What Constitutes Good Guiding?

[oymd]

58 minutes ago, drjolo said:

These spring loaded CEM40, CEM26 and similar mounts are very susceptible to perfect balancing - in RA, Dec, but also along the telescope axis. So if you have heavy focuser motor or guide scope on the side, it needs to be balanced on the opposite side of the OTA. Worm gear tension needs to be adjusted as light as possible to cancel the play, but no more. Stepper motors in these mounts are undersized and such behavior may be a reason of accumulation of microsteps that are released after some time. 

 

40 minutes ago, vlaiv said:

Ok, so if you are using iOptron commander - it is different from using EQMod.

You won't be doing PE curve preparation - mount will do that for you, or rather iOptron commander will do that for you. In this case - it must know what the needed corrections are and you need to start guiding when you hit PEC record. Commander will track where mount is supposed to be - but it will also track corrections and from the two will derive PEC curve.

You need to keep it like that for at least couple of PE cycles - let's say 5 cycles - that is 2000 seconds total, so just a bit more than half an hour.

Once you are done - when you next start RA tracking, regardless if you are guiding or not - you can hit PEC playback and it will apply recorded correction to the mount. It should keep that PE correction until you record a new one - so you should be able to reuse it very session by playing it back.

As far as 1.2" RMS - that is quite a lot, and there are couple of things that you should look into:

1. Do above advice on balancing your setup

2. See if your clamping connection is strong enough for the scope - you might need to improve that

3. You should have virtually zero backlash since you have spring loaded gears - make sure you've tuned that properly in both axis

4. Make sure your tripod is sturdy enough and have been placed on ground where it can't move - don't put it on soft ground like sand - or if you do - try to "dig it in" a bit so it is rock solid. If it has extending legs - don't extend them fully - you don't need your setup to be at "observing" position for imaging. It can be lower than that.

Thank you so much for your prompt responses

1- With regards to BALANCING, i can say with confidence that it is PERFECTLY balanced. Too balanced in fact. As the ZWO EAF sticks out too much on one side, I attached a short WO dovetail on the opposite side, and loaded it with washers to achieve perfect balance in the third axis. Balancing these CEM mounts is ridiculous! Attached is a pic showing my DIY counter weight for the focuser and my latest calibration after stripping the mount and replacing the belts.

2- everything seems to be RIGID

3- DEC has no obvious backlash. RA has a VERY slight ROCKING/WOBBLE when I manually push the counterweight to one side. IT IS IMPOSSIBLE TO GET RID OF IT. The only way I can get the RA to be solid with ZERO wobble, is to tighten the gear/worm meshing to the point that I can no longer undo the RA lock and I get stiction. Even if I then loosen the meshing by 1/1000th of a turn (exaggeration here of course ), the RA wobble immediately comes back. There is simply no sweet spot where the gear/worm meshing eliminates the wobble, yet can be unlocked and slews normally. I have spent DAYS on this!

4- Tripod is the beefy 2" one, with legs all the way in, and on concrete floor.

 

 

[vlaiv]

1 minute ago, oymd said:

Thank you so much for your prompt responses

1- With regards to BALANCING, i can say with confidence that it is PERFECTLY balanced. Too balanced in fact. As the ZWO EAF sticks out too much on one side, I attached a short WO dovetail on the opposite side, and loaded it with washers to achieve perfect balance in the third axis. Balancing these CEM mounts is ridiculous! Attached is a pic showing my DIY counter weight for the focuser and my latest calibration after stripping the mount and replacing the belts.

2- everything seems to be RIGID

3- DEC has no obvious backlash. RA has a VERY slight ROCKING/WOBBLE when I manually push the counterweight to one side. IT IS IMPOSSIBLE TO GET RID OF IT. The only way I can get the RA to be solid with ZERO wobble, is to tighten the gear/worm meshing to the point that I can no longer undo the RA lock and I get stiction. Even if I then loosen the meshing by 1/1000th of a turn (exaggeration here of course ), the RA wobble immediately comes back. There is simply no sweet spot where the gear/worm meshing eliminates the wobble, yet can be unlocked and slews normally. I have spent DAYS on this!

4- Tripod is the beefy 2" one, with legs all the way in, and on concrete floor.

What is your guide exposure (how long is it)?

Try the following:

Shoot one or several subs with your regular guiding settings and then use longer guide exposure - like at least 4 seconds and shoot another set of subs.

Compare FWHM values between two sets of subs.

If there is no significant FWHM difference (or even FWHM or second batch is smaller) but RMS got down - then it is the seeing that is at fault.

[oymd]

52 minutes ago, vlaiv said:

Ok, so if you are using iOptron commander - it is different from using EQMod.

You won't be doing PE curve preparation - mount will do that for you, or rather iOptron commander will do that for you. In this case - it must know what the needed corrections are and you need to start guiding when you hit PEC record. Commander will track where mount is supposed to be - but it will also track corrections and from the two will derive PEC curve.

You need to keep it like that for at least couple of PE cycles - let's say 5 cycles - that is 2000 seconds total, so just a bit more than half an hour.

Once you are done - when you next start RA tracking, regardless if you are guiding or not - you can hit PEC playback and it will apply recorded correction to the mount. It should keep that PE correction until you record a new one - so you should be able to reuse it very session by playing it back.

As far as 1.2" RMS - that is quite a lot, and there are couple of things that you should look into:

1. Do above advice on balancing your setup

2. See if your clamping connection is strong enough for the scope - you might need to improve that

3. You should have virtually zero backlash since you have spring loaded gears - make sure you've tuned that properly in both axis

4. Make sure your tripod is sturdy enough and have been placed on ground where it can't move - don't put it on soft ground like sand - or if you do - try to "dig it in" a bit so it is rock solid. If it has extending legs - don't extend them fully - you don't need your setup to be at "observing" position for imaging. It can be lower than that.

 

1 hour ago, drjolo said:

These spring loaded CEM40, CEM26 and similar mounts are very susceptible to perfect balancing - in RA, Dec, but also along the telescope axis. So if you have heavy focuser motor or guide scope on the side, it needs to be balanced on the opposite side of the OTA. Worm gear tension needs to be adjusted as light as possible to cancel the play, but no more. Stepper motors in these mounts are undersized and such behavior may be a reason of accumulation of microsteps that are released after some time. 

And this is what I meant by the RA wobble

[vlaiv]

4 minutes ago, oymd said:

And this is what I meant by the RA wobble

Can't really see well on that video - is the whole mount (or rather RA part) moving when you push on CW bar or is it just CW bar?

In any case - that's not looking good - there should be no wobble like that and I don't think it is caused by springs in spring loaded worm. That motion is "perpendicular" to springs themselves and should not be related to them.

[oymd]

16 minutes ago, vlaiv said:

Can't really see well on that video - is the whole mount (or rather RA part) moving when you push on CW bar or is it just CW bar?

In any case - that's not looking good - there should be no wobble like that and I don't think it is caused by springs in spring loaded worm. That motion is "perpendicular" to springs themselves and should not be related to them.

its when I wobble the CW bar to the left, the Scope and everything on it wobbles to the right, and vice versa. Its barely visible, but can be felt very easily.

Tightening the worm/gear mesh eliminates it, but then locks cannot be released

[pipnina]

On 16/06/2018 at 08:54, ollypenrice said:

Vlaiv is perfectly correct that the round stars test is almost meaningless but at least with round stars you can produce an attractive image. When setting up the first Mesu we always had round stars but by tuning the guide parameters they got smaller and smaller!

I don't think there's any reason to believe that the HEQ5 is less accurate than the 6 provided both are within payload.

Any backlash adds to the guide error and the worse the seeing the more it does so. (When corrections are few there is a reduced tendency for the mount to be sent oscillating across the backlash.) So losing the backlash is a very good idea. The other ways to combat backlash are to run slightly east heavy and slightly polar misaligned. East heavy keeps the drive in 'push' mode and the misalignment means you can disable guiding on one direction in Dec, letting the other 'push in the direction of correction.' This does work and won't stop you from doing 15 minute subs.

The duration of guide subs is also something to tune in the light of prevailing conditions. Our EQ sixes thrive on short guide subs if the seeing is good because they have pretty rapid PE. However, if the seeing is bad this has them chasing it, so we do better with longer subs which average out the position of the guide star's image. (It's worth remembering that the guide trace has no way of knowing where the real star is. It only knows where its image is. So very short subs do give a better trace but is that a trace of the real star or just its image?  Curses!! )

The other variable within the guide trace is the position of the scope between corrections. Again we have no means of knowing, but Avalon claim that with their backlash-free belt drive the correction is fed in faster so that, between guide inputs, the true location of the scope is on target for more of the time. With any backlash oscillation in play it's a good bet the situation between inputs will be worse.

Olly

I know this is a bit of a long gap for a reply, but I did some maths only a month or two ago to work out how much torque the heq5 can produce at the driven shaft. When run at 1/64 microstep I think it came out to around 0.24nm. about as much as I could apply to a 1/2" socket drive on my workplace's torque calibration machines with my finger and thumb.

 

So all in all rather poor torque for a 705:1 gear reduction!

You can increase torque by reducing the microstep ratio but it's not linear, maybe mods could improve the heq5/eq6 mounts with inexpensive higher torque motors hmmm

[ollypenrice]

20 minutes ago, pipnina said:

I know this is a bit of a long gap for a reply, but I did some maths only a month or two ago to work out how much torque the heq5 can produce at the driven shaft. When run at 1/64 microstep I think it came out to around 0.24nm. about as much as I could apply to a 1/2" socket drive on my workplace's torque calibration machines with my finger and thumb.

 

So all in all rather poor torque for a 705:1 gear reduction!

You can increase torque by reducing the microstep ratio but it's not linear, maybe mods could improve the heq5/eq6 mounts with inexpensive higher torque motors hmmm

What do you need this torque for, though? A balanced mount offers very little resistance.

Olly

[newbie alert]

1 hour ago, oymd said:

 

And this is what I meant by the RA wobble

IMG-3677.MOV

It could be from the worm carriage housing...

I've got a cem60 and had an issue where in PhD it would guide fine for a minute or so and then dive off a cliff edge and eventually lose the guide star... It caused me no end of trouble... Thinking it was belt tension I opened up the cover ( mine was on the Dec axis) and the belt looked ok, seemed to have the right tension but lucky I moved the axis and physically see the cradle move I just tightened up the 2 grubscrews and never had an issue since... Might be worth a check, assuming your 40 shares the same setup

Your backlash result pic, you know it's measuring your Dec axis, so nothing to do with Ra 

[drjolo]

2 hours ago, oymd said:

3- DEC has no obvious backlash. RA has a VERY slight ROCKING/WOBBLE when I manually push the counterweight to one side. IT IS IMPOSSIBLE TO GET RID OF IT. The only way I can get the RA to be solid with ZERO wobble, is to tighten the gear/worm meshing to the point that I can no longer undo the RA lock and I get stiction. Even if I then loosen the meshing by 1/1000th of a turn (exaggeration here of course ), the RA wobble immediately comes back. There is simply no sweet spot where the gear/worm meshing eliminates the wobble, yet can be unlocked and slews normally. I have spent DAYS on this!

If such a small difference causes RA to wobble, then maybe it is as newbie alert wrote - some movement in the worm cradle, or play along the RA worm axis. These are scenarios 2 and 3 from the screen below (comes from CEM26 adjustment guide)

[drjolo]

3 hours ago, vlaiv said:

3. You should have virtually zero backlash since you have spring loaded gears - make sure you've tuned that properly in both axis

Unfortunately spring loaded gears can have backlash depending on the stepper torque and the actual pressure made by the spring. When I did adjustment in CEM26 according to the manual (so fully tighten and then loose by 1/2 or 3/4 turn) I got backlash. I have put a printed 12 cm lever on the Dec motor shaft like this:

to monitor small movements. I have been sending pulses manually with HC at 1x speed. And the tests are as follows:
 - when the Dec clutch is disengaged the motor reacts immediately for the pulses when the direction changes, there is no backlash
 - when the Dec clutch is engaged, there is a backlash in the Dec motor axis, and after changing guide pulses direction the motor starts to move in the opposite direction after some time (500-3000ms)

This backlash is caused by the increased torque required to move Dec worm when the clutch is engaged. Microstepping torque depends on microstepping resolution and for 1/128 it is like 1% of the full step torque, and for 1/32 step it is like 4% of the full step torque. This is apparently not enough to rotate the axis during small guide corrections, and this microstepping torque needs to accumulate for some time to be able to move the worm. I have loosened clutch tension a little (1/4 and then 1/2 allen key turn), and the backlash seems to be lower but did not disappear. Once the spring load was very light and balancing was perfect, then backlash was minimal. 

 

[vlaiv]

19 minutes ago, drjolo said:

Unfortunately spring loaded gears can have backlash depending on the stepper torque and the actual pressure made by the spring.

That is technically not a backlash.

Backlash is motion in mechanical part of the system. What you've described is more like latency in stepper motor due to lack of torque - sort of torque build up.

I agree it is bad for guiding - but can also be dealt with by using beefier stepper motors that have enough torque, or using some sort of transmission prior to worm.

[drjolo]

Yes, I know it is not a backlash literally, but looks like. And PHD2 was even able to train how to remove this "backlash" in Dec.

I thought about replacing the motor, because it is regular NEMA17 size, unfortunately there is no room to put there longer stepper version in CEM26. It was my third mount model from iOptron and I am cured now of iOptron mounts (and Bresser as well - but that is different story). 

[vlaiv]

1 minute ago, drjolo said:

Yes, I know it is not a backlash literally, but looks like. And PHD2 was even able to train how to remove this "backlash" in Dec.

I thought about replacing the motor, because it is regular NEMA17 size, unfortunately there is no room to put there longer stepper version in CEM26. It was my third mount model from iOptron and I am cured now of iOptron mounts (and Bresser as well - but that is different story). 

Any chance you could limit size of microstepping?

If I recall correctly - they have very high resolution - something like less than 0.1" per micro step.

Maybe going to 1/32 instead of 1/128 would be beneficial although it is lower stepper resolution as far as angular movement goes?

 

[drjolo]

Hm, as far as I know from other solutions (not iOptron, however) microstepping resolution is controlled dynamically from the code and control lines from CPU, so with GoTo the resolution is lower to achieve larger speed. If that is the case for iOptron as well, that would require firmware changes I think. 

[pipnina]

3 hours ago, ollypenrice said:

What do you need this torque for, though? A balanced mount offers very little resistance.

Olly

Whoops, I got my mind muddled up and quoted you instead of Vlaiv, who actually did mention step-skipping (which is usually a result of insufficient motor torque)

To that end, basically stronger motors would mean skipping fewer steps, which would mean better guiding/slewing if the mount is struggling on either.

[oymd]

10 hours ago, vlaiv said:

What is your guide exposure (how long is it)?

Try the following:

Shoot one or several subs with your regular guiding settings and then use longer guide exposure - like at least 4 seconds and shoot another set of subs.

Compare FWHM values between two sets of subs.

If there is no significant FWHM difference (or even FWHM or second batch is smaller) but RMS got down - then it is the seeing that is at fault.

I’ve experimented with various guide exposures. 
 

On my AZ-EQ6 I found 2 seconds to be perfect. 
 

With this CEM40, anything over 1s, and tge RMS shoots up and PHD2 turns into an ECG of a patient with atrial fibrillation!!

 

But that is just by viewing the graph. I should try comparing FWHM as you suggest!