JUWEI 17 - impossible to get accurate guiding

[pierre-yves lechapelain]

Hi ! 

I am testing the Juwei 17 mount : 

imaging train : 130 650 reflector with QHY 294 C 
guiding train : SV165 (120mm) with ASI 585MC camera 
guide speed : 0.5x 

Link to my logs : https://openphdguiding.org/logs/dl/PHD2_logs_cHnV.zip

Fact : for now tracking is not good enough, 0.8 to 1.7 arcsec RMS : 




link to CN discussion : https://www.cloudynights.com/topic/918425-juwei-17-mounts-on-aliexpress/page-28

I do not know what exactly i am doing wrong...a lot of the periodic error is still not compensated. 

Has anyone an idea that could help ? Thanks a lot 

[pierre-yves lechapelain]

I tried changing guiding rate from 0.5x sideral to 1.0x, did a calibration : 


Configuration of predictive PEC : 






Results are still bad : 1 arcsec RMS 


log file : https://openphdguiding.org/logs/dl/PHD2_logs_5H4c.zip

[pierre-yves lechapelain]

Analysis of the log file show : 

Periodic error of +/- 25 arcsec : 

A big portion of the periodic error is still there : 

[Brian Valente]

Hi Pierre-yves

>>>>Periodic error of +/- 25 arcsec : 

Yes, that is pretty typical for these kinds of mounts.

You might look through the other message threads on this forum, there are several discussions of the Juwei 17 mount and some questionable mount performance.

In general you need to guide this really aggressively. 

use 1 second exposures or less. 

Consider using RA PPEC algorithm with a period of 422 seconds (based on your results) and disable auto adjust period. you will need to evaluate aggression settings for both reactive and predictive. There is a tradeoff between high aggression, exposure time, and possibility of introducing oscillations. You will need to find the right balance for your mount. 

PHD can effectively compensate for significant amounts of error, but there are some errors too large to address. The Juwei 17 seems to be one of those with exceptionally high error. Another customer reported a PE of 60"

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Brian Valente

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[Brian Valente]

PS

I also want to add this:

looking at the residual error (error that is uncorrectable by PHD) you will see the primary PE is highest. But it also has two other periods that are high and uncorrected: 211sec and 18 sec

It's possible the 211 second will be correctable with more aggressive guiding, but the 18 second will likely be a constraint to your RA RMS.

[Nelson Viegas]

Hello Pierre-yves,

I have a mount like yours and they need to be guided agressively. I'm using 500ms exposures with 1x sideral speed guiding with the default PHD2 guiding algorithms. It has been ok for my mount, but I see that mine has slightly lower main periodic error than yours, but I guess that shouldn't, in this case, matter too much.

In my case, I was guiding a long 127/980mm refractor with OAG (guiding camera binned 3x).

DEC was a bit noisier, as the OTA was in a "sail" position. :)

And the RAW periodic errors:

And the guided performance:

My mount has some upgraded electronics I designed which increases the motor voltage and microstepping which I guess can make some diference and also makes it go faster, up to 8º/sec, but in some tests I made, I concluded that the harmonic reducer "smoothness" is the limiting factor here.

Good luck!

Best regards,

Nelson Viegas

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[pierre-yves lechapelain]

Thanks Nelson !

I am going to try using my main imaging train at 480 mm focal length to check is using an OAG would work better. 

Nice upgrade, I am also thinking about some modifications, I would love your input on them :

- change the nema 17 motors to 0.9 degrees stepper

- switch to a fysetc E4 board to enable higher micro stepping 

Could you send me your log file (of the screen copy you shared) so that I can compare them in details with mine ?

Have a good day 

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAOumaJVDsKDuJ%3DYw_a9FA2HPdaVzW84WyVvWo6Hckp%2B5tpzD3A%40mail.gmail.com.