating that underlying ( and possibly ) mechanical error.

Andrew
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Old 18-06-2015, 05:21 PM
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To be honest, with the AO sitting in a box I wouldn't worry too much about this issue, as when that is hooked up you are going to be aiming to guide with exposures much smaller than 0.5s. If you had the same issue with the AO connected then I might go chasing things. Chances are you arent going to find a problem, and if it is not an issue with AO working, then dont fix a problem that isnt there.
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