Guiding issues with newly polar aligned EQ8

[michaelmorris]

After months of delays I thought I was finally ready to start CCD imaging with my Atik 460 camera and my EQ8 mount.

Alas, it was not to be. I'm now getting all my star trailling.  PHD2's guide assistant suggests the problem is that it calculates that the mount requires a whopping 31 seconds of backlash compensation on Dec.  

I've tried running the compensation in North or south only, but this doesn't seem to help.  Ideas please?

[Tim]

Hi Michael

How did you polar align? Are you certain of the accuracy of the alignment?

[michaelmorris]

Hi Tim

I used Sharpcap's polar alignment tool. After doing this alignment, a 9 mm e.p.in the 9.25" was holding a star in the centre of a reticle for a good 5 minutes then very slowly drifting off to the west.

Thinking about it, I've realised that the balance is out in Declination axis by quite a lot. Could that be the problem?

[michaelmorris]

Okay.  I balanced the mount and tried again.  Unfortunately it made virtually no difference to the egg-shaped stars (all all with a same direction to their axis) and PHD2 still reported DEC backlash of around 30 secs.  Tried 'north only' and 'south only' Dec guiding and south only appeared to make a big improvement in the DEC guiding graph.  However, cloud rolled in before I could tell definitively whether this translated in to nice round stars or not.

Anyway, it is some progress.  This leaves me with a number of definite actions.

1 - Check the polar alignment - can anyone suggest the simplest alternative to Sharpcap's polar alignment routine to use as a check?

2 - Check for possible differential flexure

3 - Eventually have to try to carry out the DEC backlash adjustment

[andrew s]

One option is to deliberately miss polar align (slightly) so the dec axis always drives in the same direction.

Regards Andrew 

[ollypenrice]

11 minutes ago, andrew s said:

One option is to deliberately miss polar align (slightly) so the dec axis always drives in the same direction.

Regards Andrew 

Don't forget to disable the unused guiding direction. I've found this trick often works well, though.

Olly

[r3i]

Hi Michael,

Have you tried the drift alignment tools in PHD2 to see if they give you a different (hopefully better) PA?

[michaelmorris]

Thanks for the tip.

[michaelmorris]

We seem to have success. ?

After rebalancing the rig to be slightly east heavy in RA, and slightly camera heavy in DEC, and having PHD2 only guiding in DEC with north direction I now have nice small round stars.  Thanks for all the advice - it really did the job!

Now to get rid of these pesky clouds .

PS just worked out that if scope is on west side of pier, DEC guiding should be set to North guiding only, and when scope is the the east side of the pier DEC guiding needs to be set to South guiding only.  Every day a school day.

 

[Stub Mandrel]

The 'right' answer seems to be very specific to your individual setup.

Sorting my EQ3 in DEC needed a fair bit of work! I fitted roller thrust bearings and a stronger higher-resolution stepper, but the final cure was... better polar alignment! It's now consistently achieving about 0.6" in DEC and 1.0" in RA, which isn't world class but is creditable for an EQ3.