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Celestron CGX Mount Slew Issue

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#1 GlendaleGuy

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Posted 13 February 2020 - 09:00 PM

    I've been having a problem with PHD giving an error during it's calibration phase which it indicated might be backlash. I decided to take everything out tonight and see if I could tell what is going on. I believe I see the symptom now, but not the cause.

 

    When slewing west at slow speed everything looks fine, but when going east after letting up on the slew button the scope continues to move east for approximately 2 seconds at a very slow speed compared to the slew rate. I set the motor speed down to 3 and it was very noticeable how going west it stopped immediately when released, but continued drifting east if the slew was going east. It didn't matter if it was a 10 second slew or just a tap of the button.

 

    I played around with the backlash settings and found no improvement. After messing around with it I decided to reset to factory settings. The problem continued.

 

    I also tried biasing the balance slightly west heavy and east heavy, again no difference.

 

    Has anyone run across this problem or know how to alleviate it? I haven't put in a support request yet because frankly I seem to get better information here than from Celestron.

 

 

   Mount: CGX

   Scope: C11 Edge HD

   Software Ver.: HC: GEM 5.31.9102

                           MC: 7.15.9120


Edited by GlendaleGuy, 13 February 2020 - 09:22 PM.


#2 Cfreerksen

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Posted 13 February 2020 - 11:04 PM

I am sure that is not the backlash that PHD2 is warning about. E & W backlash is really not a concern. The mount when guiding is constantly going to the west. Even when a guide pulse to the east is initiated it only slows the westward travel. It never makes the mount reverse direction.

 

Upon closer review you will no doubt notice that it is complaining about the Declination backlash. The way the mount motors are made it is impossible to eliminate DEC backlash on the CGX. It can be minimized, but not eliminated. Take a look at your calibration results. I am sure that is the DEC. It is really not a big deal as long as it is not huge. If it is a bother and you are getting poor guiding results in the DEC axis, some folks intentionally make their PA slightly off and guide in only one direction on DEC. 

 

What kind of results are you seeing when you guide?

 

Chris



#3 freestar8n

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Posted 14 February 2020 - 01:07 AM

This can certainly be backlash if the slew speed used was faster than sidereal.  The OP describes slewing with a button - and I assume that is separate from some calibration process for the mount.

 

There may be two unrelated issues here - one involving backlash when slewing in RA at faster than sidereal rate - and another that causes odd calibration results due to dec. backlash at guide rate.

 

I have a cgx-l and I do see this effect when slewing faster than guide rate - and I assume it is due to backlash and either imbalance or balance that is too good.  But it has no detrimental impact because guiding happens for me at 0.5x sidereal - and backlash shouldn't happen in RA.

 

I guide with MetaGuide, which I wrote and is free, and it does a simple calibration of the mount and isn't concerned about how exactly dec. behaves.  I see no point in worrying about it because it will always have some backlash involved and dec. response is hard to handle cleanly.  But at the same time dec. errors tend to be slower than RA errors and you have more time to chase after any errors in dec.

 

In short - this all sounds normal and shouldn't impact guiding.  If it is backlash in RA, then some intentional imbalance may help.  But it won't matter for guiding.

 

Frank


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#4 fgraus

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Posted 14 February 2020 - 08:30 PM

 

 

    When slewing west at slow speed everything looks fine, but when going east after letting up on the slew button the scope continues to move east for approximately 2 seconds at a very slow speed compared to the slew rate. I set the motor speed down to 3 and it was very noticeable how going west it stopped immediately when released, but continued drifting east if the slew was going east. It didn't matter if it was a 10 second slew or just a tap of the button.

 

  

   Mount: CGX

   Scope: C11 Edge HD

   Software Ver.: HC: GEM 5.31.9102

                           MC: 7.15.9120

When going east, are you watching the mount moving or are you looking through the scope and seeing the motion of the stars? Have you looked through the window at the pulleys and belt when you release the button to see if the pulley stops when you release the button and then starts turning in the other direction? I wonder if it is programmed in the mount to reduce load on the gears when reversing direction. 



#5 GlendaleGuy

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Posted 15 February 2020 - 11:01 PM

I am sure that is not the backlash that PHD2 is warning about. E & W backlash is really not a concern. The mount when guiding is constantly going to the west. Even when a guide pulse to the east is initiated it only slows the westward travel. It never makes the mount reverse direction.

 

Upon closer review you will no doubt notice that it is complaining about the Declination backlash. The way the mount motors are made it is impossible to eliminate DEC backlash on the CGX. It can be minimized, but not eliminated. Take a look at your calibration results. I am sure that is the DEC. It is really not a big deal as long as it is not huge. If it is a bother and you are getting poor guiding results in the DEC axis, some folks intentionally make their PA slightly off and guide in only one direction on DEC. 

 

What kind of results are you seeing when you guide?

 

Chris

   OK, there was a time of no clouds so I ran it tonight. I just stuck the Loadstar x2 right into the back of the scope, no OAG or anything, and picked a fairly bright star. I also tweeked the Dec backlash setting up to +/-3 to make sure there was no hesitation before installing the Lodestar. I forced a cal and didn't get the error today like I have every other time. I also did a screen shot of PHD with the statistics. I guess I'm going to have to go back to the full equipment setup and see where I am and what makes the difference if it starts acting up again.

Attached Thumbnails

  • CGX 2_15_20 PHD.jpg

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#6 Cfreerksen

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Posted 16 February 2020 - 12:16 AM

Looks like your guiding is not too bad. 0.74RMS is pretty good for a CGX. I have seen better on a night of good seeing. Have seen much worse too. On a couple of nights I could barely get an autofocus to run because everything was jumping around so much. Guiding was about 2.1RMS. Just packed it in for the evening. how steady the sky is will make a huge difference.

 

I had a good night last night. Albeit a very northerly target. 0.57RMS was about average. I think the low was about 0.39 See attached.

 

You should watch for the backlash setting causing jumps in the guiding if set too high. I use the setting in PHD2. I had to do the setting myself as it's recommendations were way too aggressive.

 

Glad things are working out for you. Guiding settings can be a pain to get just right with lower quality mounts.

 

Chris

Attached Thumbnails

  • 2020_02_14_21_08_45.jpg

Edited by Cfreerksen, 16 February 2020 - 12:21 AM.



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