Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

CEM 60 Periodic RA drift

Equipment Mount Imaging
  • Please log in to reply
7 replies to this topic

#1 dsullivan

dsullivan

    Sputnik

  • *****
  • topic starter
  • Posts: 42
  • Joined: 07 Jan 2018
  • Loc: West Hartford CT

Posted 03 July 2022 - 07:41 AM

I'm having trouble with what I believe to be backlash in my CEM 60's RA. Since the backlash is periodic I believe the issue could be caused by damage to the RA gearing. The RA assembly has been dismantled and inspected, however I am unsure if there is damage to the ring or worm gear.  

 

  • Previous sessions ran flawlessly with the exact same OTA/setup/configurations and I am unaware of any incident that may have caused this issue to begin occurring
  • All imaging sessions from the past month now produce unusable subs (300s) displaying star drift in both directions
  • This occurs when imaging targets in different sky locations
  • When the RA is engaged, there is no observable backlash or play when pressure is manually applied in either direction 
  • Backlash occurs periodically, suggesting damage/flaw with RA gearing

 

 

RA Worm Gear Inspection Video

There doesn't appear to be any damage to the worm gear.
 

RA Ring Gear Inspection Video

There appears to be some wear to the ring gear but no drastic damage, though I'm not entirely certain of this assessment.
 

PHD Guiding Assistant Backlash Example Video
You can see the severity of the backlash, and how it occurs in regular intervals.


Imaging session examples
PHD showing periodic backlash:

heUiLM0.png

 

5min sub with star drift from backlash:

unknown.png



#2 michael8554

michael8554

    Vanguard

  • -----
  • Posts: 2,184
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted 03 July 2022 - 08:03 AM

Have you carried out a PHD2 Guide Assistant run of several worm turns, to show the mount's inherent RA behaviour ?

 

Have you got PPEC engaged ?

 

Is it a good model ?

 

Is RA better without PPEC ?

 

Couldn't open your videos, a PHD2 GuideLog would be more instructive.



#3 dsullivan

dsullivan

    Sputnik

  • *****
  • topic starter
  • Posts: 42
  • Joined: 07 Jan 2018
  • Loc: West Hartford CT

Posted 03 July 2022 - 08:46 AM

Have you carried out a PHD2 Guide Assistant run of several worm turns, to show the mount's inherent RA behaviour ?

Yes, the PHD Guiding Assistant Backlash Example Video I linked to shows 11 minutes worth of tracking, I'm assuming that's a few worm gear turns, and you can see the RA behavior repeating itself a few times.

https://drive.google...s_MKNgfsKa/view

 

Have you got PPEC engaged ?

I have the base level CEM60 which does not have PPEC 

 

Is it a good model ?

Not applicable

 

Is RA better without PPEC ?

Not applicable

 

Couldn't open your videos, a PHD2 GuideLog would be more instructive.

They are links to videos on Google Drive. I tested them on different devices and accounts. They might take a second to load but they provide much more information for troubleshooting. 



#4 Der_Pit

Der_Pit

    Mercury-Atlas

  • *****
  • Posts: 2,658
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 03 July 2022 - 05:48 PM

PHD2 has a PPEC algorithm for RA guiding that should take care of such periodic variations, however, as they are quite large and steep, you'd need a quite fast cadence, i.e., short exposures.

Usually the culprit for periodic error is the worm gear, for strong excursions sometimes even only some dirt (or bug...).  RA backlash shouldn't be there for a CEM60, unless the belt tension is wrong (*) or the magnetic meshing isn't adjusted properly.

 

(*) have you checked that?  In doubt attached is the iOptron manual for doing that

 

 

Attached Files



#5 rjsc2000

rjsc2000

    Mariner 2

  • -----
  • Posts: 237
  • Joined: 21 Sep 2006
  • Loc: Portugal

Posted 04 July 2022 - 03:48 AM

I don't have the mount but if that error is periodic, can you mark the worm with a marker when it makes that spike? And check when it happens again, if it's in the same spot. You can also check the bearings



#6 michael8554

michael8554

    Vanguard

  • -----
  • Posts: 2,184
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted 04 July 2022 - 04:12 AM

The GA video in post #3  shows some fast RA excursions.

 

Use the GA  Frequency Analysis to measure the frequency.

 

Some belt-drives have fast harmonics, eg 7 secs, that are too fast for PHD2 to correct.

 

Often reduced by belt adjustment, or moving the belt along a few teeth.



#7 Malbrough

Malbrough

    Sputnik

  • *****
  • Posts: 42
  • Joined: 17 Feb 2021

Posted 04 July 2022 - 02:38 PM

Been experiencing this on my cem60ec. Just did RA adjustment mentioned above, will report results soon.

#8 Malbrough

Malbrough

    Sputnik

  • *****
  • Posts: 42
  • Joined: 17 Feb 2021

Posted 04 August 2022 - 12:59 PM

My problem turned out to be a combination of bad seeing conditions and my polar scope crosshairs being out of line. I did the ioptron polar scope alignment, and all is well.

Was a bit tedious of an alignment, but figured it out. Removal of the polar scope was not needed, just removed the bubble level and tweeked the 3 hex screws little by little to center the crosshairs.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Equipment, Mount, Imaging



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics