Pattern of RA Spikes after Good Calibration - Ideas to Resolve?

[gary]

Hi Folks

Last night I performed a Calibration of the iGuider on my iOptron CEM70G mount. The location for the calibration was quite close to the intersection of the meridian and Dec 0.

After the calibration, guiding seemed to be going nicely until a pattern of 3-4 regularly-space sharp RA 'spikes' started to occur. At one point there was also a Dec spike which I believe only occurred once. The guiding 'RMS Total' was quite low at around 0.4 but I have to believe the spikes are a problem that I need to solve and I hope you can help me with that.

The link to the guiding log is below along with an attached image of the calibration info. If you need any further info, just let me know.

Thank you for any observations or ideas that might help me understand and correct the problem. If the spikes can go away, I'll have the pretty decent guiding that I need for 3-5hr exoplanet transit imaging runs

Thanks!

Gary

Guiding log:

https://openphdguiding.org/logs/dl/PHD2_logs_WKze.zip

[Brian Valente]

Hi Gary

is this an encoder mount?

This looks like a thorny one. 

Couple observations:

first, your calibration/expected rate is about 1/2 of what is expected. not sure if you are fiddling with the settings or something else is going on there

Regarding the spikes, they are not coming from PHD, they are fairly large, instantaneous, and return immediately (although there is a guide pulse at the same timing, I don't think that is the cause of the return, it's too precise). 

This kind of thing to me is usually some sort of mount-based setting, possibly dealing with encoders, periodic error, or other issues. when they happen they seem to be regularly spaced at 50 second intervals.

I'm not sure of the source, you may want to share this with ioptron. 

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[Gary Shaw]

Hi Brian

This is a straight CEM70G - no encoders. 

I’ve never seen these spikes before and I’ve used the mount for several years. It was just following the calibration that they appeared. 

Only thing different last night was the dew shield I added to the OTA (8” F4 Newtonian). I rebalanced everything after adding that and before doing the calibration. 

Thank you!

Gary

On Aug 9, 2022, at 11:23 AM, Brian Valente <bval...@gmail.com> wrote:



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[Brian Valente]

I suggest setting up a new profile and going through the profile wizard, just to make sure the calibration turns out correctly

It probably won't impact your spikes, but the mount should behave as expected re: calibration

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[gary...@gmail.com]

Hi

I set up a new profile before doing the calibration and running the Guiding Assistant last night. I can certainly do that over again but if you don’t think it would rid me of the spikes, is there another benefit from re-doing everything – perhaps just seeing if the problem repeats ?

G

 

From: open-phd...@googlegroups.com <open-phd...@googlegroups.com> On Behalf Of Brian Valente
Sent: Tuesday, August 9, 2022 12:07 PM
To: open-phd...@googlegroups.com
Subject: Re: [open-phd-guiding] Pattern of RA Spikes after Good Calibration - Ideas to Resolve?

 

I suggest setting up a new profile and going through the profile wizard, just to make sure the calibration turns out correctly

 

It probably won't impact your spikes, but the mount should behave as expected re: calibration

 

 

On Tue, Aug 9, 2022 at 11:04 AM Gary Shaw <gary...@gmail.com> wrote:

Hi Brian

This is a straight CEM70G - no encoders. 

I’ve never seen these spikes before and I’ve used the mount for several years. It was just following the calibration that they appeared. 

 

Only thing different last night was the dew shield I added to the OTA (8” F4 Newtonian). I rebalanced everything after adding that and before doing the calibration. 

Thank you!

Gary

 

On Aug 9, 2022, at 11:23 AM, Brian Valente <bval...@gmail.com> wrote:



Hi Gary

 

is this an encoder mount?

 

This looks like a thorny one. 

 

Couple observations:

 

first, your calibration/expected rate is about 1/2 of what is expected. not sure if you are fiddling with the settings or something else is going on there

 

Regarding the spikes, they are not coming from PHD, they are fairly large, instantaneous, and return immediately (although there is a guide pulse at the same timing, I don't think that is the cause of the return, it's too precise). 

 

This kind of thing to me is usually some sort of mount-based setting, possibly dealing with encoders, periodic error, or other issues. when they happen they seem to be regularly spaced at 50 second intervals.

 

I'm not sure of the source, you may want to share this with ioptron. 

 

 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i6grjv7F4wP3Be7sr89DqhwQn4s0ii6oZhvZ5MkcOeCqw%40mail.gmail.com.

[Brian Valente]

Gary,, as i mentioned i don't think it will impact your spikes. When they happen they happen on a period of about 50 seconds. It's worth reviewing your mount/user group to see if that's a known period for your mount

Regarding the new profile, you should end up around 12 calibration steps in each direction when pointed at meridian/ c equator. Yours are more like 24. So I don't know if that's an issue with your profile settings or something else, but it doesn't seem right. Again, nothing to do with your spikes. Or maybe it does, don't know ;) 

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[gary...@gmail.com]

Brian:

I just reviewed the profile and found that I had entered the FL as 250mm rather than the correct one at 120mm.   I’ll redo the profile and rerun the calibration and Guiding Assistant tonight and see what happens.

 

I think this might explain the difference between the ‘Expected’ and actual RA and Dec rates.  Any thoughts on whether that FL goof could be the culprit behind the spikes?

 

Thank you…

Gary

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[Brian Valente]

>>>I think this might explain the difference between the ‘Expected’ and actual RA and Dec rates.  Any thoughts on whether that FL goof could be the culprit behind the spikes?

Nice, that would explain it

no, I don't think that would explain it. As I mentioned before, the first thing i'd check is if 50 seconds is a known period with that mount. if it is, then at least you know something is up with the mount mechanics, maybe a belt tooth is slipping or similar.

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[gary...@gmail.com]

Brian

Here’s the spec sheet on the mount. It notes the worm period is 348 secs.

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[David Kerber]

There are many more potential sources of periodic errors in a mount than just the worm period. Any of the reduction gear stages are potential (though less likely) sources, as is the belt (tooth frequency, specifically).

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[Brian Valente]

>> There are many more potential sources of periodic errors in a mount than just the worm period. Any of the reduction gear stages are potential (though less likely) sources, as is the belt (tooth frequency, specifically).

yep - this is correct. I'm not just referring to the primary pe. 

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[gary]

Hi 

By redoing the calibration with the correct FL for iGuider in the profile, the spikes seem to have disappeared and the Guiding Assistant gave the identical recommendations as it did the prior night. The annoying thing is that the total RMS was far lower the first evening while the spikes were evident. Last night total RMS was around 0.8 whereas it was in the 0.3. - 0.4 range. I had done the calibration on the same star in about the same location on both evenings and our skies and seeing appeared mainly the same for both evenings. 

If the spikes were causes somehow by the incorrect FL entry, perhaps I should do that purposefully if it results in superior guiding - even with those RA spikes ( lol ).

Thank you

Gary 

[Brian Valente]

>>>If the spikes were causes somehow by the incorrect FL entry, perhaps I should do that purposefully if it results in superior guiding - even with those RA spikes ( lol ).

hmm... that's surprising to me, but it's hard to argue with results ;) Good job hunting that down Gary

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[Bruce Waddington]

Hi Gary.  I think you're winding yourself around the axle a bit. <lol>  If you specified a focal length that was 2x the correct value, the reported guiding statistics would have been 1/2 the actual amount in units of arc-sec.  That's just a reporting problem, the actual guiding performance was the same.  And I don't see any way that this could have had anything to do with the appearance or disappearance of spikes in RA, that's something else.

Regards,

Bruce

[Gary Shaw]

Hi Bruce

So, that’s about what the RMS from both nights tells us. With the incorrect FL of 250mm, it was around 0.4 and with the corrected 120mm FL, it was in the area of 0.7-0.85 - depending on the min move I was testing at the time. I’m glad the spikes are gone but still a bit haunted by not know the cause.

Cheers

Gary

On Aug 10, 2022, at 8:07 PM, Bruce Waddington <bw_m...@earthlink.net> wrote:



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