CEM70 guiding problems - Forums

Hello there,

I just bought a new CEM70. I previously used a HEQ5 with the Rowan Belt Mod for my Takahashi E130. I was quite satisfied with the average RMS 0.5" guiding accuracy of the HEQ5 (using multistar guiding of the ASIAIR), but with 12.5kg load it was at its load capacity. I thought I could improve the situation with the CEM70.

However, with the CEM70 I now have a RMS value of 1.5" with outliners well past 4". This is showing up in the images with the E130, even though it only has 430mm focal length. I am shocked and desperately looking for ideas to find what is going wrong here. Maybe/hopefully its only a matter of the guiding parameters of the ASIAIR, which ought to be modified wrt to the HEQ5 settings ?

Any help is appreciated. Working settings of CEM70+ASIAIR owners most welcome.

Goetz

Hello Gotz,

You will have to check the tension of the belts of the CEM70 as well as the mesh adjustment on RA and DEC for backlash.

First thing to do is check if the movement from the motors is immediately transferred to movement on the axis. The belt on my DEC was lose so I had to tighten it in order to remove backlash.

A guide on how to do that can be found here.
https://usermanual.wiki/m/04d095dd466567e2210f5413328843c9ac0401eb8de50e98c81589bdd75c76c8

After you have checked for the tension of the belt the next thing is to adjust the meshing of the gears - if needed. There is a guide for this in the manual of the mount. Recently there was backlash on my RA axis so I eliminated it through adjusting the mesh.

Hope this helps. Otherwise the CEM70 is a great mount.

CS,
Ivan

Thank you, Ivan.

It seems that guiding is better close to the zenith and worse for objects close to the horizon. Also, increasing the exposure time to 2sec did improve things.
For the currently running image of SH2-115 RMS is now down to 0.55" which is OK.

I will check the mount tomorrow and report any adjustments I make - and also if I get any improvements

Götz

Hi Götz,

I also upgraded my mount recently and had issues with guiding initially.
Since you change the mounting of the scope, make sure that you have checked for cone error and fixed it if needed. It will affect a lot of the mechanics and cause issues in many places.

Cheers
Gernot

@Ivan Raichev :I just checked the belts, they are both nice and tight, i.e. they move by less than 2mm when I excert moderate pressure on them.

@Gernot Schreider: Shouldnt the cone error produce a long term effect continuously changing over hours and only be relevant when you dont guide ?

The problems I see occur every 1-2 minutes. RA or DEC deviate by 2-4" and the guiding doesnt manage to get it back for 8-12 seconds, even though corrections do take place. As far as I remember last night, the direction of the deviations alternates, e.g. a deviation to the north is followed by a deviation to the south about a minute later.

Maybe this is a problem in the interaction of the ASIAIR and the CEM70 mount ?

Hello Gotz,

Such changes indicate backlash in my opinion. My suggestion is to make a test. I can think of two ways to make it:
1/ during the day - point the telescope to somewhere in the distance - a mountain or something. Set the camera to a lower exposure time - several frames per second in order to be able to instantly detect movement. Take the remote. Set the slew speed to minimum and see if the image moves in the moment that you press a direction on the remote. If it doesn't this means that it takes time the movement to be transferred to the axis which indicates backlash. It can be fixed either by tightening the belt or by adjusting the meshing.

2/ watch this video - https://www.youtube.com/watch?v=emjXwHHVlF4Recently I found out that I had backlash in RA with this method and adjusted it with the hex keys and the holes on the mount.

CS,
Ivan

Hello!
I´ve had problems with my guiding the last night aswell, even tough nothing obvious had changed since the last imaging sessions. I'm guiding with the ASI Air aswell, and your guiding problems sound pretty much the same as mine. After checking the mount many times and also the guiding set up I couldn't figure out what the problems was.

My problems where solves after I did a reset of the guiding calibration in the Abi Air itself, after that the guiding was back to 0,5", as I was used to.

maybe the guider calibration in the Asi air could be the reason for your problems aswell.

Franzi

Götz Golla:
@Gernot Schreider: Shouldnt the cone error produce a long term effect continuously changing over hours and only be relevant when you dont guide ?

The problems I see occur every 1-2 minutes. RA or DEC deviate by 2-4" and the guiding doesnt manage to get it back for 8-12 seconds, even though corrections do take place. As far as I remember last night, the direction of the deviations alternates, e.g. a deviation to the north is followed by a deviation to the south about a minute later.

Maybe this is a problem in the interaction of the ASIAIR and the CEM70 mount ?

Hi Götz,

as far as I understand it the issue of cone error can affect the polar alignment and thus the guiding performance.
You could check the guiding assistant in PHD2 and review what it says about polar alignment error.

Cheers
Gernot

Well, I dont use PhD but ASIAIR, sorry.

I am using a Losmandy tail with the telescope connected to it with the standard Takahasi mount plate and rings. I dont see any obvious source of cone error and I didnt touch this setup in any way. If there is a cone error, it shouldnt differ from my HEQ5 setup. I never had problems with guiding with the HEQ5.

Hi

Dont use the ASIAIR anymore but for me the minimum exposure time for proper guiding is at least 3sec exposure to average out seeing effects. Believe the ASIAIR uses a sort of PHD2 light. The ASIAIR should still produce a guidelog but not sure anymore how you get it off the device, if i recall they get put on the same location as your session lights and you can use https://adgsoftware.com/phd2utils/log viewer to analyze the calibration etc. It should be called something like PHD2_GuideLog_date_etc

If you can share something like that we can have a look.

CS
Sean

Sean van Drogen:
Hi

Dont use the ASIAIR anymore but for me the minimum exposure time for proper guiding is at least 3sec exposure to average out seeing effects. Believe the ASIAIR uses a sort of PHD2 light. The ASIAIR should still produce a guidelog but not sure anymore how you get it off the device, if i recall they get put on the same location as your session lights and you can use https://adgsoftware.com/phd2utils/log viewer to analyze the calibration etc. It should be called something like PHD2_GuideLog_date_etc

If you can share something like that we can have a look.

CS
Sean

Yes, indeed, the PHD Guidelog is there on the ASIAIR. Thanks for the hint. Its in /asiair/logs. I will look into compiling phdlogview for Debian asap since this will be handy also in the future.

I managed to compile phdlogviewer on my main workstation and can analyse the phd logs of the ASIAIR now. This is SO great

Thank you @Sean van Drogen If weather permits I will have an observing session tonight and show the phd data here.

Götz Golla:
I managed to compile phdlogviewer on my main workstation and can analyse the phd logs of the ASIAIR now. This is SO great Thank you @Sean van Drogen If weather permits I will have an observing session tonight and show the phd data here.

Glad I could help

Guiding varies so much with atmospheric conditions. On a clear night recently my guiding on an HEQ5 Pro (heavily loaded) was around 0.55 arc secs. Last night it was nearer 1 arc secs as there were whispy clouds everywhere. No changes to my tripod, mount or scope at all in between.

Not sure it's related, as you seem to be able to gyuide your CEM70 already with the ASIAIR Pro, but I recently had a problem when updating the firmware of the CEM70 : I couldn't perform the calibration anymore from the ASIAIR Pro. Same problem for other people, as described in theses posts :
https://bbs.astronomy-imaging-camera.com/d/12397-calibrationbacklash-issue-solved https://bbs.astronomy-imaging-camera.com/d/12858-guiding-calibration-issue-with-asiair/9
I indeed observed on the controller of the CEM70 that it was switching from guiding to tracking randomly... so clearly a comm problem between the mount and the ASIAIR.

I solved it using the tips of a guy in those threads (no need to unplug as someone said) : you just take a preview, platesolve it and align the mount just before performing the calibration. Then the guiding just works well for me...

Hope this helps (though if you can guide already, that means you performed well the calibration step....).

Here are some news and questions about the CEM70 guiding:

Last night I tried PEC recording. The mount recorded the PEC for just one worm gear period and stopped recording then, which suprised me. I heard that you would want to record PEC over several periods to smooth out seeing and other effects. Mayby next time I should choose a higher exposure time of 4sec during the recording for the smoothing ?

Anyway, I then turned on PEC playback and it appeared to me that guiding was much better. I got a total RMS of 0.45", which is very good. I was confused, however that guiding worked at all, since many people say that the mount will not accept guide pulses when PEC playback is on ?

Additionally, I found the following excellent tutorial about guide settings of the ASIAIR: https://eastwindastro.blogspot.com/2021/02/how-to-adjust-asiair-guide-aggression.html.The main thing is that with guiding you dont want to correct for seeing. Therefore its OK to choose longer exposures (2-3secs) and also not to guide too aggressive. I have to admit that this was new to me (and I am always happy to learn).

Some more news:

I have tuned the guiding parameters of ASIAIR now with excellent results with the CEM70:

Exposure time for guiding is 3sec
Max DEC and Max RA duration is 2500ms
Guiding rate now is 0.75x
Aggressiveness is 130% for RA and 50% for DEC

With these values I get an RMS of 0.2 - 0.3" in DEC and 0.4 - 0.5" in RA.

The RA RMS is higher because the periodic error in RA needs to be compensated. Thats why the RA aggressiveness is higher and this leads to some more random fluctuations. For lower RA aggressiveness I get less short term fluctuations but a longer term fluctuation since the periodic error cannot be compensated completelly. The effect looked a little bit like backlash, but it wasnt.

I didnt find any difference in the RMS values with PEC playback on or off. I think the reason for this is that PEC recording only lasts one worm period when starting it from the hand control unit. Other options do not easily exist with the ASIAIR. But I shouldnt complain. The RMS values are now better than I was hoping for.

Thank you all for your support and ideas.

Götz

If you ever decide to go with PHD2 and other capture software this is the run I had last night with my CEM70

Main difference to the ASIAIR is that for RA I run PPEC as guide algorithm

Just food for thought.

CS Sean

Sean van Drogen:
If you ever decide to go with PHD2 and other capture software this is the run I had last night with my CEM70

Main difference to the ASIAIR is that for RA I run PPEC as guide algorithm

Just food for thought.

CS Sean

Hello Sean,

here comes a little follow-up. Since I have some spare time and its full moon time I spent some time to implement NINA and utilize PHD2. With the PPEC algorithm I can almost get rid of the periodicity of the RA curve. To achieve this I had to increase the maximum guiding pulse in RA to 8000ms for an exposure time of 1.5sec. For shorter guide pulses of longer exposure times it looks like PHD2 isn't able to correct the PEC curve.

If I let the PPEC algorithm determine the period length I am getting 115sec only, which is way off from the worm gear period of 348sec. Should I fix the period in PHD2 to 348sec ? I will try this next.

The best results for now are RA=0.5arcsec RMS and DEC=0.25arcsec RMS, which is not much better than with ASIAIRs PHD2 Light. I will continue to play with it.

There is too little information that can be tell from what you said. Post your phd2 log and or the screen shot of the guiding graph here for analyzing. Also a calibrating graph would help too. My CEM70 reliably guides under 0.4 arcsecs after some fine tuning.

Minh Lết:
There is too little information that can be tell from what you said. Post your phd2 log and or the screen shot of the guiding graph here for analyzing. Also a calibrating graph would help too. My CEM70 reliably guides under 0.4 arcsecs after some fine tuning.

Hi Minh,
I will try to retrieve the phd2log next time. Are you using the PPEC algorithm and have set the period parameter to 348sec or do you let phd2 estimate it ? Your other PHD2/PPEC parameters would be very helpful to me.
Regards, Götz

Götz Golla:
Minh Lết:
There is too little information that can be tell from what you said. Post your phd2 log and or the screen shot of the guiding graph here for analyzing. Also a calibrating graph would help too. My CEM70 reliably guides under 0.4 arcsecs after some fine tuning.

Hi Minh,
I will try to retrieve the phd2log next time. Are you using the PPEC algorithm and have set the period parameter to 348sec or do you let phd2 estimate it ? Your other PHD2/PPEC parameters would be very helpful to me.
Regards, Götz

I do not use PPEC on the mount, it does not work with PHD2 guiding. I did try PRedicttive PEC on PHD2 and it keeps the RMS in the 0.3" total. Do not let phd2 estimate the worm period if you know it for a fact. I use Hyteresis daily and the rms is still in the 0.4" which is good enough for me.
i just set the worm period and leave every thing else default (may be min move in RA to 0.1 and adjust the dec min move to 0.3 if the dec keep switching side)

Minh Lết:
Götz Golla:
Minh Lết:
There is too little information that can be tell from what you said. Post your phd2 log and or the screen shot of the guiding graph here for analyzing. Also a calibrating graph would help too. My CEM70 reliably guides under 0.4 arcsecs after some fine tuning.

Hi Minh,
I will try to retrieve the phd2log next time. Are you using the PPEC algorithm and have set the period parameter to 348sec or do you let phd2 estimate it ? Your other PHD2/PPEC parameters would be very helpful to me.
Regards, Götz

I do not use PPEC on the mount, it does not work with PHD2 guiding. I did try PRedicttive PEC on PHD2 and it keeps the RMS in the 0.3" total. Do not let phd2 estimate the worm period if you know it for a fact. I use Hyteresis daily and the rms is still in the 0.4" which is good enough for me.
i just set the worm period and leave every thing else default (may be min move in RA to 0.1 and adjust the dec min move to 0.3 if the dec keep switching side)

With PPEC I meant Predictive PEC on PHD2. Thats what I am using with NINA. Yesterday I ran a frequency analysis with phdlogview and found that the dominant period is 115sec, just what phd2 is also finding. I will use this value as a fixed value.
I have a guess why my guiding was bad the previous night. It probably was because the guiding rates for RA and DEC were set to 0.5. I hope that when I set the RA guiding rate to 0.75, P(r)PEC will be able to fully correct for the periodic error. I just need another clear night to verify this.

Sean van Drogen:
If you ever decide to go with PHD2 and other capture software this is the run I had last night with my CEM70

Main difference to the ASIAIR is that for RA I run PPEC as guide algorithm

Just food for thought.

CS Sean

I've been wrestling with mine a bit too. I have RMS around the 0.4-0.5 but I also see peaks a bit like yours (sometimes up to 2.5")
I feel like I should not have these - I am lucky my imaging scale is 2.2"/pxl right now but if I got a bigger scope, these peaks seem like they will affect the image

I am hoping that by tightening the mesh just a little it will help (loosening it definitely played havoc with guiding) but it's been frustrating to get rid of them and I am not even sure what I should reasonably expect

My setup is pretty light - an AT60ED so it's nowhere near the payload limit of the mount

Nick Ambrose:
Sean van Drogen:
If you ever decide to go with PHD2 and other capture software this is the run I had last night with my CEM70

Main difference to the ASIAIR is that for RA I run PPEC as guide algorithm

Just food for thought.

CS Sean

I've been wrestling with mine a bit too. I have RMS around the 0.4-0.5 but I also see peaks a bit like yours (sometimes up to 2.5")
I feel like I should not have these - I am lucky my imaging scale is 2.2"/pxl right now but if I got a bigger scope, these peaks seem like they will affect the image

I am hoping that by tightening the mesh just a little it will help (loosening it definitely played havoc with guiding) but it's been frustrating to get rid of them and I am not even sure what I should reasonably expect

My setup is pretty light - an AT60ED so it's nowhere near the payload limit of the mount

For me on the DEC it was mostly the belt that was the biggest culprit trying to get this down to an acceptable level. On the RA it was really the gear mesh.
For the RA I am still not completely happy as its about double of DEC sometimes.

I also image with a relatively light load with my Zenithstar 73, but from time to time I use my C9.25 SCT. With the heavier load i get worse guiding, but my PA was not as spot on either

Think with the heavier and bigger load for me also wind is a much bigger factor as with the dewcap that thing is basically a sail.

CS Sean