Help with my CEM 70 Guiding issues.

[anthon...@gmail.com]

hello!

I've been having some guiding issues with my CEM 70 so I thought I'd share my findings here maybe someone can give me an insight for what might be going wrong.

Basically, the main issue with the mount is instability; I get acceptable guiding on some nights with stable values between 0.4 and 0.6"/px (very smooth curves) and bad guiding on most other nights with RA peaks. My first reasoning was seeing conditions but I have some reasons to believe it might be mount mechanics or PHD2 settings.

Before I delve into the details, I've tried:

- different balancing techniques

- all possible guide settings in PHD2 using mainly Hysteresis and PPEC

- I adjusted gear mesh many times

- I tried with both a guidescope and an OAG

- I made sure everything is tight and stable 

- I've recently replaced the RA/DEC boards sent by iOptron that reduce EMI 

So here are my findings along with some questions. I'm hoping someone has had some experiences with these issues that could

shed some light and help me figure this out.

I will also be sharing some guidelogs throughout this thread.

Here are my findings/problems/questions:

1- During Calibration, I've noticed that the RA sometimes tends to "fall" towards the east first before responding to the west pulses.

It looks very weird and I've never had this issue before with other mounts. I tried to tighten the belt thinking maybe

it could be backlash but that didn't solve it. Adjusting gear mesh doesn't seem to solve it either unless I'm unknowingly

far from the sweet spot which I doubt.(Btw you might find in the guidelog some calibration trials that are far from dec 0

but I tried to calibrate close to dec 0 with the same results)

2- While trying PPEC, I've noticed that PHD2 sometimes fails to detect the actual worm gear period of 348 seconds, and

it tends to use values which have a 4:1 3:1 2:1 ratio. So sometimes it might detect 87s as the period, other times

it might detect it as 174 and sometimes it even surpasses the 348s mark.

3- When using the frequency analyzer in PHDlogviewer, I've noticed that there is one period, 87s, which tends to have

a very high amplitude, even higher than the worm gear and I suspect that PHD2 is not reducing this error as much as it's 

reducing the worm gear PE.PEC training doesn't seem to reduce it. At first I thought maybe that was the gearbox but

 after doing some research, I found out that the motor's period is aroud 115s. 

Does anyone know what could the component with an 87s period be in the CEM 70? knowing that its ratio is 4:1 wrt to the

worm gear.

4-  I am getting very high minmo recommendations using GA from what I was used to. To my knowledge, higher minmo is usually

recommended in bad seeing conditions, but could it also be caused by some random errors and vibrations in the mount?

I've attached some guidelogs and screenshots. The guidelogs are not very tidy, but you can check the parts with the longest guiding time for analysis as well as the multiple calibration trials I've tried. 

In summary, I'm not sure if the mount has some mechanical problems, some internal play/vibrations inducing random errors

that show up from time to time which is why I need your help analysing all the data I have. I feel like the frequency

analysis will help a lot in trying to troubleshooting.

[Brian Valente]

Hi Anthony

see below re: your questions

On Sat, May 1, 2021 at 2:35 PM anthon...@gmail.com <anthon...@gmail.com> wrote:

hello!

I've been having some guiding issues with my CEM 70 so I thought I'd share my findings here maybe someone can give me an insight for what might be going wrong.

Basically, the main issue with the mount is instability; I get acceptable guiding on some nights with stable values between 0.4 and 0.6"/px (very smooth curves) and bad guiding on most other nights with RA peaks. My first reasoning was seeing conditions but I have some reasons to believe it might be mount mechanics or PHD2 settings.

Before I delve into the details, I've tried:

- different balancing techniques

- all possible guide settings in PHD2 using mainly Hysteresis and PPEC

- I adjusted gear mesh many times

- I tried with both a guidescope and an OAG

- I made sure everything is tight and stable 

- I've recently replaced the RA/DEC boards sent by iOptron that reduce EMI 

So here are my findings along with some questions. I'm hoping someone has had some experiences with these issues that could

shed some light and help me figure this out.

I will also be sharing some guidelogs throughout this thread.

Here are my findings/problems/questions:

1- During Calibration, I've noticed that the RA sometimes tends to "fall" towards the east first before responding to the west pulses.

It looks very weird and I've never had this issue before with other mounts. I tried to tighten the belt thinking maybe

it could be backlash but that didn't solve it. Adjusting gear mesh doesn't seem to solve it either unless I'm unknowingly

far from the sweet spot which I doubt.(Btw you might find in the guidelog some calibration trials that are far from dec 0

but I tried to calibrate close to dec 0 with the same results)

I"m not sure what you mean by 'falling' - your second calibration looks okay, although the number of RA steps seems high. It's not critical that the calibration steps are exactly on the axis line, although showing something too far from the line (or with a lot of variability) could be an indication of a mechanical problem  

2- While trying PPEC, I've noticed that PHD2 sometimes fails to detect the actual worm gear period of 348 seconds, and

it tends to use values which have a 4:1 3:1 2:1 ratio. So sometimes it might detect 87s as the period, other times

it might detect it as 174 and sometimes it even surpasses the 348s mark.

I think you may misunderstand the PPEC algorithm. It doesn't "detect your actual worm gear period". It watches the periodicity of errors and consistently corrects/updates what it sees. If you know the worm period of your mount, you should enter it and disable auto adjust 

3- When using the frequency analyzer in PHDlogviewer, I've noticed that there is one period, 87s, which tends to have

a very high amplitude, even higher than the worm gear and I suspect that PHD2 is not reducing this error as much as it's 

reducing the worm gear PE.PEC training doesn't seem to reduce it. At first I thought maybe that was the gearbox but

 after doing some research, I found out that the motor's period is aroud 115s. 

Does anyone know what could the component with an 87s period be in the CEM 70? knowing that its ratio is 4:1 wrt to the

worm gear.

you are looking at the residual error, and in my experience (and looking at many logs) it's very typical to see the faster frequencies being the ones that are more difficult to guide out, and therefore are higher. That is not unusual

Regarding that specific period for your mount, that's something that you will probably find answers for in ioptron user group (or maybe some ioptron users here).

 

4-  I am getting very high minmo recommendations using GA from what I was used to. To my knowledge, higher minmo is usually

recommended in bad seeing conditions, but could it also be caused by some random errors and vibrations in the mount?

 Generally it's seen conditions. It's a recommendation but of course you can dial it in to whatever value you like. 

I've attached some guidelogs and screenshots. The guidelogs are not very tidy, but you can check the parts with the longest guiding time for analysis as well as the multiple calibration trials I've tried. 

Regarding mount performance, it's more informative to look at a long-ish unguided run (yours are pretty short) or the Raw RA performance in the log viewer (below):

 

From this it appears your 87 second error is your primary mechanical challenge. 

Looking at the residual error (what PHD can't correct out) it's still that period, although of course much smaller:

I suspect it may be something you can address via mechanical adjustments, belt, etc. but that is specific to your mount, so talking to other mount owners would help here

 

I've seen reports the CEM mounts can be prone to issues when guiding at faster guiderates, you may want to try a guide exposure of 2-3 seconds. other CEM70 owners can chime in here as well

you seem to have been fiddling a lot with small changes to the guiding parameters: don't do this. At best it's going to waste your time. These kinds of small changes aren't going to make any measurable difference. 

Also i encourage you to use dark frame libraries, it appears you don't have one enabled. it may help avoid future issues

Brian

In summary, I'm not sure if the mount has some mechanical problems, some internal play/vibrations inducing random errors

that show up from time to time which is why I need your help analysing all the data I have. I feel like the frequency

analysis will help a lot in trying to troubleshooting.

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Brian Valente

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