What on Earth is going on in Dec on my Losmandy G811

[c172...@gmail.com]

Hi,

  I just returned home from a 3 night star party.  I have gotten a little closer to good guiding, but.... not so much.

   I have a Losmandy G811 mount that I have been struggling with in order to get good guiding.  Early this year, I sent the RA axis to Losmandy for a fix that corrects some mount periodic error.  I had a 76 second periodic error that now seems corrected.   

  What is going on in dec in my guide log?

  My setup.  I have a C9.25 CST on a Losmandy G811 mount.

  I am using an OAG with the ZWO 174MM mini.  

 

  After creating a new profile a few nights ago and getting a good calibration, I switched to predictive pec in ra.  Looking at my mounts backlash, I then decided to switch off dec guiding and only dither in ra.  Over the next few nights I think I have done that task correctly.    On my last night, Sep 17th, I setup a 4 hour guide run.  

   I should have set PHD2 to guide dec in only one direction.  It was near the time for a meridian flip.  I then went to sleep and hoped for the best....

  If you look at the pic below, I feel that the RA axis (blue) is working perfectly fine.  The dec axis (in red) is doing some sort of weird step in some sort of periodic looking pattern.

  What could this problem be?  My mount started off as a G9 and then after a series of upgrades, it is now a Losmandy G811 or maybe a "Losmandy G911."

  I have kept my origional dec axis and that, perhaps needs some upgrading.  

  My questions are... what is happening to the dec axis in red below?  

  Is this something mechanical like a cable getting stuck somewhere?  If that is the case, why is the ra axis (blue) unaffected and looking so perfect?

  Is it something in the oag setup, could a screw be loose on a prism and the oag optics wobble and cause this anomoly? If so, Why not RA?

  Is my setting of guiding direction correct?  After the meridian flip, does my "guide north" setting need to become "guide south."

  I am considering buying a new dec axis to fix this.

  My thoughts would be that if I have dec axis guiding switched off in PHD2, then the drift should be slow and linear in dec, not stepwise line in the log below?

  Is it a PHD2 bug?

  Please advise on what can best correct the dec axis so that it guides like the ra?

   

Regards,

Jeff

https://openphdguiding.org/logs/dl/PHD2_logs_RFXT.zip

[Brian Valente]

Hi Jeff

a couple things

First, your guiding in one direction is in the wrong direction for the guidelog you uploaded. You'll notice that there are a few small corrections south, and in that regard, it seems to be doing okay there

On the giant steps, this looks like something is mechanically amiss. Remember at this point there is no motor running, so all these movements are equipment shifting, cable snagging, mirror flop, or something loose. 

It would be helpful if you included the calibration run as well. 

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Brian Valente

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[Open PHD Guiding]

Hi Brian, 

  Thanks for responding.  I uploaded the first and only calibration done on 9/14  

https://openphdguiding.org/logs/dl/PHD2_logs_YZSc.zip

  The orientation of my equipment optics was not altered during the 3 night star party.

  

I realize I set the guide direction incorrectly.  

With regard to "guide direction", Losmandy Gemini/PHD2  software, and a meridian flip.....will the guide direction behave/be set correctly after a meridian flip.  Meaning, if it is  (or should be) "guide north" before a meridian flip, will it need to be (or should be ) "guide south" after a meridian flip?  Does PHD2 work this out correctly from whatever was initially setup?

On the giant steps, mechanically amiss point.   That's the part that really has me wondering about causes in the context of a stationary dec axis.

I am under the assumption that my dec axis was not active for most of my guiding except for the few corrections.  

If my dec axis has jaggies from some kind of mechanical impact, why does my RA axis not look the same?  If there was a cable snag or mirror flop, shouldn't it affect both axis in exactly the same way?  

I am using an old 9.25 SCT. It is not the edge version.  I guess there could be mirror flop in my setup.

My old Losmandy G9 (G8) dec axis is what separates my mount from a brand new G11.  I am considering a new dec axis if that can help, sounds like it wont.

I was not planning on a new scope.  How can I test mine/fix it in some way to eliminate those periodic jaggies.

My assumption is that mirror flop is mostly in some parts of the sky and is fairly infrequent over say 5 or 10 minutes.  What I see in my guide log looks like it has a pattern.  I did assume mirror flop affects both ra and dec in the same way.  My ra axis is working beautifully.  I just don't know how to proceed with my existing 9.25 SCT setup.

Jeff

[c172...@gmail.com]

There was someone in the Losmandy group suggesting "Maybe clutch slippage due to oil or insufficient torque" as well as perhaps 

"Mechanically, your transfer gears may be binding up"

does any of this make sense as a possible cause for a dec axis that should now have moved?

Jeff

[Brian Valente]

Hi Jeff

>>>There was someone in the Losmandy group suggesting "Maybe clutch slippage due to oil or insufficient torque" as well as perhaps

Clutch slippage is possible. you can check by trying to move it yourself by hand. if it moves easily, then you should clean the clutch pads, there's a video on the losmandy youtube channel for that. I would also doublecheck your dec balance

It's not insufficient torque, as the motors are not running

>>>"Mechanically, your transfer gears may be binding up"

The motor is not running at these times, so not this one

Additional thoughts/questions:

How are you handling mirror flop on your SCT?

For an older style axis without spring loaded worm, you will probably want the axis slightly biased towards the camera end

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/c57ebbf5-a937-4d85-8583-87ca9ef1e298n%40googlegroups.com.

[Brian Valente]

Jeff what I suggest is doing some investigative testing

Use the calibration assistant to run a fresh calibration, and immediately following that do a 30 minute guiding assistant run, followed by 30 minutes guiding. 

send the resulting log, that will isolate what the mount does with no guiding vs. guiding

But again, I suspect there's something loose somewhere, or it could be mirror flop. If you have a refractor you could also put on and test that would help

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/be089bda-99d8-4457-9b02-2e245beb4523n%40googlegroups.com.

[Open PHD Guiding]

Thanks Brian, I will try that.

Jeff

[Bruce Waddington]

Coming back to your questions about uni-directional guiding, I think it's explained pretty well in the User Guide:

https://openphdguiding.org/man-dev/Supplemental_Info.htm#Declination-Uni-directional_guiding

That said, I agree with Brian that you have something loose in the guiding assembly or, more likely, you haven't done a careful enough job of cable routing.  As a result, uni-directional guiding is unlikely to help you much at this point.  There is no reason at all to assume that cable drags or mirror flops are going to affect both axes - it all depends on the details of the underlying mechanical problems.

Bruce

[c172...@gmail.com]

Thanks Bruce, I'll rereview that material.  I'll make more sure clutches are tightened and cables are not likely to snag. 

Regards,

Jeff