High peaks with iOptron GEM45

[Christian S.]

I am observing high peaks up to 10'' when guiding with PHD2 on my iOptron GEM45.

To me it seems like PHD is sending an correction in DEC and RA is over-reacting on it.
When this happens I can observe the guiding star jumping.

I double checked everything and running out of ideas. Can somebody help?

https://openphdguiding.org/logs/dl/PHD2_logs_nsbR.zip

CLS,

Christian

[Christian S.]

Here is a screenshot of the issues I am observing. Any hints what I should check or try?

[bw_msgboard]

Hi Christian.  I think there's definitely something wrong with the mount mechanics here.  What you're seeing is that a large Dec backlash compensation guide pulse is causing a big excursion in RA.  In other words, the mount is "rocking" on the RA axis when a large guide pulse is given to Dec.  We've seen this before on some other mounts - it can be caused by a problem in a bearing or possibly when there's a big static resistance on the Dec axis.  To avoid this for now, you will need to disable Dec backlash compensation and put up with the reversal delay in Dec when the direction of guiding changes.  You can try increasing the guide speed in the mount from the current 0.5x sidereal to a value closer to 1x sidereal.  This should mitigate the reversal delay and may help to push through any resistance.  From there, it would be good to investigate the Dec axis mechanics.  Are you sure the scope is well-balanced in Dec for all pointing positions?  Does it rotate smoothly and easily or is there some form of static resistance there?  If something is sticking or rubbing, can you eliminate it?

 

The changes you made in the guiding algorithms were mostly counter-productive, so you should revert to the default settings while you're trouble-shooting the mount situation.  What you're seeing isn't a guiding problem.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Christian S.
Sent: Thursday, September 02, 2021 10:26 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] High peaks with iOptron GEM45

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[Christian S.]

Hello Bruce,

thanks for your reply. It's probably really only (mostly?) a balancing problem.

Yesterday I first tried to calibrate near Celestial equator/Meridian in the south. I was able to guide smooth for about 15 minutes without seeing any peaks.

So I slewed to my target M 92 which was more in direction zenith and here I saw the big peaks again.

I recalibrated and I started playing around with the paramaters but nothing improved till I changed the DEC Guiding direction.

With only one direction of DEC guiding I was able to guide acceptable for approx 20 minutes. Than the DEC position started to vanish.

Finnaly your message received me. I was removing all cables and balanced DEC axis in the current RA axis position. There was indeed some unballancing and the Scope was moving direction east when I opened the clamp. Not much but still. After re-balancing, I reset parameters and recalibrated. And voila:

My HEQ5 was more forgivable about not so well balanced loads. I still see the same kind of spikes but this time much smaller (< 3''). What still makes me wonder that the RA spike looks always like an inverted DEC Spikes.

May the other things you told me, like increasing the guiding speed, help to smooth things out some more?

You can find the log files attached here: https://openphdguiding.org/logs/dl/PHD2_logs_SKtQ.zip

Thank you for you help and CLS,

Christian

[Christian S.]

Hello,

I meanwhile had contact to the reseller and he suggest to use the handcontroller to proof it's not a problem in guiding.

I could reproduce the spikes also with the hand controller. Everytime the direction of DEC is changing, there is a jump in both axis.

I read that a lot of people unbalance their DEC into direction camera to guide DEC only in one direction. Do I face the same problem here or is there something more wrong?

I really appreciate any help you can provide.

Christian

[bw_msgboard]

This is a problem in the mount, quite possibly a bearing, so it seems like it should be fixed by the manufacturer.  Didn't the reseller offer that option?

 

In order to avoid direction reversals in Dec guiding, you can use uni-directional guiding.  This has nothing to do with balance, you should keep the scope well-balanced in Dec.  In order to make uni-diectional guiding easier, you can intentionally introduce some polar alignment error, perhaps up to 10 arc-min of error.  This will cause the guide star to drift in declination and then you choose the Dec guiding direction to correct for that.  Depending on the details of the polar mis-alignment, the direction of Dec drift will change at some point in the sky.  When that occurs, you will need to change the PHD2 Dec guiding direction from north-only to south-only or vice-versa.  If you make most of the alignment error in altitude (as opposed to azimuth), the direction reversal will occur fairly close to the time you cross the central meridian.  If you run the Guiding Assistant for a few minutes, it will measure the direction of Dec drift and tell you what it is.  Many people use uni-directional guiding very effectively, there's no reason you can't produce good images this way.  In addition to side-stepping problems like this RA/Dec interaction, it also eliminates any effects from large Dec backlash.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Christian S.

Sent: Wednesday, September 08, 2021 9:08 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] High peaks with iOptron GEM45

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[Christian S.]

Hello Bruce,

thank you very much for your help. I contact the reseller with the results again and waiting for an answer.

CLS,

Christian