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PHD2 RA Spikes

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#1 TheoM

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Posted 07 March 2021 - 05:13 PM

Greetings everyone I hope y'all doing well,

 

For the past few days my guiding is not going well. First let me kick off by saying that my polar alignment is almost perfect (done it with Sharpcap and 00°00'03" PAE) and I did balance very well toward the East to compensate those spikes. I've done the maintenance of the mount (HEQ5 Pro) very recently because there was some play in the RA when the mount was lock tight. I this a series of test and tonight I've done it towards Capella. Things were better from the past few nights ago but i can't get near 1" of RMS. I know that Skywatcher said that the RMS should be around that number but I manage to get 0.70" RMS month ago. I'm using ASCOM and pulse guiding combine with APT. The gears looks great and I've got the mount last year. Notes that i've been using PHD2 for 5 month now and I'm new to it. I try to read and get as much information as I can but I can't manage to get a proper guiding. I hope that some of you will light me up.

PS : My mount has not been Belt-modified but I really considered to buy the Rowan Belt mod.

Thanks a lot,

Theo

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Edited by TheoM, 08 March 2021 - 05:55 AM.


#2 IonClad

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Posted 07 March 2021 - 06:08 PM

Have you tried increasing the guide rate for RA/DEC in EQMOD? I believe the default is 0.4 or so, and I use 0.7 with that same mount and achieve about + or - 1'' oscillations. That would allow faster correction of RA oscillations. The RA oscillations are about 13 seconds, and by my understanding correlated to the teeth on the smallest gear directly attached to the motor. You may consider ensuring that meshing with the adjacent gear is good.

 

I believe you should also have the option of 16-bit data with that guide camera, which would greatly increase star SNR. If overexposing with the ZWO drivers try decreasing the gain to 0.



#3 TheoM

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Posted 07 March 2021 - 06:48 PM

I use it at .7 too. I'll try to modify the data tomorrow. Thanks



#4 rgsalinger

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Posted 08 March 2021 - 12:37 AM

I've never ever seen guiding issues solved by going up / down a bit in guide speed, but there's always a first time.

 

To give you useful (IMHO) help what's needed is for you to post the guiding log from PHD so that we can all see what your parameters are and what the guiding actually looks like. Most guiding problems (if they are serious and not just a bad seeing night) come from a bad calibration. So, that's one other thing to consider. It's best to calibrate at the intersection of the celestial equator and the meridian or as near to that as you can. 

 

Rgrds-Ross



#5 TelescopeGreg

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Posted 08 March 2021 - 01:00 AM

I've never ever seen guiding issues solved by going up / down a bit in guide speed, but there's always a first time.

My AVX has an annoying 11 second RA periodic error, and on a whim I thought I'd try changing the default 50% Guide Speed to 70%, to see if it allowed PHD to make more timely corrections.

 

That made things worse.

 

So, by reverse inference, or some such pseudo-science lingo, some speed settings might improve things over others.  That said, I think you're probably right that a proper calibration would take all this into consideration, and an increase in speed would probably be offset in the software by a decrease in something else.  (I did not re-calibrate after my test change, BTW.  As noted, it was just on a whim...)



#6 TheoM

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Posted 08 March 2021 - 05:53 AM

rgsalinger my bad, i thought the file did upload... Uploading it right now .



#7 rgsalinger

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Posted 09 March 2021 - 06:58 PM

Log is still MIA.



#8 michael8554

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Posted 10 March 2021 - 05:25 AM

Try moving the gear mesh one tooth at a time  and then test.

 

Or you could try the PHD2 PPEC algorithm with the period set to 13.6 seconds.



#9 TheoM

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Posted 10 March 2021 - 01:40 PM

I will try PPEC tonight. Michael, do you know by any chance the Period lenght of the HEQ5 ?



#10 TheoM

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Posted 10 March 2021 - 01:41 PM

It will be auto-corrected right ?



#11 rgsalinger

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Posted 10 March 2021 - 02:39 PM

The log posted shows that the calibration for the guider was done at DEC +46 which is not optimal. It may or may not work well if you are imaging at the same place. If you try to image elsewhere the cosine correction the PHD will apply will not be as accurate as it could be. Best practice is to calibrate the guiding system at or near 0 DEC. Then if you don't remove the guiding system from the OTA there's no need to recalibrate even if you break down and set up each night. Can be a nice time saver. 

 

There is evidence of RA backlash in the long guiding sequence that's at the end of the log. That's really not possible since the RA axis never reverses. So, it must be something else. You can try PEC but I don't think that's the problem here. Is there any chance that there's a dangling cable of some kind affecting the mount's ability to move in the RA direction? Of course, if the mesh is really bad then perhaps the mount just can't slow down - to much momentum fast enough to make the correction. Using a lower guiding speed and perhaps a bit longer guiding cadence might help a bit as well. 

 

Rgrds-Ross



#12 TheoM

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Posted 11 March 2021 - 04:57 AM

Greetings everyone,

 

Just tried PHD2 PPEC last night and I did manage to get less than 1" RMS. Note that there was a lot of wind also. I also played with gearing and found that there was a slight misalignment on the RA gearing, plus getting a perfect balance (maybe try ti balance East heavy next time ?). What do you guys think ?

 

Theo

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#13 michael8554

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Posted 11 March 2021 - 07:04 AM

I will try PPEC tonight. Michael, do you know by any chance the Period lenght of the HEQ5 ?

The point of using PPEC in your case is to try and correct the 13.6 second oscillation.

 

So as I said, that's the setting for the Period, with auto adjust unticked.


Edited by michael8554, 11 March 2021 - 07:09 AM.


#14 michael8554

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Posted 12 March 2021 - 05:49 PM

From the same discussion on "Stargazers Lounge", it seems my recollection of using PPEC for a 13.6 period is faulty, apparently it won't work on that short a period.




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