EQ5 pro mount - Excursions on Dec axis

[Jon Foulkes]

Could really do with some help....
I've always tended to get poor DEC guiding with my EQ5 pro mount, with large transients occurring every 10 mins.  In trying to understand the behaviour a bit more, I ran the PHD2 guiding assistant recently for an extended period.  As the GA disables guiding (so just allows RA tracking with no DEC drive at all) you would tend to expect a fairly straight line response drifting up or down depending on the PA error.  Alas, not with my mount.  I get a near perfect sine wave of period 600 secs - which suggests that the guidecam is picking up movement in the DEC axis corresponding to the periodic error of the RA drive.  With guiding then enabled, PHD2 makes a pretty decent job of dealing with the RA periodic error, but unfortunately doesn't seem to be able to cope with the DEC axis, where clear residual transients still remain.
Would welcome any pointers to what could cause the RA axis drive to affect the DEC axis.

Logfile:
https://openphdguiding.org/logs/dl/PHD2_logs_djAe.zip

Top pic - Analysis of unguided section
Bottom pic - Guided section (from 0100 hrs onwards)

Equipment: EQ5 pro mount, ASI mini guidescope & 120mm camera.  Z130 (5" f5 reflector) scope, Canon 500D imaging camera.  Mount controlled using ST4 connection.
Rig levelled & balanced, PA using Sharpcap.  
 

[michael8554]

Hi Jon

As you said, RA and Dec guiding is off during a GA run.

For SGL readers, the PHD2 guys suggested that between Cal and the GA run, the guidecam rotated.

Then every move of the mount had a RA and Dec component, as registered by the guidecam.

So the RA PE would show as Dec movement too.

Michael

[Jon Foulkes]

4 hours ago, michael8554 said:

Hi Jon

As you said, RA and Dec guiding is off during a GA run.

For SGL readers, the PHD2 guys suggested that between Cal and the GA run, the guidecam rotated.

Then every move of the mount had a RA and Dec component, as registered by the guidecam.

So the RA PE would show as Dec movement too.

Michael

Thank you for your reply Michael.  Yes, I think I understand how the in-phase & quadrature components can be manifest in this way, though I confess I’m not sure how such movement has occurred (the GA runs were performed immediately after calibration, without moving the mount).  I guess as it’s probably only a few microns, it does give me some food for thought on how this might happen.

[michael8554]

If guidecam rotation was the cause, I think it would have to be considerable to get such a large translation between the axis.

I'm not clear whether this is a new or ongoing situation ?

What Dec was the Cal and GA at ?

Michael

[Jon Foulkes]

1 hour ago, michael8554 said:

If guidecam rotation was the cause, I think it would have to be considerable to get such a large translation between the axis.

I'm not clear whether this is a new or ongoing situation ?

What Dec was the Cal and GA at ?

Michael

I’m certain there was no discernible rotation of the guidecam.  
Since I started guiding a few months ago, Dec has always been problematic.  Only recently tried running the GA for an extended period, which was when I noticed the sine wave (previously I’d only let GA run for 2 or 3 minutes).

As I’m using ST4 I have to calibrate at the target, which in this case was M13.  So I guess Dec was at around 26 deg or so.