EQ6-R Pro and PHD2 strange dec backlash issue

[mos]

Hi everyone,

I have a new EQ6-r Pro mount that is giving me some headaches. I'm guiding with an ASI290mm through a WO 50mm guide scope attached to my WO GT81.

PHD2 calibration is inconsistent and the backlash test has a strange plateau halfway through the returning south movements at which point the DEC motor goes audibly quiet. This was repeatable in different parts of the sky (close to the meridian and equator where I was calibrating and then south where my target for the night was)  with 3 examples below. 

There are no cables dragging, clutches are not loose and scope balance is good not being perfect so I don't think there are any issues there.

I've attached guide logs however they do contain dithering which might not make things so clear. There are definitely points where DEC backlash appears and PHD2 fails to bring the mount back.

I'm really at a loss as to what could be causing this and would love anyone's insights.

- Thanks, Matt

 

PHD2_GuideLog_2019-09-27.txt

[michael8554]

Hi Matt

I can't see anything inconsistency in your three Calibrations, they all seem to show roughly the same amount of Backlash ?

There are hours of guiding in the logs, it would help if you went through them and pinpointed where your problems were.
From what I've seen your guiding is good, and RA and Dec are similar, so you should be getting round stars - if you aren't you may be having Differential Flexture?
The Guide Assistant recommended to apply Backlash Comp, but the logs show you didn't (Backlash Comp = Disabled) ?
Your mount for the most part chugged along in Dec keeping pretty much inside the Min Move setting (the two horizontal red lines in the image below), with only a few Dec corrections required.
Your reported PA started at about 1 arcmin but peaked at 9 arcmins by the end of the sessions - is something sagging?
I've found in Section 19 one period where the mount struggled to get Dec back inside the red Min Move limit:

You can see that the first Dither moves Dec upwards and is nicely corrected by the Fastswitches (it says "Settling failed" but it was pretty good by the end of the settling period).
The second Dec Dither is in the other direction, and although the Fastswitches get Dec back initially, the mount starts heading down again and requires many Dec corrections to bring it back.
So it looks to me that after the first dither the scope balance helped to pull Dec back, in the other the scope balance fought against the corrections.

If you can pinpoint the problem sections that would help, and I would try the recommended Backlash Comp.

You could also keep the scope slightly unbalanced, which takes up the backlash all the time, and only Dither in RA,

Michael

Edited September 28, 2019 by michael8554

[mos]

Hi Michael, 

Thanks very much for the detailed reply!

Sorry for any confusion - my main concern is definitely the platau part way into in the south movements of the backlash test and what might be causing it.

And secondly as you've highlighed in your example of the DEC axis struggling to come back in line.  This is my second EQ6-R after the first had issues, and I'm worried this mount is faulty as well or at least in need of a decent adjustment.

The mount is doing a good job of guiding as far as round stars go but FWHM measuremnts range from 4 or 5 to 9 arc/s. 

I have previously tried backlash compensation but it caused very large overshooting and went into oscillations trying to correct each way.  I will give it a go again next time though and see if PHD dials down the amount. 

Thanks, Matt