Guiding problems after meridian flip

[Denis Zsebi]

Good evening,

I am turning to you for help with an issue I am not able to solve myself for a couple of weeks.

Equipment: sky watcher eq6r pro, explore scientific 102 Ed, zwo asi294mc pro, zwo oag, zwo 120mm mini (guide cam), remote controlled windows mini pc for control

Software used: phd2 latest version, eqmod, sharpcap for polar alignment, Nina for acquisition 

Information: my equipment stays outside usually weeks. If I experience any guiding problems, I create a new profile and recalibrate (always near intersection of meridian and celestial equator) on the start of almost every session I run the guiding assistant and apply suggestions, my balance is really good in all 3 axis.

Problem: I start my session as usual. Normally I capture targets that are at least 50deg high and most of the time on my average seeing I am able to get .35-.65 total rms error. I preferably pick targets that cross the meridian in the middle of the night if available. 

Nina controls the meridia flip - when it occurs everything goes well, flip, autofocus, recenter, resume guiding - here is when he problem begins. 

After the flip my dec guiding starts doing spikes. It starts going for example north, I see phd2 sending corrections I the correct direction, but dec goes further and further - usually up to 8 arc seconds of error and the suddenly returns to 0. After a couple of exposures this repeats to the same direction or the opossite direction. My RA guiding stays in acceptable numbers before and after the flip.

I know there is a tool to check if I need to reverse dec output after flip - I don’t. 

I always try to do near perfect polar alignment in sharpcap - so if I deactivate the dec output after the flip I am able to collect usable exposures but is less accurate. 

My mount has almost no dec backlash, I measured it using guiding assistant and the graph is 98% ideal.

Guiding after the flip worked a couple of weeks back, but I can’t seem to remember if I changed something in eqmod or phd2 settings/

My link to the uploaded guide log - https://openphdguiding.org/logs/dl/PHD2_logs_CRBY.zip

Thank you for your help.

Denis

[Denis Zsebi]

[Bruce Waddington]

Hi Denis.  I agree, your problem has nothing to do with the "reverse dec after meridian flip" option.  Here's a look at what is happening after the meridian flip (Dec in green):

This is a pretty classic example of static friction (stiction) on the Dec axis - a situation where there is large static resistance to getting the axis moving, followed by a "break-through" when the resistance is overcome, followed finally by a release of the stored energy and an over-shoot in Dec.  That raises the question of what starts this oscillation sequence - what is the origin of the first large excursion.  We can see that here:

The two red arrows point to large Dec excursions that don't seem to be related to the stiction/over-shoot activity.  In both cases, the guide star just makes a spontaneous large move.  These are not associated with guide commands, you can see that the guide commands were small and in the opposite direction.   The first place I would look is with the cable routing.  If there is any sort of binding or pulling caused by cables, you can see this pattern.  And of course meridian flips are well-known for exposing cable routing problems that aren't seen in other pointing positions.  I recommend manually putting the scope/mount through these meridian flips and looking very carefully at what's going on with the cables.  Even if you've done a careful job of cable routing, you may have things like cable ties or ribbed surfaces on cable sheath materials that can briefly catch then release as the cable rubs across a stationary surface.  You'll have to keep in mind that these "large" excursions correspond to movements at the guide camera sensor of about 15 microns - 1/3 the thickness of a human hair.  For reference, the Dec pointing position during these times was +58 degrees.  While you're at it, you should also check to make sure the OTA is well-balanced in Dec at this pointing position and the axis can't rotate on its own.

Good luck,

Bruce

On Monday, September 12, 2022 at 3:11:40 PM UTC-7 denis...@gmail.com wrote:

[Denis Zsebi]

Hi Bruce,

Thank you very much for you reply. 

In the case that it would be a cabeling issue (I think it could be the case) is it possible that when I turn dec guiding of in phd2, it stabilizes?

For the balance, it is really good as I spent countless hours training and learning about 3rd axis the hard way - but I will check!

Here’s a pic of my equipment. I have only 3 cables going from mount to OTA, everything else is connected to the mini pc and power delivery hub so I could rule them out since they are in stable fixed position no matter where the scope points.

So if the problem is with these remaining cables that should be an easy find /fingers crossed/

I will report back when I have a clear night for testing. Thank you

S pozdravom Denis Zsebi

On 13 Sep 2022, at 04:57, Bruce Waddington <bw_m...@earthlink.net> wrote:



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[Denis Zsebi]

Hi Bruce,

I released all those cables that could be responsible for those issues. I’ll just let them hang, make sure they don’t touch anything, rebalance and give it a shot.

But those pesky clouds…..😬😬😬

I’ll be back

Kind regards Denis Zsebi

On 13 Sep 2022, at 04:57, Bruce Waddington <bw_m...@earthlink.net> wrote: