Guiding accuracy differences between targets

[Graham Smith]

Hi

On the same night with no changes other than moving to a different target my guiding numbers look very different. 

The 1st target starts at 9:33 and ran for 75 mins with RMS values of RA 1.39", Dec 0.64", total 1.53". These are fairly good numbers for my setup (an old EQ6R Pro with a ED127 OTA). 

The 2nd target starts at 23:13 and also ran for 75 mins with RMS values of RA 2.39, Dec 0.75", total 2.51", not so good.

I believe the logs show where the I was pointing and the pier side per session (west then east). There were no obvious differences in sky quality per target.

I believe guiding accuracy can differ per regions of the sky, but this difference seems too high. Appreciate any pointers to how I can improve. 

https://openphdguiding.org/logs/dl/PHD2_logs_qL79.zip

[Brian Valente]

Hi Graham

It looks like your mount's RA axis has some pretty significant issues that would explain what you are seeing

First, here's your 3 minutes unguided result. The RA axis (in blue) should be a smooth oscillating sine wave, but instead it's a stair-step in one direction moving nearly 300" (an enormous amount for a PE)

 

this behavior shows up as large excursions in RA (again in blue)

Your first target the measured primary period error in RA is about 10" peak-peak:

When you move to the second target, that error is nearly doubled to 19.3" peak-peak:

This is pretty obviously a mechanical problem in RA. I suspect something is loose (maybe the bearing pre-load?) but an unguided like what you have... I don't think i've ever seen something like that.

Probably best to share the details with iptron and see what they recommend for adjustment/repair/replacement.

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Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[Graham Smith]

Thanks for the quick reply. 

I ran guide assistant after the calibration and I think that is the 1st 3 minutes. Would that create the stair step graph?

Would loosening off the backlash adjustment on RA be a good experiment to see if the bearing is overloaded?

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[Brian Valente]

>>>I ran guide assistant after the calibration and I think that is the 1st 3 minutes. Would that create the stair step graph?

Yes, that's what I am referring to "unguided run/GA = guiding assistant"

That graph represents your RA axis moving at sidereal speed. As to what would cause that, as I mentioned before we can't tell from guiding graphs, but from the guiding assistant run and your follow-on guide charts, it definitely is mechanical.

You need to make adjustments to your RA, beyond that I can't really say. 

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