Big spikes during guiding - "perfect" and then random spikes

[Richard T]

Hi All

I'm at a loss and reaching out for help. I've been trying to get great guiding for two years now and just can't crack it.

Last night's log attached.

Log 19. Remove the spike and Total RMS Error is 0.64. I think PEC OFF.

Log 20. Two shots, stating 22:59 and 23:09 - fantastic and with remainder of guiding removed, RMS Error 0.49". New PEC curve done and PEC ON.

Issue is the spikes. Ruins all other shots.

Balance is as good as it can be. RA balanced, head heavy, weight heavy - same result.

PA as good as I can get.

Clutches tight. All in focus.

DEC is spring system. Bit weird is 2000+ms Backlash Comp but it works. Bearing blocks tight. No sign of backlash when manually twisting DEC or RA. Rock solid, no play.

Multi Star guiding usually picks a bloated star. Also tried single nice profile star.

Appreciate any "guidance" 

Regards Richard

[Ken Self]

Hi Richard

Your guide log show a Calibration issue of "Rates". There is no calibration in the log so we cannot analyze what went wrong there. A good calibration is critical for guiding. 

The log also shows you are not using darks for your guide camera. That wont necessaily cause your problems but is not good practice.

Please read the best practices guide at: https://openphdguiding.org/phd2-best-practices/

I'd suggest to create a new profile using the profile wizard and follow all the prompts. It will walk you through the creation of a darks library.

Then do a calibration and run the Guiding Assistant. If the calibration is still a problem, come back here with the guide log including the calibration.

The spikes in the graphs are always synchronized in both RA and Dec which is odd. A movement of the tripod/pier would normally give different movements in RA and Dec. So off the top of my head I would imagine something in the guider optical train. Maybe some looseness in the guide scope mounting. It need not be much - just a few microns.

Cheers

Ken

[Brian Valente]

Hi Richard

I hope i understood all your details

your spikes are likely caused by a bad PEC curve. Try turning it off. if the spikes go away, that confirms it.

See how that goes

 Brian

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Bryan]

Ken

There is a calibration ... section 12.  It looks adequate to me.

The coincident RA and Dec spikes struck me as odd, as well.

Bryan

[Richard T]

Thanks Guys

Thinking what Ken said, all is mounted quite tight etc BUT then I thought, I do have a guide system focuser ZWO EAF changing focus position, temperature compensation. I bet the spikes are at the same time as focus temp compensation is executed. 

Something to check anyway. Would explain why both suddenly spike. MAYBE ??

[Ken Self]

@Bryan - oops didn't see the calibration and it does look like a "keeper". Odd that the Rates issue comes up. Does the camera have rectangular oixels?

If the focuser moves that could be enough to move things. Another thing to look at is if you have trailing stars on your images.

[bw_msgboard]

I don’t think the calibration qualifies as a “keeper”.  I would say that you didn’t manually clear the Dec backlash before you did the calibration and that led to a measured Dec guide rate that is too low.   If you have substantial Dec backlash, which appears to be the case, you *must* manually clear that backlash before you do a calibration.  *Every time*.  In this case, the computed Dec guide rate is 25% *lower* than the RA guide rate.  That should never happen, the RA guide rate should be less than or equal to Dec depending on the pointing position, never greater.  That’s why you got the alert about suspicious rates.  That doesn’t explain the spikes, it just answers the question about the calibration.  This looks very much like a repeated external event that has nothing to do with guiding but your idea about the focuser moving sounds promising.  Micro-tuning the guide system focus isn’t a normal practice, what has convinced you that it’s necessary?  Anything that intentionally moves on the guiding assembly is likely to create more problems than it’s worth in my experience – the focuser will sag unless it’s locked in position.

 

Bruce

 

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[Richard Triglavcanin]

Thanks Ken & Bruce

 

Re calibration I think its something to do with spring loaded DEC design with the G11. I just accept calibration and seems OK.

 

The DEC compensation of 2000+ ms is odd but again works. Don’t like it and will work on it but for now seems OK. It may throw off the Guiding Assistant results, not sure.

 

I used to get oblong stars but not so much anymore. Guide camera is 2.4 x 2.4 pixels.

 

These days I get the stars as two or maybe three stars during the 10 min shot. Guiding jumps and …

 

I just checked the APT log file, APT Camera 3 which just does the Temp Compensation focus moves. The timings for focus compensations and spikes match.

 

Will try again in a couple of days – WITHOUT temp comp.

 

 

 

From: open-phd...@googlegroups.com <open-phd...@googlegroups.com> On Behalf Of Ken Self
Sent: Tuesday, 26 January 2021 9:29 AM
To: Open PHD Guiding <open-phd...@googlegroups.com>
Subject: Re: [open-phd-guiding] Big spikes during guiding - "perfect" and then random spikes

 

@Bryan - oops didn't see the calibration and it does look like a "keeper". Odd that the Rates issue comes up. Does the camera have rectangular oixels?

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