HEQ5 DEC guiding problem

[Pompey Monkey]

Please can someone shed some light on my problem?

I've taken quite a few unguided subs on my HEQ5 pro since I got it a few weeks back and seemed to be getting star trails on every 10th to 12th image or so.

I have just built myself a finder-guider from a QHY5 and a Meade 8*50 finder to try to improve matters. Tonight I managed to calibrate and get a 40 or so minute test guiding session (I am actually very pleased to get this far as it's quite a steep learning curve!). This is the resulting tracking graph:

The RA is not perfect, but not bad either. The DEC, however, exhibits an almost square wave appearance, which correlates nicely with the sporadic nature of my trailing unguided pictures. This strongly suggests to me that it is some sort of mechanical issue with the mount. Am I right? If so what could it be?

BTW, the spike about two thirds the way along was my crude attempt to check for a backlash issue by hanging a small weight from the front of the primary OTA!

[Pompey Monkey]

Well, I think I found the answer!

In my haste to focus the guider, I had managed to pinch (or something) the objective lens and the stars were not round (no pictures, sorry). Did some more guiding tonight and it was fine!....

Until my laptop battery died after only 1 forty second and 2 five minute subs

[ollypenrice]

I would say that the square wave would be consistent with backlash. The tops and bottoms of the wave on the x axis would be generated as the mount sat on the same side of the free play. Then it might fall quickly to the other side and stay there for a spell. It would never hover in the middle.

On a scattergraph (a very useful tool that should be in all guide programmes) this would show as two clusters of guide input positions rather like this OO but probably overlapping slightly. Each O is created on its own side of backlash.

You could try disabling one side of the guiding in Dec. You'd need trial and error to see which one you could disable but the one you keep is the once correcting polar misalignment, whichever that is.

I don't think the star shape issue would explain your graph.

Olly

[Pompey Monkey]

I am at work at the moment. When I get home, I will post both of the logs: the bad one and last nights good one. 

I can see what you are saying, but unless I can replicate the same polar mis-alignment, I will have little chance of seeing the same backlash issue, I guess. Interestingly, PhdLab suggested that I was 327 minutes of arc out of polar alignment! 5 1/2 degrees? Really? Does it calculate this from the excessive control inputs produced by PHD?

Much too learn, methinks!

[Pompey Monkey]

Here you go:

bad guidelog.txt

good guidelog.txt