need help analysing my guiding and mount periodic error

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Dec 21, 2021, 10:46:22 AM12/21/21
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hellow, i recently bought a new mount - ioptron gem28. i manage to sometimes get around 0.9rms and i have a lot of spikes that hinder my captures. i attached a log showing my guiding and also a 2 worm cycles of guiding assitance.

hopefully i can get some info at how to improve there and more important reduce the spikes.

i have less than 2 minutes polar error and my dec is pretty solid yet has spikes too. RA is usually much more jumpy.

help please :)

wave...@talktalk.net

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Dec 21, 2021, 2:57:29 PM12/21/21
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Hello benzam_ _,
 Still learning myself, so what I see in your GuideLog is by no means an expert's view, but the mount seems to have around 20arcsec peak-peak native periodic error in RA. You should be able to reduce that with some training to make guiding a bit simpler. However, one cause of irregular errors./spikes may be from your guidescope arrangement. At 120mm focal length it's making the guide image scale pretty coarse at over 6arcsec/pixel. The mounting of these small scopes is often a source of trouble.. I suspect there's also some backlash in the Dec drive that might need to be considered. Did the Guiding Assistant's Baseline session throw up any warnings/recommendations?
Cheers,
- Jack

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Dec 21, 2021, 3:02:14 PM12/21/21
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thanks for the respond. 
about traaining i assume you mean pec? the mount has software pec only so i need to re-train every setup and im not sure how much help it is.
the recommendation were to add backlash compensation of 2200ms - i did that
and min move for ra and dec that i applied as well.
mounting is done by the guidescope bracket and the main scope has a 288focal lengh so it should be fine with this guider :\

ב-יום שלישי, 21 בדצמבר 2021 בשעה 21:57:29 UTC+2, wave...@talktalk.net כתב/ה:

bw_msgboard

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Dec 21, 2021, 11:17:26 PM12/21/21
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I think you probably have a couple of limitations here.  First, the native RA tracking isn't great and may fall outside the specs for the mount.  Here's a view of the uncorrected periodic error:
 
 
Some of the sections are fairly steep, with a tracking error of around 20 arc-sec in an elapsed time of only 43 seconds.  Much of this can be guided out but you will probably have to guide pretty aggressively with 1-second exposure times.  As a result, you will also have to keep a close eye on your MinMove values to avoid chasing the seeing conditions.  The frequency picture looks like this:
 
 
The large peak is around 164 seconds, probably coincident with the native worm period.  But the other one (left arrow) is more problematic - that's a 1 arc-sec error at a frequency of 21 second.  That's going to be quite difficult to guide out so it will limit the tracking accuracy you will get in RA.  I think you should ask iOptron about these things and see if there are adjustments that can be made.
 
Your second limitation is the one Jack pointed out, your use of a guide camera on a finder-scope.  It's probably ok from an image-scale point of view, which is what you may have been told as part of the sales pitch.  But you are probably going to have problems with the lack of rigidity and mechanical stability.  Any mechanical shift of 4 microns at the guide camera sensor will create a guiding error of 6.5 arc-secs - and a ruined image.  4 microns is less than 1/10th the thickness of a human hair.  You will probably need to do a lot better with the mechanics of the guide scope mount, especially if it's a typical finder-scope stalk mount with Delrin-tipped thumb-screws.  There are many places in your log that show abrupt guide star excursions that have nothing to do with guiding (green is Dec):
 
 
Many of these events disrupt both axes so they probably originate outside the mount: wind gusts, cables pulling or dragging, anything that can cause tiny movements of the wheezy guiding assembly.  If you are skeptical about this, just create a new PHD2 profile and do some guiding sessions with the guide camera connected to the main scope (no imaging) - see if these big excursions get a lot better.
 
Good luck,
Bruce

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Sent: Tuesday, December 21, 2021 12:02 PM
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Subject: [open-phd-guiding] Re: need help analysing my guiding and mount periodic error

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Dec 22, 2021, 3:43:43 AM12/22/21
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i really appreciate the help. i have sent ioptron support a message.
i need to know your opinion before i take my next step, is this a lemon? do i need to start pushing for a replacement? or a simple tune will do the trick.
i am pretty disappointed as i thought ioptron is a good company and they seem not to stand in their own specs claim.

about the guide camera, i have attached a picture of how its on. basically the bracket on the scope and a simple holder. 
i do have a second rig with a 130pds and a 60/240 guider attached in a very firm mount on top. should i try that too or its definitely a mount problem.

ב-יום רביעי, 22 בדצמבר 2021 בשעה 06:17:26 UTC+2, bw_m...@earthlink.net כתב/ה:
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bw_msgboard

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Dec 23, 2021, 12:52:44 AM12/23/21
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I can't really give you an informed opinion about what to do.  I think you should give iOptron the opportunity to look at the problem and decide what they want to do.  If you look on other forums such as the 'Mounts' section on Cloudy Nights, you will find a lot of discussion about these mounts and many different outcomes from people who have encountered problems.  It's really a moving target based on how long the mount has been available, what level of firmware is being used, all the problems associated with shipping in the pandemic, and presumably a growing body of experience for iOptron.
 
Regarding the guide scope arrangement, I wouldn't choose to work this way.  The cable routing is pretty poor - the one connected to the guide camera is just flopping around and can easily move the back of the camera by a few microns.  And of course there's only the single mounting point with a handful of thumbscrews and a long overhang of the telescope beyond the stalk.  In my opinon, it looks weak.  Here's a recent rant I posted on this subject that you can think about (or not):
 

Until a couple of years ago, imagers who used separate guide scopes generally took it seriously: a real refractor telescope mounted close to the main OTA using a dovetail plate and two well-separated clamshell-type mounting rings for the guide scope.  And they were also using guide cameras with larger pixels, in the range of 6-9 microns.  They often went to extra lengths to be sure that one of the mounting points was close to the focuser and guide camera so those elements wouldn’t move around or sag.  As a result, we generally didn’t see many problems with unwanted movement in the guiding assembly, so the ratio of image scales between guiding and imaging was the primary consideration.  But now we’re in a new world of tiny-pixel cameras and slick sales campaigns that urge you to fasten your guide camera on the back of a finder-scope, a use it was probably never intended for.  And more often than not, that little finder-scope is on a central stalk, probably poking a considerable distance above the main OTA, with a handful of  thumbscrews to hold things tight.  As a result, it can be a substantial source of instability.  Even the hanging weight of a guide cable can cause the camera sensor to move by a few microns  (you should try to visualize  how small that is, a human hair is roughly 50 microns thick) and you can get a 6- 10 arc-sec guiding deviation as a result.

 

Good luck,

Bruce


Sent: Wednesday, December 22, 2021 12:44 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Re: need help analysing my guiding and mount periodic error

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Dec 25, 2021, 6:33:17 PM12/25/21
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Thanks for the assistance. I have done some tuning to the mesh and the PE is much better now. I did some tests and fir my surprise i found that at 0.5sec guiding exposures i get a pretty good if not excellent performance of around 0.7rms sometimes as low as 0.58. I wonder if thats normal or has any impact on the data i gather and what does it say on mu moint performance? At 1sec i get around 1rms 

ב-יום חמישי, 23 בדצמבר 2021 בשעה 07:52:44 UTC+2, bw_m...@earthlink.net כתב/ה:

bw_msgboard

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Dec 25, 2021, 10:36:49 PM12/25/21
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It's hard to say without seeing your logs.  If you still have a considerable amount of periodic error, you probably will get better results by guiding aggessively like this.  However, short exposure times usually create other difficulties.  The signal strengh of the stars will diminish, meaning you will probably have fewer stars for multi-star guiding to use.  Guiding accuracy degrades accordingly.  It also leaves you more vulnerable to chasing the seeing, so you will have to be very careful about the MinMove values you use, recognizing that the seeing effects will vary hour-to-hour.  Did you get specific advice from iOptron?  Did they say that a 20 arc-sec peak-peak tracking error was normal?
 
Regards,
Bruce

Sent: Saturday, December 25, 2021 3:33 PM
To: Open PHD Guiding

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Dec 26, 2021, 1:04:52 AM12/26/21
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My question to ioptron is still pending respond.
i did get respond from the vendor and in a group of ioptron users. they verified that that the PE is normal curv of such mounts and that i should set gear mesh on the soft side since they suffer from stiction. i did and the ra spikes settled abit. i do have both axles spikes that i suspect are from wind/cables but i am more worried on the shorter exposures as a requirement from great guiding. added a new log from yesterday that includes both calibration and guiding assistant (and few hours of guiding)


בתאריך יום א׳, 26 בדצמ׳ 2021, 05:36, מאת bw_msgboard ‏<bw_m...@earthlink.net>:
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bw_msgboard

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Dec 26, 2021, 6:06:06 PM12/26/21
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I don't think I can see a statistically meaningful difference between your guiding at 0.5 sec and 1.0 sec.  But you should do what you want. With the short exposures, the multi-star guiding used an average of 3 stars.  With the longer exposures, the average was 6 stars. It all seems a bit moot to me as long as you have these crazy large guide star deflections on both axes, none of which have anything to do with guiding.  But depending on what sort of imaging you're trying to do and the image scale of your main scope, your current situation may be good enough if you're willing to discard frames that are ruined by the large deflections.
 
Good luck,
Bruce

Sent: Saturday, December 25, 2021 10:05 PM
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Dec 26, 2021, 11:19:52 PM12/26/21
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When you say atar deflection you mean terrible PE and mount performance?

בתאריך יום ב׳, 27 בדצמ׳ 2021, 01:06, מאת bw_msgboard ‏<bw_m...@earthlink.net>:
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bw_msgboard

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Dec 26, 2021, 11:35:18 PM12/26/21
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No, I'm talking about the same things I showed you in the original message:
 
 
I'm talking about the apparent movement of the guide star on the guide camera sensor - that's what's being plotted here, that's what I'm calling a "guide star deflection".  When these occur, PHD2 issues a bunch of guide pulses to correct them.  So the stars in your main image are going to be moving around by as much as 3-10 arc-sec in the example above.   As I said, these things have nothing to do with PE or mount performance, they are caused by what you dismissed as "wind/cables".  I suggested they are caused by your flimsy guide scope arrangement.  Whatever the source, you should probably fix them or decide to live with what you've got, they are currently the major limitation in your guiding results.

Sent: Sunday, December 26, 2021 8:20 PM
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Dec 26, 2021, 11:49:21 PM12/26/21
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Ah i see. I havent dissmissed it. I take everything you say into account. Already ordered zip ties, guide scope(60/240) + mount and power box to makes things tighter. Thanks alot.

בתאריך יום ב׳, 27 בדצמ׳ 2021, 06:35, מאת bw_msgboard ‏<bw_m...@earthlink.net>:
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bw_msgboard

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Dec 26, 2021, 11:54:32 PM12/26/21
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Ok, sorry, I took your earlier reply to mean you didn't think those were important - my bad. :-(  I think if you do what you're already planning here, you will probably be in reasonable shape unless you're trying to do high-resolution imaging. 
 
Hope things go smoothly,
Bruce

Sent: Sunday, December 26, 2021 8:49 PM

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Dec 27, 2021, 12:16:37 AM12/27/21
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First of all you needn't be sorry you have been the greatest help i have recieved considering ioptron are yet to reply.
I wanted to make sure this isnt a faulty mount as its new and i was even considering a return.
In everything else i can deal myself.

The communities are the ones who solved my problems 
And for those that will follow with the same problem, i found out the the springed gear meshing on the gem28 comes usually too tight from factory creating sticktion and after going 3/4 out on both dec and ra the spikes were dramatically reduced and guiding were able to deal with it smoother, i am now getting around 0.7rms pretty steady with some deflection cause by cables or guidescope unfixed problems. These will be solved soon :)


בתאריך יום ב׳, 27 בדצמ׳ 2021, 06:54, מאת bw_msgboard ‏<bw_m...@earthlink.net>:
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