Help needed guiding iOptron CEM60EC

[old_eyes]

Last Friday night was clear and a good opportunity to do some setting up and testing in my new observatory. I am having some problems guiding the mount reliably. SInce it is just as likely to be a fundamental misunderstanding or error on my part, I throw myself on the mercy of the community and hope someone has an idea what to do next.

This is a completely new set up and therefore lots to confuse or go wrong from software to cables.

Rig consists of:

• Pier mounted iOptron CEM60EC
• Equinox ED80 scope
• QHY168C camera
• Skywatcher Evoguide finder/guider
• QHY5-II mono guide camera
• PHD2 guide software
• APT control and acquisition software

All run by a pier mounted Minix Z83-4 computer controlled over ethernet using TeamViewer.

So, what did I do?

• Rough polar align using PoleMaster (overlaying hte circle and boc reported perfect alignment)
• Used the drift align tool in PHD2 to fine tune. I followed the instructions and did azimuth adjustment on a star at meridian/equator (Menkar in this case), and roughly east/equator for altitude. Got pretty flat Dec lines for both, but again followe instructions and did not stress about getting them absolutely flat as I would be guiding.
• Used the Guide Assistant and accepted it's recommendations (exposure nudged up to 3.5 sec)
• Calibrated PHD2 on Menkar yielding this calibration:

• Guided on Menkar for a period yielding this graph:

• Dec looks OK, but RA all over the place, although most of the guide corrections are on one side.
• The shifted to M45 and collected data for an hour giving this guide trace:

• Very similar result, also testing 4sec and 10 sec exposures.

My question is where should I go next to improve? Reading SGL and CN there are a large number of comments on how to guide CEM mounts, but everyone seems to come to a different conclusion. It is very confusing.

So can anybody shed some light on possible issues and which order I should address them in?

• Is this poor polar alignment?
• Is it a balance issue?
• Is this the guide system fighting the mount using the encoder?
• Is this a firmware issue?
• Do I need to set up PHD2 differently (note PHD2 suggested settings have ranged from "take hysterysis to zero, exposure long, minmo to zero and control via RA aggression" to "standard settings work fine").
• Should I just turn off RA guiding?

This seems to be a minefield and any up to date suggestions from anyone using a CEM60EC or expert in PHD2 would be gratefully received!

The relevant PHD2 log is here:

PHD2_GuideLog_2019-11-29_204054.txt

[fwm891]

I don't have an EC version of the CEM60 but have a similar set-up.

You've run a PA drift align and got fairly flat traces for both axii so that should be fine.

The CEM's are dead easy to balance so I presume you've balanced first the dec then RA axles - ruling out balance problems

The encoders are just providing a constant drive rate (PPEC) they're not sensing against the stars so your guide pulses should be OK. However your problems do look RA oriented.

I have a Williams 73 ZS with finder guider, my calibration steps are much higher circa 2500 and calibration steps are usually 14/15 west, 6 east and 8/10 for north/south steps. At 500 it would take probably 30-40 steps to reach the +25 threshold. Is the focal length of your guider set correctly?

Hysterysis I set to 30, guiding 1 rarely use anything other than 2 seconds, minimum mo 0.15 (dec & RA) aggression 70% RA 30% dec.

To me your Calibration steps are too short (Brain / Guiding tab) and your minmo set to zero doesn't allow the mount to settle and it's always chasing itself.

Cables: are they dragging or fouling anything...

Edited December 1, 2019 by fwm891
text added

[old_eyes]

18 minutes ago, fwm891 said:

I don't have an EC version of the CEM60 but have a similar set-up.

You've run a PA drift align and got fairly flat traces for both axii so that should be fine.

The CEM's are dead easy to balance so I presume you've balanced first the dec then RA axles - ruling out balance problems

The encoders are just providing a constant drive rate (PPEC) they're not sensing against the stars so your guide pulses should be OK. However your problems do look RA oriented.

I have a Williams 73 ZS with finder guider, my calibration steps are much higher circa 2500 and calibration steps are usually 14/15 west, 6 east and 8/10 for north/south steps. At 500 it would take probably 30-40 steps to reach the +25 threshold. Is the focal length of your guider set correctly?

Hysterysis I set to 30, guiding 1 rarely use anything other than 2 seconds, minimum mo 0.15 (dec & RA) aggression 70% RA 30% dec.

To me your Calibration steps are too short (Brain / Guiding tab) and your minmo set to zero doesn't allow the mount to settle and it's always chasing itself.

Cables: are they dragging or fouling anything...

I will try your suggestions. Cables are fine. I am using through mount cabling and have checked that any cables on the mount are not in the way.

[Freddie]

The EC versions have well documented issues with guiding. The mounts section of the cloudy nights forum has plenty of info. relating to this issue.

[old_eyes]

21 minutes ago, Freddie said:

The EC versions have well documented issues with guiding. The mounts section of the cloudy nights forum has plenty of info. relating to this issue.

Unfortunately, everyone who has solved the problem seems to have come up with a totally different solution. There is no consistency, so I could spend a long time cycling through approaches. I was hoping that someone would recognise the problem and give me a shortcut to reasonable guiding.

[cotak]

If you can return it, I'd suggest just get the non-EC version.

The EC is very hard to get guiding well. Generally the analysis of data suggest that the best bet is to go for longest exposure you can use. And lowest guiding rate.

When I have and OAG running with a long focal length I find it works quite well. With shorter guide scope now on an 80mm for wide field, I am finding PhD seems to think there are larger deviations. And the guiding is worse, to the tune of 0.9 for 6 second exposures. I just took delivery of a zwo OAG which is thin enough to work with the 80mm and reducer. If it works better like that I'll report back.

[michael8554]

Guidescope FL of 242mm is correct, guidecam pixel size is correct too, but PHD2 is taking 25 steps to Calibrate instead of 10 to 12, so I'm not sure if the guidespeed of 7.5 arcsecs/sec reported by the mount is correct.

Is there a guidespeed setting somewhere else in your setup, like APT ?

The Guide Assistant should be run AFTER a successful Calibration, I see you had notification that something was wrong with the Cal?

Try upping the Calibration Step as Francis suggested, your guidescope is not fully focused - HFD is 7, use the Star Profile to get it nearer 3 to 4.

Then re-calibrate and run the Guide Assistant.

Your RA guiding is yoyo-ing, I think because PHD2 is applying over-large corrections based on your 25 step calibration, which I think is the result of a wrong guidespeed setting.

Michael

[cotak]

So we know from the analysis on CN that the EC mounts will decide on its own how to actually react to a guide pulse. This makes the calibration not always the way PhD expects.

[michael8554]

Really ? Isn't that going to make accurate guiding impossible?  Is that reaction dependant on the Dec position?

Michael

[cotak]

You don't really guide this thing. It's more nudging it for drifts.

With the SDE it's doing it's little dance around the center point anyhow so it's not exactly going to be where you pointed it.

 

[cotak]

Hi Folks

A friend from CN looked at your logs (Andrew from down under) and he thinks your mount might have some really out of spec SDE to the tune of 8-10 arc-sec.

You need to run a fast capture run (0.5s or faster and subframe mode) in GA and see if you really have such an out of spec copy. If so you need to return it asap.

My personal experience has been not so great with iOptron in regards to SDE on my mount. I am in the final stage of checking if my encoder ring fall off. And if not then it might be a return for repair situation for me as well.

Edited December 3, 2019 by cotak

[old_eyes]

Thanks.

6 hours ago, cotak said:

Hi Folks

A friend from CN looked at your logs (Andrew from down under) and he thinks your mount might have some really out of spec SDE to the tune of 8-10 arc-sec.

You need to run a fast capture run (0.5s or faster and subframe mode) in GA and see if you really have such an out of spec copy. If so you need to return it asap.

My personal experience has been not so great with iOptron in regards to SDE on my mount. I am in the final stage of checking if my encoder ring fall off. And if not then it might be a return for repair situation for me as well.

Thanks, I thought that was worth trying. Will do so when the next bit of clear sky appears.

[old_eyes]

On Sunday I followed @fwm891's recommendations on settings.

I started the whole process over again with rough polar align using PoleMaster followed by drift align with PHD2.

When doing the drift align I noted that throughout the process the variation on the RA axis was much higher than on the DEC axis.

Calibration went better with the larger step size as recommended.

However, the end result was no better:

I tried increasing the exposure time up to 10 seconds, but no real improvement. I tried messign with the aggression and hysterysis a bit (but with no clear plan in mind) also with no improvements.

Time for further thinking

[cotak]

You should do your data capture for GA soon. Your graph is pretty extreme and you'd want to know if it's a mount issue or if it's your configuration.

I plunked on my OAG and it improves my PhD results fair bit so it does seems does not work well with EC mounts and short guide lengths, but why I don't really know as of yet.

Next for me is to check encoder ring.

[old_eyes]

The next step was to see if there was some sort of weird oscillation and flexing in the guide camera system. The Evoguide finder/guider is by default mounted on a stalk on a finder shoe, with guidescope rings that use screw pressure to hold everyhting in place. Not the most flexure proof configuration.

So I thought I would see what happens if I guide using the main scope and camera, which is much more tightly tied down (Equinox ED80 + QHY168C)

I just gave PHD2 the parameters for this scope and camera and let it do its thing. It didn't particularly like it as the full frame takes so long to download, but it did calibrate OK and attempt to guide.

Pretty much the same behaviour, so I think that it is not differential flexure, oscillation or resonance in the telescope mounts.

[old_eyes]

Last thought was to check what happens when I turn on and off the RA filtering using the handset.

According to the manual with RA allowed, the mount uses the RA guide pulses from the guiding software. With RA filtering on these guide pulses are ignored.

So, with RA filtering on (guide pulses ignored):

The shape of the RA error curve suggests to me that the 2 sec exposure is sampling a faster process that is going on within the mount.

If I watch the guide star position in PHD2 you can see it jump slightly to one side and then back towards its original position with each exposure.

If I turn RA guiding back on I get this:

To me this makes sense as PHD2 is sampling different points on a rapidly changing mount pointing position. Its correction pulses will be directionally correct (because the guide star always moves in the same direction), but not the right magnitude, because the mount is no longer pointing where it was by the time the pulse arrives. The result is a total mess.

 

PHD2_GuideLog_2019-12-01_215730.txt

Edited December 3, 2019 by old_eyes

[old_eyes]

Finally for comparison are a 60 sec and 300 sec exposure of the Pleiades taken without any guiding, just relying on the mount itself. These are raw, undebayered single frames of a faint star part of the image zoomed in to 6:1 in Pixinsight.

60 second

300 second

The interesting thing is that the elongation is of approximately the same length and in the same direction in each frame. This is not polar alignment drift as the 300 sec exposure would have a trail 5x that of the 60 second exposure. Whatever is going on is happening inside the mount.

Also interesting that particularly in the 300 sec exposure you have a 'comet' head and tail effect. The mount is spending more time at the head end of the variation. it is constantly being moce away from that position and being pulled back.

[old_eyes]

Things to do tonight (if clear).

Following @cotak's suggestion I will run the guide camera as fast as possible with RA guiding turned off to see if I can get a better estimate of the frequency of this oscillation.

I will look again at balance. With the original arrangement I had the guide scope directly on top of the main scope. With a Sesto Senso focuser on one side of the main scope it coudl not be completely balanced, so I left it a bit camera heavy as the best compromise. I can rotate the guide scope round to one side to balance the focuser better, although it wtill won't be perfect.

I don't want to go berserk with outboard counterweights in serach of the ultimate balance until I am convinced that is the problem.

I have been following general advice to turn the clutches to hard on and then back off 1/4 turn. This gives a solid feel, but avoids noisy complaints on slewing. I coudl experiment with that a bit.

The manual recommends 0.5x sidereal for tracking on both axes. I could try adjusting the RA number.

I am also starting a conversation with the vendor in case there is a mount fault.

Anything else?

[old_eyes]

Forgot to mention that some commentators on CN have said that the CEM60-EC doesn't like the way that PHD2 does calibration and that they have had more success with other guide programmes.

On Sunday I quickly tried MaxGuide and AstroArt 7 (the demo version). Essentially the same results, although I just used them with default settings and a quick setup. Since the errors seemed the same there did not appear to be any point in taking time to optimise them.

[cotak]

1 hour ago, old_eyes said:

Things to do tonight (if clear).

Following @cotak's suggestion I will run the guide camera as fast as possible with RA guiding turned off to see if I can get a better estimate of the frequency of this oscillation.

I will look again at balance. With the original arrangement I had the guide scope directly on top of the main scope. With a Sesto Senso focuser on one side of the main scope it coudl not be completely balanced, so I left it a bit camera heavy as the best compromise. I can rotate the guide scope round to one side to balance the focuser better, although it wtill won't be perfect.

I don't want to go berserk with outboard counterweights in serach of the ultimate balance until I am convinced that is the problem.

I have been following general advice to turn the clutches to hard on and then back off 1/4 turn. This gives a solid feel, but avoids noisy complaints on slewing. I coudl experiment with that a bit.

The manual recommends 0.5x sidereal for tracking on both axes. I could try adjusting the RA number.

I am also starting a conversation with the vendor in case there is a mount fault.

Anything else?

In my experience the balance only impacts random deviations, not constant. 

And we are talking balancing 44-50lb of newt that I see the deviation from balancing issue. 

[old_eyes]

Just now, cotak said:

In my experience the balance only impacts random deviations, not constant. 

And we are talking balancing 44-50lb of newt that I see the deviation from balancing issue. 

That is how I think it should be. This is a light rig and shoudl not be troubling the mount even if slightly out of balance.

[fwm891]

Just a mad thought but you have set the commander software to 'sidereal' not lunar or solar rates? Just seems that whatever your doing regardless of exposure the results look similar - hence the wrong guide rate thought...

[michael8554]

If your RA graphs were symmetrical about the Xaxis I'd say that RA was overshooting due to Agression being too high, but RA is predominantly wanting to head south as if PA was way out, or as if PE was high and on the south part of the curve. 

Just for fun I'd try reducing RA Agression way down and see what way RA heads. 

Michael 

[old_eyes]

Thanks for all the replies and suggestions. Did some work this evening. Updates tomorrow.

[old_eyes]

So, the vendor suggested I try the latest beta firmware (190716). I am not generally a fan of beta software, feeling that I should not mess with something that has not been approved for general use without knowing what I am doing.

However, it was worth a go, so I did the updates. Slightly alarmingly the handset reported a comms error for each of the four updates, but since there was a note on the instructions that some of hte updates might give an error message, I assumed htat was true for all of them.

This update immediately seemed to visually lead to smaller errors on RA and larger errors on DEC.

The main problem was that it was now impossible to calibrate with PHD2. I tried a nuimber of calibrations and they all behaved in the same way. Example below:

The problem is in RA where the pulses do not result in equal movement. Sometimes the guide star does not move for a couple of pulses and then jumps forward.

This is consistent with a recent thread started by OzAndrew on Cloudy Nights (https://www.cloudynights.com/topic/681210-calibrating-ioptron-ec-mounts/?hl=+cem60 +calibration) where he reports that both PHD2 and the mount are applying pulses and not communicating with each other. He traces that to the mount not reliably setting a flag that it is still moving. @cotak - I think this is the Andrew you mentioned in your ealrier post.

PHD2 reports after calibration that there is a sifference in guiding rate between RA and DEC that is greater than is acceptable and the calibration should not be relied on. Ot os easy to see why when you look at the plot, there are three pulses with almost no movement.

However, with cloud heading and no way to improve the calibration process I perservered.